Google
This is a digital copy of a book that was preserved for generations on Hbrary shelves before it was carefully scanned by Google as part of a project
to make the world's books discoverable online.
It has survived long enough for the copyright to expire and the book to enter the public domain. A public domain book is one that was never subject
to copyright or whose legal copyright term has expired. Whether a book is in the public domain may vary country to country. Public domain books
are our gateways to the past, representing a wealth of history, culture and knowledge that's often difficult to discover.
Marks, notations and other maiginalia present in the original volume will appear in this file - a reminder of this book's long journey from the
publisher to a library and finally to you.
Usage guidelines
Google is proud to partner with libraries to digitize public domain materials and make them widely accessible. Public domain books belong to the
public and we are merely their custodians. Nevertheless, this work is expensive, so in order to keep providing this resource, we liave taken steps to
prevent abuse by commercial parties, including placing technical restrictions on automated querying.
We also ask that you:
+ Make non-commercial use of the files We designed Google Book Search for use by individuals, and we request that you use these files for
personal, non-commercial purposes.
+ Refrain fivm automated querying Do not send automated queries of any sort to Google's system: If you are conducting research on machine
translation, optical character recognition or other areas where access to a large amount of text is helpful, please contact us. We encourage the
use of public domain materials for these purposes and may be able to help.
+ Maintain attributionTht GoogXt "watermark" you see on each file is essential for informing people about this project and helping them find
additional materials through Google Book Search. Please do not remove it.
+ Keep it legal Whatever your use, remember that you are responsible for ensuring that what you are doing is legal. Do not assume that just
because we believe a book is in the public domain for users in the United States, that the work is also in the public domain for users in other
countries. Whether a book is still in copyright varies from country to country, and we can't offer guidance on whether any specific use of
any specific book is allowed. Please do not assume that a book's appearance in Google Book Search means it can be used in any manner
anywhere in the world. Copyright infringement liabili^ can be quite severe.
About Google Book Search
Google's mission is to organize the world's information and to make it universally accessible and useful. Google Book Search helps readers
discover the world's books while helping authors and publishers reach new audiences. You can search through the full text of this book on the web
at |http : //books . google . com/|
;
'/-
Jf
THE FROG:
AN INTRODUCTION TO ANATOMY.
HISTOLOGY, AND EMBRYOLOGY.
I^
THE FROG:
AN INTRODUCTION TO
ANATOMY, HISTOLOGY, AND
EMBRYOLOGY.
BY THE LATE
A. MILNES MARSHALL, M.D., D.Sc, M.A., F.R.S.,
.»-
FORMERLY FELLOW OF ST. JOHNS COLLEGE, CAMBRIDGE; PROFESSOR IN THE
VICTORIA UNIVERSITY ; BEYER PROFESSOR OF ZOOLOGY
IN OWENS COLLEGE, MANCHESTER.
EDITED BY
F. W. GAMBLE, D.Sc,
ASSISTANT LECTURER AND DEMONSTRATOR IN ZOOLOGY,
UNIVERSITY OF MANCHESTER
NINTH EDITION
REVISED AND ILLUSTRATED
LONDON
DAVID NUTT
57-59 LONG ACRE *
1906
HARVARD MEDICAL LIBRARY
IN THE
FRANCIS A. COUNTWAY
LIBRARY OF MEDICINE
PREFACE TO THE NINTH EDITION.
This little work is intended to supply the student with a
practical guide to the study of elementary anatomy, histology,
and embryology. For this purpose the frog is the animal
chosen, as being easy to obtain, convenient to dissect, and
a fairly typical example of the great group of vertebrate
animals. Where, from its small size, or for other reasons,
the frog has proved unsuitable, other animals have been
substituted for it. For convenience of reference, and in
order to definitely stamp the practical character of the
' work, directions for dissection, <fec., have been printed in
italics.
The late Professor Marshall had so complete a grasp of
the difficulties which coofront the student of elementary
^ zoology, and so great a sense of proportion and arrange-
^ ment, that no substantial alteration can increase the
educational value of his work. In the present edition,
however, I have revised the chapter on development and
added some figures. I am indebted to Mr. J. W. Jenkinson
) of Exeter College, Oxford, for help in the preparation of
r the new figures, and to Dr. Hans Gadow of Cambridge for
kind assistance. Through the courtesy of Messrs. Smith,
^ Elder & Co. I have been able to reproduce seven additional
^ illustrations from Professor Marshall's " Vertebrate Embry-
^ ology."
F. W. GAMBLE.
Manchester, January 1906.
r
V
?\
CONTENTS.
Introduction.
FAGB
Apparatus required. Dissection. Drawing. Use of the
Microscope. Preparation of Microscopical Objects.
Section cutting. Table of Histological Processes . . 1-14
CHAPTER I.
General Anatomy of tub Frog.
External Characters. Buccal Cavity. Abdominal Viscera.
Peritoneum. Digestive Organs 15-23
CHAPTER IL
The Vascular System of the Frog.
The Heart. The Veins. The Arteries. The Structure of the
Heart. Microscopical Examination of Blood . . 24-39
CHAPTER III.
The Skeleton of the Frog.
The Axial Skeleton. The Appendicular Skeleton . . . 40-54
CHAPTER IV.
The Muscular System of thS Frog.
Mnscles of Trunk. Muscles of Head. Muscles of Hind-
limb 55-67
CHAPTER V.
The Nervous System of the Frog.
The Central Nervous System. The Peripheral Nervous
System. Histology of Nerves 68-S6
vii ^
Vlll CONTENTS
CHAPTER VI.
The Eye tiSD Ear.
PAGb
The Eye of the Frog. The Eye of the Sheep or Ox. Histology
of the Eye. The Ear of the Frog 87-95
CHAPTER VII.
The Reproductive Organs and the Cloaca.
The Male Frog. The Female Frog ...... 96-98
CHAPTER VIII.
Development of the Frog.
General Account. Formation of the Egg. Maturation of the
Egg. Fertilisation. Segmentation. Formation of the
Germinal Layers. Development of the Nervous System.
Development of the Sense Organs. Development of the
Alimentary Canal. The Gill Clefts and Arches. The
Vascular System. The Muscular System and the Coelom.
Development of the Skeleton. Development of the
Urinary System 99-150
CHAPTER IX
Elementary Histoiogy.
Epithelium. Glands. Muscle. Connective Tissue. Carti-
lage. Bone 1 51-163
Index ...... 165-172
LIST OF ILLUSTRATIONS.
FIG. PAGE
1. The Common Frog^ Rana temporaria {Jrom Ecker) . . • iS
2. Diagrammatic section of the posterior part of the Body ... 19
3. General view of the Viscera ao
4. Diagram of the Venous system . 25
5. Diagram of t?ie Arterial system 29
6. Dissection of the Heart 33
7. Zii^ Skeleton 41
8. The Skull from the ventral surface 44
9. The Skull from the right side 46
10. Diagrammatic section of the posterior part of the Head ... 48
11. The superficial Muscles of the Hind-limb 63
12. The Nervous System of ^BXidi ^scM\t.i\i9. {from Ecker) ... 69
13. The Brain t from the dorsal suff ace 71
14. The Brain t from the ventral surface 71
15. Diagrammatic liorizontal section of the Brain {from Ecker) . . 72
16. Dissection of the Cranial Nerves of the right side .... 78
17. Diagrammatic section of the Human Eye 89 •
18. Section of the wall of the Eye 92
19. The Internal Ear , , 94
20. Stages in the Development of the Frog . Between pages 98 and 99
20A. Tadpole Just hatched To face page 100
21. Segmentation of the Egg {from Haddon) no
ix
LIST OF ILLUSTRATIONS
KIG. PACK
22. Section of the Segmented Egg iii
22A-25A. Formation of the Mesenteron . . Between pages iiq and 113
[Figures 23 to 37 refer to the embryo at various stages.]
23. Sagittal section {commencement of invagination) . . . .112
24. Sagittal section (formation of mesenteron) 113
25. Sagittal section (completion of mesenteron) 114
26. Transverse section {stage with open neural groove) . . .118
27. Sagittal section {diastopore open f neural groove closed) . . . 119
28. Sagittal section {stage of Fig. 20, 2) 120
29. Sagittal section {stage of Fig. 20t e) . . I2X
30. Transverse sections {development of eye) 124
30A. Phatynx, ear and heart Tofacepctge 129
31. Diagram of branchial vessels and pronephros {stage 6*5 mm. long) . 13,1
2^ Diagram of heari and branchial vessels {stage 6. 5 mm. long) . . 132
33. Dissection of a 12 mm. tadpole 134
34. Diagram of heart and branchial vessels {stage 12 mm. long) . . 135
^ 34BIS. The tadpole* s Skull To face page 141
34A. Skull of tailed frog To face page 142
35. Dissection ofa/^o mm. tadpole . . 144
36. Dissection of Urinogenital system {stage of Fig. 20, g-i©) . . . 146
37. Transverse section at time of hatching {stage of Fig. to, n) . . 147
2^, Section of the cardiac end of the do^ s stomach ..... 155
INTRODUCTION.
L— LIST OF APPARATUS BEQUIBED.
The following apparatus is reGommended to the student of
Elementary Biology :
1. Two or three scalpels or dissecting knives of different
sizes.
2. Two pairs of forceps, one large and one small. Both pairs
should be straight, and should have the tips roughened in order
to secure a firmer hold.
3. Two pairs at least of scissors ; one pair large and strong,
for cutting bone and other hard tissues ; the other pair small,
for fine dissections. A second small pair may have the blades
bent at an angle (elbow scissors). In selecting scissors be care-
ful to see that they cut quite up to the points of the blades.
4. A pair of stout needles, firmly mounted in handles.
5. A pair of the finest sewing needles, mounted in handles:
only about a quarter of an inch of the needle should project.
They are used for teasing histological preparations.
6. A seeker, t.6., a blunt needle mounted in a handle, and
bent at an angle half an inch from the end.
7. A metal blow-pipe : and a glass cannula with india-rubber
cap.
8. A pocket lens, containing two or three lenses mounted in
a handle, and giving when combined a magnifying power of at
least six diameters. A strong '' watchmaker's lens " is most
useful to those who ean hold it at their eye, as leaving both
hands free to dissect.
9. Slides and coverslips, for mounting microscopical speci-
mens. The coverslips should be the thinnest sold (No. 1).
Square covers are easier to handle than circular.
10. A blank note-book, for drawing in ; an KB pencil, and
a piece of indian-rubber.
11. A cheap pair of compasses, for measuring the dissections.
DISSECTION AND DRAWING
II.— ON DISSECTION.
The object of dissection is to separate the several parts and
organs from one another, so as to deJ&ne their boundaiies and
display clearly their mutual relations. Dissection consists
mainly in removing the " connective tissue " which binds the
several parts together.
The following rules should be carefully attended to :
1. Pin down the animal firmly to the dissecting board
Never attempt to dissect a specimen that is not so fixed.
2. In pinning out a dissection stick the pins in, not vertically,
but obliquely, so that their heads do not get in the way or
obscure the dissection.
3. Never cut away anything until you are quite certain what
it is you are removing.
4. Put the part you are dissecting slightly on the stretch ;
6.(7., when dissecting the bloodvessels or nerves of the throat,
distend it by passing a small roll of paper or lumps of cotton-
wool down the oesophagus ; or when dissecting the muscles of
the leg, pin out the leg in such a position as to stretch the
muscles you are cleaning.
5. In cleaning bloodvessels or nerves always dissect along
them, and not across them ; and avoid laying hold of them
with the forceps. Similarly when cleaning muscles, dissect
along their fibres and not across them.
6. Fine dissections should be done imder water, which sup-
ports the parts and greatly facilitates the operation. A stream
of water ^dlowed to play gently on the dissection from time to
time is often a valuable aid.
7. The dissection of muscles, and still more of nerves, is
greatly aided by placing the specimens in spirit for a day before
dissecting.
8. Keep your instruments clean and sharp. Be careful not
to blunt your fine scissors or scalpel by using them for cutting
hard parts.
9. If you get in a muddle, stop and wash the dissection
thoroughly under the tap before proceeding further.
m.— ON DRAWING.
It is absolutely essential to draw your dissections, and this
must on no account be omitted. Keep a separate book for
THE USE OF THE MIGROSOOPB 3
your drawings, and draw every dissection you make. Do not be
discouraged if you find it difficult at first: you will never
regret time spent on it.
The following rules will be useful to those who have not
learnt to draw systematically :
1. Make your drawing to scale, «.«., either the exact size of
the natural object, or half or double or treble that size, as the
case may be, remembering always that a drawing can hardly be
made too large.
2. In commencing a drawing, first determine by careful
measurement the positions of the principal points, and sketch
in lightly the whole outline before fiTii«hing any one part.
3. If the object you are drawing is bilaterally symmetrical,
draw a faint line down the middle of your paper, and sketch in
the left-hand half first ; by measuring from your median line
it will be very easy to make the two halves symmetrical.
4. Name on your drawing the several parts shown, and mark
also the scale adopted. If your drawing be of the natural size
mark it thus — x 1 ; if it be double the size of the object mark
it X 2 ; if half the size, x ^, and so on.
5. Draw on one side of the page only: and write an explana-
tion of your drawing on the opposite page.
6. Always make your drawing in pencil first, since much
clearer outlines can be obtained with pencil than with chalk,
but for complicated drawings coloured pencils are very useful,
and water-colour paints still better. Keep certain colours for
particular organs or tissues ; e,g.y when drawing the skeleton
colour the cartilage blue, the cartilage bones yellow, and the
membrane bones either red or white ; when drawing the blood-
vessels colour the arteries red and the veins blue.
7. Draw only what you see.
IV.— THE USE OF THE MIOROSCOPB.
The microscope consists essentially of a stcmd and a hodAf, the
latter of which bears at its ends the lenaea by which the magni-
fying power is obtained.
The stand is an upright pillar, the lower end of which is
attached to a heavy foot to ensure steadiness. A little way
above the foot the stand supports a horizontal plate — the stage
— on which the object to be examined is placed. The stage is
4 THE USE OF THE MICROSCOPE
perforated in the middle by a hole, the size of which can be
varied by means of diaphragnns. Throu£;h this hole ]ish.t is
directed^ the object nf^aminedly means of a^irxor
attached to the stand below the stage. Above the stage the
stand supports a vertical tube, in which the body of the micro-
scope slides up and down.
The body is a tube, in the upper end of which is placed a
combination of lenses, known as the eyepiece^ while to the lower
end is screwed another combination of lenses — the objective,
A microscope is usually provided with a couple of eyepieces
and a couple of objectives of different magnifying power. An
objective magnifying only a small number of times is called a
hw po^jner; one magnifying many times (200 diameters or more),
a high power. Siimlarly eyepieces are spoken of as high or low
according to their magnifying power.
In order that an object may be seen clearly the objective
must be at a certain definite distance from the object, this
distance (the " focal length ") varying with different objectives,
and to a slight extent with different observers. The higher
the power employed the closer must the objective be brougth
to the object. As the position of the object on the stage of the
microscope is fixed, this distance is regulated by mov ing the
body of the microscope up and down in the tube in which it slides.
This process of focussing is effected in two ways:
(1) By simply sliding the body up and down by hand in the
stand, or by screwing it up and down with a rack and pinion,
according to the type of microscope employed. This is known
as the coarse adju^stment. The sliding of the body should he
performed with a slightly screwing motion, and can only be used
when low powers are being employed.
(2) With high powers the objective has to be brought so
close to the object that a more delicate method of adjustment
is necessary. Thisjme adjustmerU is effected by a screw with a
milled head placed at the top of the vertical pillar forming the
stand. By turning the head from right to left, in the direction
of the hands of a watch, the body of the microscope is lowered
and the objective brought nearer to the object : by turning in
the reverse direction the objective is raised.
In using the microscope attend to the following rules :
I. Always examine an object first with the low power. Having
adjusted the eyepiece and objective, direct the light up the tube*
THE USB OF THE MIGBOSOOPS 6
of the microsoope by means of the mirror, and then place the
object on the stage. Twist down the body until the objective
is about a quarter of an inch from the cover-glass ; look down
the microscope, and gradually twist the body up until the
object becomes visible. Focus accurately by means of the fine
adjustment screw. . .
2. When using a high power begin with the objective dose to
the cover-glass, and then focus with the fine adjustment. It
will facilitate the process if, while focussing with the right
hand, you move the object about slightly with the left hand.
3. Take extreme care never to let the objective touch the
cover-glass ; and never to touch or allow any dirt to get on the
face of the objective. The face of an objective cannot be cleaned
without doing harm to it.
4. Should by any chance a drop of glycerine get on the face
of the objective, wash it carefully with water from a wash-
bottle, and wipe it gently with a silk handkerchief or piece of
chamois leather. Should Canada balsam be allowed to get on
the objective, do not attempt to dean it yourself, but hand it
at once to the assistant.
5. See that the body of the microscope slides smoothly in its
tube. If it does not, remove it, and clean it by rubbing with a
few drops of olive oil : wipe off the oil before replacing the body
in the tube.
6. Keep both eyes open when looking through the micro-
scope : a very little practice will enable you to do this, and it
wiU save you much fatigue. Also get into the habit of using
either eye.
7. With a high power, use a small diaphragm : the amount
of light will be somewhat diminished, but the deamess and
definition of the object much increased.
8. When examining an object, keep one hand on the fine
adjustment, and keep screwing it up and down slightly the
whole time : in this way parts of the object at different depths
are brought into focus successively, and a clearer idea of the
object is obtained.
9. If the object appears dim or dirty, find out where the
fault lies in this way :
While looking down the microscope, turn round the eyepiece
with your right hand. If the dirt turns round too, remove and
clean the eyepiece. If the fault is not in the eyepiece, move
6 THE PREPARATION OP MICROSCOPICAL OBJECTS
the slide about gently ; if the dirt moves with the slide, remove
the slide and clean it. If the dirt does not move with either
the eyepiece or the slide the fault is almost certainly in the
objective, which should be removed and examined ; if dirty, it
must be cleaned very carefully with a piece of silk or chamois
leather.
v.— THE PBEPABATION OF MIOBOSOOPIOAL OBJECTS.
In mounting microscopical objects be careful that your slides
and coverslips are thoroughly clean. Slides should be labelled
as soon as they are prepared, and should be kept in a box or
cabinet in which they lie flat.
A. Methods of Mounting.
There are various media in which objects may be mounted.
The method of procedure is much the same with all. Put a
small drop of the fluid in the middle of the slide, place the
object in the middle of the drop, and arrange it with needles in
the position desired. Then place the cover-glass carefully on
the top, letting it rest by one edge on the slide and supporting
the opposite edge by a needle : withdraw the needle ^:adually
so as to let the cover-glass down slowly, and drive out any air-
bubbles there may be in the fluid. If any air-bubbles still
remain, leave them alone, as they will probably work out by
themselves. Be careful not to use too large a drop of your
mounting medium, and above all things be careful not to let
any of it get on the top of the cover-glass ; should this happen,
the cover-glass must be removed at once and the specimen
mounted afresh with a dean one.
The most miportant moimting media are the following :
1. Normal Salt Solution : a 0*75 per cent, solution of com-
mon salt in water. This is veiy useful in the examination of
fresh specimens of animal tissues, as, unlike water, it has
practically no action on them. It cannot be used, however,
for making permanent preparations.
2. Qlycerine can be used either pure or diluted with its own
bulk of water. If the preparations are intended to be per-
manent, a narrow ring of cement must be painted round the
edge of the cover-glass to flx it to the slide. For permanent
THE PREPARATION OF MICBOSOOnCAL OBJECTS 7
preparations it is better to use glycerine jelly ; a drop of ^^^n
should be melted on the slide, and the object transferred to it
from glycerine : ring with cement as before.
3. Canada Balflam is the most generally useful medium for
permanent preparations, as requiring no cement. Specimens
that are to be mounted in balsam must first be deprived of all
water they may contain by placing for an hour or so in
absolute alcohol, and should then, before mounting, be soaked
for a few minutes in oil of doves or turpentine in order
to clear them, i,e,, render them permeable by the balsam.
Canada balsam, if too thick, may be diluted with chloroform
or benzole.
B. Teasing.
The object of teasing is to separate the several parts of a
tissue or organ from one another in order to show their minute
structure.
The fragment to be teased should be placed on a slide in a
drop of the medium in which it is to be mounted, and then
torn up into shi*eds by means of a couple of needles held one
in each hand. The process is often greatly facilitated by
placing the slide on a piece of black paper, which renders the
particles easier to see. When torn up as finely as possible, a
cover-glass is placed on as before. The two rules to be borne
in mind in teasing are the following :
1. Take a ven/ small fragment to commence with ; hold it
with one needle, and tear it with the other.
2. Tease it as finely as you can. Your object is to sepcu:ate
the component parts from one another.
0. Maceration.
The process of teasing is in many cases facilitated by pre-
viously macerating the specimen, ue,. soaking it in some fiuid,
which, while preserving the individual cells, tends to loosen
them from one another. The most important macerating fluids
are as follows :
1. Banvier's Alcohol : a mixture of one part of strong spirit
with two parts of water. The specimen should be put fresh
into the mixture and allowed to remain twenty-four hours or
more.
8 THE PREPARATION OF MICROSCOPICAL OBJECTS
2. Mailer's Fluid ; a solution of bichromate of potash with
a little sodic sulphate in water.
D. Staining.
Various reagents are employed for the purpose of staining
preparations ; some of these merely dye the whole preparation
more or less uniformly, but the most useful ones are those which
stain certain parts of the cells only, or at any rate stain these
much more strongly than the other parts (selectiye stains). The
most important are the following :
1. Hsematozylin. There are various pi*eparations of hsema-
toxylin, or logwood, used in microscopical work: the best is that
proposed by Delafield. It is prepared thus : dissolve 4 grammes
of crystallised hsematoxylin in 25 cubic centimetres of strong
alcohol ; add this solution to 400 c.c. of a saturated solution of
ammonia alum, and expose to the light in an unstoppered bottle
for 3 to 4 days. Filter, add 100 c.c, glycerine and 100 c.c. of 90
per cent, alcohol.
The specimens, which must be perfectly free from all trace
of acid, should be cut into small pieces, and passed through
weak spirit to water. They should then be left in the h»ma-
toxylin in a covered vessel or stoppered bottle for from one to
twelve hours, according to the size of the specimen and the
depth of staining desired, and then brought up through water
and weak spirit, and left in strong spirit for some hours before
mounting. Haematoxylin stains the nuclei of cells much more
strongly than the other parts.
2. Boraz-Oarmine. This, which is perhaps the most gener-
ally useful of all the stains in ordinary use, -is prepared as
follows. Dissolve 2 parts of carmine and 4 parts of borax in
100 parts of water: add an equal volume of 70 per cent,
alcohol ; let the mixture stand for a couple of days, and then
filter.
Specimens may be left in borax-carmine for from one to
twenty-four hours, or even for two or three days : on removal
they should be placed in acid-alcohol — i.e., 70 per cent, alcohol
to which a few drops of hydrochloric add have been added —
until they become a bright scarlet colour, when they should be
transferred to 70, and then to 90 per cent, alcohol, in which
latter they may be left till required. The time of immersion in
THE PREPARATION OF MICROSCOPICAL OBJECTS 9
add-alcohol will vary, according to the nature and size of the
specimen, from a quaiiier of an hour up to a day or more.
3. Picro-Cannine is a very useful, and to a certain extent a
differential stain, and it colours the several tissues different
tints. It may be prepared thus. Dissolve 1 gramme of carmine
in 4 C.C. of liquid ammonia and 200 cc of drilled water. Add
5 grammes of picric add; shake the mixture well for some
minutes, and then decant from the excess of add. Leave the
decanted liquid for some days, stisring it occasionally: then
evaporate it to dryness, and to every 2 grammes of the dried
residue add 100 c.c. of distiUed water.
Picro-carmine answers best with specimens preserved in 70
per cent, alcohol. They should be brought through weak spirit
to water, and left in the stain for a day ; then brought up to 70,
and afterwards to 90 per cent, alcohol. Some specimens give
better results when washed freely with water on removal £rom
the picro-carmine, and then placed in 1 per cent, acetic acid
for an hour before transferring to alcohol.
4. Magenta stains very rapidly but diffusely: the colour
also is not permanent.
5. Silver Nitrate. A } per cent, solution in water stains the
intercellular substance, which binds together the several cells of
a tissue, much more strongly than the cells themselves, and is
therefore chiefly used when we wish to render prominent the
outlines of the individual cells. The spedmens should be placed
fresh in the silver solution for from two minutes to a quarter
of an hour, then washed thoroughly with distilled water, and
exposed to the light until stained sufficiently deeply, when they
may be mounted in glycerine. Such preparations are rarely
permanent, as the reduction of the silver, to which the staining
is due, continues until the specimens ultimately become too
dark to be of any use.
6. Osmic Acid. A 1 per cent, solution of osmic acid in water
forms an extremely useful staining reagent. It is especially use-
ful for the detection of fat, which is stained by it a dark brown
or black colour. Specimens, which must be quite fresh, should
only be left in it a few minutes, and may then be mounted in
glycerine, or else washed, dehydrated, and mounted in balsam.
7. Acetic Acid. Although not strictly a staining agent, in-
asmuch as it does not colour the spedmens, acetic acid may
10 PBESEBVING AND HABDENING
conveniently be mentioned here, as it is used for the same pur-
pose as the true stains, i.e., for the sake of rendering certain
parts of the cells especially distinct. Acetic acid, of which a
1 per cent, solution is employed, causes the protoplasm of cells
to swell up and become transparent, and brings the nuclei into
special prominence. It is used with fresh specimens.
VL— ON PBESEBVING AND HABDENING.
The reagents in common use for killing and preserving small
animals are valuable also from their power of " fixing " the
tissues, i.e,, of coagulating the protoplasm of the cells. The
objects to be attained are to effect this coagulation quickly,
before the tissues can undergo any alteration, and thoroughly,
i.e., throughout the whole thickness of the object to be hardened.
They are as follows :
1. AlcohoL Specimens may be placed at once in 70 per
cent, alcohol ; and thence transferred after a couple of days to
90 per cent., in which they may be left till required.
2. Osmic Acid. For this purpose a 1 per cent, solution in
water is used : it acts almost instantaneously, and so allows no
change to occur in the tissues ; it has also the merit of stainipg
the tissues as well as hardening them. It can, however, only
be employed when the specimens are very small, as it hardens
the surface layers so rapidly that it is unable to penetrate
beyond a very slight depth. A few minutes' immersion is
usually sufficient. The vapour must not be inhaled.
3. Corrosive Sublimate. This is by far the best general
reagent for killing and fixing small animals. A saturated solu-
tion in water is employed, in which the object is placed for
half an hour or more.
4. Chromic Acid. A 0*25 to 0*5 per cent, solution of chromic
acid in water is a useful hardening reagent ; it acts much more
slowly than osmic acid, but penetrates to greater depths.
Specimens should usually be left in the solution for one or
more days, and then thoroughly washed for some days in
increasing strengths of alcohol. These processes should be
carried out as far as possible in a dark drawer or cupboard.
6. A Mixture of chromic acid with a few drops of osmic acid
WASHINO AND SIECTIOK CUmNG 11
is often very useful, as it combines the advantages of both
reagents.
6. Picric Acid is a very valuable hardening reagent, of which
the best preparation is Elleinenberg's. Specimens should be left
in it from 12 to 24 hours. It is prepared thus : with 100 c.c.
of water make a cold saturated solution of picric acid : add 2
c.a of concentrated nitric acid : filter, and add to the filtrate
three times its volume of water.
Vn.— ON WASHING.
Specimens that have been hardened in any of the preceding
reagents (except alcohol) should be placed in water on
removal from the reagent, then in 80 per cent, alcohol, then
in 50 per cent, alcohol. They should then be washed in alter-
nate strengths of 50 per cent, and 70 per cent, alcohol in order
to remove all traces of the hardening reagent, and should be
stored in 70 per cent, alcohol. The object of using graduated
strengths of alcohol is to avoid distortion of the tissues by too
sudden changes of specific gravity. These remarks apply also
to treatment before and after staining.
Vm.— ON SECTION CUTTING.
Many tissues and organs can only be studied satisfactorily by
cutting them into thin sections, and this method of investiga-
tion is of such importance as to requiie special notice. There
are three chief stages: Hardening, Imbedding, and Cutting,
which will be noticed in succession.
A. Hardening.
Before the object can be cut into sections it is necessary to
harden it ; this may be effected by freezing, but the more
usual plan is by means of reagents, which have been discussed
under previous headings.
B. Staining.
The hardened specimens, if not too large, may now be stained
with either hsematoxylin, borax-carmine, or picro-carmine; they
should then be brought to 70 or 90 per cent, alcohol. If the
specimen is too large to stain whole, the sections must be
stained after they are cut (see p. 13).
G. Imbedding.
The preparation of sections is greatly facilitated by imbedding
12 SECTION CUTTING
the specimen in some waxy substance. For this purpose various
materials have been employed, but by far the most useful is
paraffin-wax, which is uJd L the following manner :
The stained specimen is placed in absolute alcohol for an
hour or two in order to completely dehydrate it. It is then
transferred to turpentine or benzole, in which it is left for half
an hour or more until completely saturated. From the tur-
pentine or benzole it is transferred to melted paraffin, which is
kept by means of a water-bath at a temperature just above
its melting-point (about 56^ C). In this it is left for several
hours, or even for a whole day, in order that it may be
thoroughly permeated. It is then placed in a small box of
paper, or other material, filled with melted paraffin. By means
of hot needles it can readily be arranged in any desired posi-
tion ; and the paraffin should then be cooled quickly.
D. Section Gutting.
When thoroughly set the block is removed from the box, and
the paraffin pared away with a knife until the specimen just
.comes into view.
The block is then placed in a microtome, and cut into thin
sections. These may be transferred one by one to a slide, but
a great saving of time is effected by the method of cutting
continuous ribbons, devised by Mr. Caldwell.
This depends on the fact that if the paraffin is of proper
consistency the successive sections, as they are cut, will stick
together at their edges, so as to form a ribbon. To ensure this
the edge of the razor should be placed at right angles to the
direction of stroke, and the edges of the block of paraffin cut
parallel to one another, and to the edge of the razor. If for
any reason it is desirable to imbed the specimens in a paraffin
too hard to form ribbons, the block shoiUd, before cutting, be
coated with a layer of soft paraffin, by dipping it for a few
moments in a dish of melted soft paraffin. This outer coating
should be left on the sides of the block parallel to the edge of
the razor, but cut away from the sides at right angles to it.
The razor should be used dry : and the sections, when cut,
placed on slides painted, just before they are used, with a thin
layer of a mixture of collodion and oil of cloves in equal parts.
The slide is then heated by a water-bath to a temperature not
exceeding 55° to 60° C, so as to melt the paraffin and evaporate
SECmON GUTTING 13
the oil of cloves. The melted paraffin should then be washed
oS by turpentine, when the sections will remain fixed to the
slide by the collodion, and may be mounted in balsam in the
usual manner.
Instead of the mixture of collodion and oil of doves, a solution
of shellac in absolute alcohol may be used: this should be
spread over the slide in a thin layer by means of a glass rod,
and allowed to dry. Immediately before being used the slide
should be brushed over with oil of cloves.
If it is desired to stain the sections on the slide, the thinnest
possible layer of glycerine and albumen (pure glycerine and
fresh white of egg in equal parts, filtered) should be smeared
over the slide, and the sections laid on it in order. The
paraffin, when melted, should be washed off with turpentine or
benzole, and the sections dehydrated with absolute alcohol ; the
desired stain can then be used in the usual manner. Dehydrate
with absolute alcohol, followed by oil of cloves, before mounting
in Canada balsam.
14
INTRODUCTION
Vm.— TABLE OF THE CHIEF PBOOESSES ALREADY
DESCRIBED.
A. EHUing and Fixing.
Alcohol Osmic or Obromic Acid
or Corrosive Sablimate
I
Water
30 %<
60%
I
^ 70 % (Storage Fluid).
Picric Acid
B. Staining.
Borax
Carmine
50%
80%
Water
;
Ha3matox,\lin
or Picro- Carmine
Silver Nitrate
I
■*■ Water •*■
30%
y
60%
^70%
G. Section cutting qq^
and Mounting. i"*
Absol. Ale.
Turpentine
or Benzole
A.oid spirit
Weak
Glycerine
Melted
Paraffin.
4-
Canada
Balsam.
Strong
Glycerine.
Glycerine
Jelly.
CHAPTER I.
GENERAL ANAT0U7 OF THE FROCI.
Fig. i.-llie Common Frog {A'ana lemfioraria) {from Ecker).
A. External Oharacteis.
Lay the frog on a board be/ore you ; note, and make drawtngt
thotoing the/dllowing points ;
1. The division Into head, tnmk, and limbs ; and the absence
of neck and tail.
2. The two great siwfaces.
&. !Ftae dorsal snr&ce, or back, is directed upwards when
the frog is in the natural position.
b. The ventral surface, or bell;, is directed downwards
towards the ground.
16 GENERAL ANATOICT OF THE FROG
3. The skin is moist and smooth ; and devoid of hairs, scales,
and claws. The colour of the skin is variable in different
specimens and at different times : it is mottled on the dorsal
surface, paler on the ventral.
4. The head is flat and triangular, with a blunt apex directed
forwards.
At the sides of the head are the eyes, which are large and
prominent. £ach eye has two eyelids, of which the upper is
thick, pigmented, and almost immovable, while the lower is
semi-transparent and freely movable.
Behind the eye on either side is an obliquely placed elongated
patch of a dark colour, in the middle of which is a circular
area — ^the tympanic membrane — supported by a firm marginal
ring.
5. The limbs. There are two pairs of limbs, fore and hind ;
each limb being composed of three segments.
a. The Fore limb presents the following divisions :
i. Ann.
ii. Forearm.
iii. Hand, with four digits, corresponding to the four
fingers of man ; the thumb being vezy small and
inconspicuous. In the male frog there is a pig-
mented thickening along the inner edge of the first
digit, specially developed at the breeding season.
b. The Hind limb is much longer than the fore limb, and
divided into the following parts :
i. Thigh.
ii. Leg.
iii. Foot, with five toes webbed together. The shortest
toe corresponds to the big toe of man, and the
longest to his fourth toe.
6. External apertnres : or openings on the surface of the
body.
a. Median apertures.
i. The Month is a wide horizontal slit.
ii. The Oloacal aperture is a small hole at the posterior
end of the body, between the legs : it lies slightly
on the dorsal surface, just behind the bony projeo-
tion formed by the posterior end of the urostyle.
THE BUCCAL CAmT 17
b. Paired apertures.
i. The Nostrils or anterior nares are two small open-
ings on the dorsal surface of the head, dose to its
anterior end.
B. The Buccal Oavity.
Open the mouth to its fuM eosient : note the wide buccal or mo uth
cavity y of which the hinder jxxrt or pharynx is continued back into
the cBSophagus, Note aiUo the following structures :
1. On the Eoof of the Mouth,
a. The Teeth.
i. The maxillary teeth are a row of fine teeth, atta ehed
round the edge of the upper jaw to the maxillae
and pre-maxillaB.
ii. The vomerine teeth are two small patches of sharp
teeth in the fore part of the roof of the mouth and
near the middle line, attached to the vomers.
b. The posterior nares are two small holes lying to the
outer sides of and slightly in front of the two patches
of vomerine teeth.
Pa^s bristles through the nostrils, a/nd see that they
come ovi through the posterior nares into the buccal
cavity.
e. The Eustachian tubes or recesses are a pair of much
larger holes, at the sides of the posterior part of the
buccal cavity. Each hole opens into a slightly dilated
chamber — ^the tympanic cavity — ^which is closed ex-
ternally by the tympanic membrane already seen on
the surface of the head.
Ferforaie the tympcmic m^mbrams on one side with a
needlcj cmd pass a bristle or seeker tlirough the hole and
down the Eustachian tube into the mouth,
d. Two rounded prominences at the sides of the roof of
the mouth are caused by the eyeballs.
Press down one of the eyes unth yomr finger, and note
that it can be made to project very considerably into t/ta
buccal cavity.
2. On the Floor of the Mouth.
a. The lower jaw, which is devoid of teeth, forms a bony
margin to the floor of the mouth : the rest of the floor
18 GENERAL ANATOMY OF THE FROG
is soft and fleshy, but is slightly stiffened by a car-
tilaginous plate — ^the body of the hyoid.
b. The tongue, which is thin and fleshy, is attached to
the front part of the floor of the mouth, and has its
free bilobed end turned backwards towards the throat.
Twm the tongtis fonvards with the forceps,
C The glottis, or aperture of the larynx, is a longitudinal
slit in the floor of the posterior part of the mouth,
and is stiffened laterally by the arytenoid cartilages.
FcK8 bristles through the glottis into the hmgs. If
cmy d^jffUfulty is experienced in fmding the glottis
snip through the angles of the mouth with scissors^
so as to allow th^ rrumth to he opened more widely,
C. The Abdominal Viscera.
Lay the frog on its hack tmder watery and fasten it down to
the dissecting hoofrd hy pins through the Urnhs, Cut through the
shin^ along the middle Une, the whole length of the ventrcd sur-
face. Separate the skin from ike v/nderlying parts, noticing its
very loose attachment to these parts, amd the large space — a lymph
cavity — heneath it. Tvm the flaps of shin outwards, and pin
ihem hack. Notice :
a. The muscles of the body-wall.
b. The pectoral or shoulder girdle : a bony arch running
across the body, opposite the fore limbs.
Pinch up wiJCh forceps the muscular hody-waU, and cut through
it into the hody-cavity or ccdom, urith scissors a little to each side
of the median Une, heing careful not to injure the anterior ahdo-
minal vein which ru/ns along the inner surface of the hody-wall
in that Une,
Contiv/ue the cut of one side backwards to the hinder end of
the body, and forwards to the jaw, cutting through the pectoral
girdle with strong scissors, amd taking care not to injure ^ parts
beneath.
Tie Ugature-thread twice round the anterior abdominal vein,
cut between the two ligatures, amd t%im back the two halves of the
vein in the body-wall.
ABDOMINAL VISCKSA 19
Inflate the limgi with a Uote-pipe iJvrough Ae glottit, and inflate
the bladder through the doaetd aparlure.
Note anddrato tite general aavaa^gemetU of the vi»eera,ikov}mg
thefoUowing stntctwree :
1. The iaait, enclosed in the pericardium, is situated in the
middle ventral line, ttnd in tlie natural condition of the parts
is covered hy the pectoral girdle and the sternum.
2. The Uvvr is a lai^ reddish-browB bilobed organ, behind
and at the sides of the heart.
3. The Inngs are two thin-walled elastic sacs at the sides of
the heart : they lie dorsal to the liver, and are often hidden hy it.
ifote tie hrietiee already paeaed into the Ivngt tlirough the glottit.
i. The small intestine is a light-coloured convoluted tube ;
in the middle line behind is the much wider large intestine.
5. The bladder is a thin-walled bilobed sac at the posterioi
end of the body cavity.
Fia 9,— A dlagrammaiic transverse seclion across the posterior
pan of [he body of a female frog.
a, tirostyle ; *, muscles of body wall : rf, large intesline ; d.a, dorsal
aorta i i, ilium ; /, lymph space between the skin and the muscuUc
body-wall (dstema iTraphfttka magna) ; «, spinal nerves ; o, kidney ;
ev, oviduct ; f, peritoneum ; i, skin ; t, fold of skin at grom ; k, ureter ;
!•, posterior vena cava.
6. In the female frog note, in additirm to the above parte,
a. The ovaries : two large bodies of irregular ^pe, ea«h
consisting of a mass of spherical black and white
^gs, like small shot.
20 OKHIEAL ANATOMY OF THB FROO
b. Theorldncts: twolong,veiy much convoluted tubes with
thick white w^lg, lying at the sidee of the body cavity.
7, In the male frog note,
a. The testes; a pair of ovoid bodies of a pale yellow
colour, attached to the dorsal wall of the body cavity.
Kotice the thin pigmented membrane — the peritonenm —
which lines the body cavity or caeloni, covered by ccelomic epi-
theUum. Trace this to the mid-dorsal line, where it is reflected
downwards as a double layer — the mesmteiy — which embraces
at its edge the ^imentaiy canal, and binds its several coils
together. (See Fig. 2.)
Kotice also that all the abdominal viscera are really outside
the peritoneum, which forms a closed sac into which the viscera
are as it were pushed from without.
£. The Digestive Organa.
ri^ht side.
a, stomach: '. bladder; e, small intestine; el, doacal aperture;
d, large iatesiine ; i, liver:/ bile duct : g, gall bladder; h, spleen;
i, lung: i. laryni; /, fal bodj; m, testis; n. ureter; o, kidney;
p. p&Qcreas ; r, pelvic sjrmpbysis : t. cerebral hemisphere ; if, spinal
DIOESTHTE ORGANS 21
Twm the liver foruKtrdSy cmd note the stomach lying beneath its
left lobe. Pass the hcmcUe of a seeker through the mouth and
down the oeaophagus into the stomach,
[If the specimen be a female, remove the ovaries cmd oviducts
completely y taking care not to damage the alimsntary canal.'\
1. The Alimentary Oanal.
a. The oesophagus is a short wide tube leading from the
buccal cavity to the stomach.
b. The stomach is a wide tubular sac about an inch and a
half in length : it is narrower behind, and separated
from the duodenum by a distinct pyloric constriction.
Cut open the stoma>ch longitudinally along its left side, and
wash out its contents : note^the handle of the seeker already in-
serted through the movJth; also the Imgitudinal folds of the
m,vAiou8 Tnembrane lining the stomach, which increase the extent
of its eurfa>ce,
c. The duodenum is the first part of the intestine, rather
more than an inch in length : beyond the pylorus it is
bent back so as to lie parallel to the stomach. At its
further end it is continuous with the small intestine.
d. The small intestine is a slender convoluted tube about
four and a half inches long, opening at its distal end
by a small orifice into the large intestine.
e. The large intestine is a short straight tube about an
inch and a quarter long : it is very much wider than
the small intestine, and opens behind to the exterior
at the cloacal aperture.
f. The cloaca in the frog is continuous with the large
intestine, into it open the renal and genital ducts
as well as the bladder : it will be described more
fully when considering the urinary and reproductive
organs. (See Chap. VIII.)
2. The Liver.
The liver is a large reddish-brown organ, divided into right
and left lobes, connected together by a narrow bridge of liver-
tissue. Of the two lobes the left one is much the larger, and
is again subdivided into two.
22 eENEBAL AKATOMT OF THE FBOO
a. The galLbladder is a small spherical greenish sae lying
between the right and left lobes of the liver.
b. The bile duct is a slender tube leading from the liver
and gall-bladder to the duodenum, into which it opens
about half an inch beyond the pylorus, and on the
inner or concave side of the loop formed by the duo-
denum and stomach. The distal half of the bile duct
traverses the pancreas : it has rather thick white walls
and is easy to see ; the upper half is more slender and
more difficult to trace.
To trctce the hUe duct turn the liver forwards 90 that the point
of attachment of the gall-bladder is clearly seen ; cmd slightly
stretch the d%wdenwm hy a pin passed through the pylorus. De-
termine the position of the two ends of the bile duct from the
description given abovCy and dissect with a scalpel along the line
thus indicated.
To see the opening of the bile duct^ slit up the first three
quarters of an inch of the duodenum along its convex border and
wash out its contents : squeeze the gaU-bladder so as to drive the
bUe dUmg the duct into the duodenum : note the point at which
it entersy and insert a bristle through the opening into the duct.
Notice also the strong wa/oy transverse folds of the mucous mem'
brans of the duodenwn.
3. The Pancreas.
The pancreas is a whitish irregularly lobed mass lying in the
loop between the stomach and duodenum, and best seen by
turning the whole loop forwards. The pancreatic ducts are
numerous and open into the bile duct, which passes through
the pancreas to reach the duodenum.
Gut through the mesentery along its attachment to the intestine :
uncoil the intestine, leaving it attached at both ends, and spread
it out on your dissecting board : measiure the lengths of the
several portions and draw them to scale,
F. Other Abdominal Viscera.
1. The Kidneys are two flat elongated oval bodies of a red
colour attached to the dorsal body-wall, close to the middle line,
one on each side of the backbone or vertebral column. They
lie in the large lymph space behind the peritoneum, and, like
ABDOMINAL TISCEBA 23
the other viscera, are outside the abdominal coelomic cavity.
(See Fig. 2, p. 19.)
a. The ureters, or ducts of the kidneys, are a pair of white
tubes arising from the outer edges of the kidneys at
about a quarter of their length from their hinder
ends, and running back to open into the dorsal wall
of the cloaca, opposite the opening of the bladder.
In the male frog a pouch -like dilatation, the
▼esicola seminalis, is present on the outer side of
each ureter, close to its opening into the cloaca.
b. The adrenal bodies are small yellowish-red patches on
the ventral surface of the kidneys.
c. The corpora adiposa^ or &t bodies, are two bright
yellow tufts of flattened processes attached to the
dorsal wall of the body cavity ; they vary much in
size, and usually come to the surface just behind
the liver.
2. The Spleen is a small round dark-red body lying in the
mesentery, opposite the commencement of the large intestine.
G. Thyroid and Thymns.
1. The thyroid is a small paired gland attached to the
base of the extei-nal jugular vein, a little way in front of the
heart.
2. The thymns is a small ovoid paired gland behind the
tympanum. It can be seen by reflecting the hinder border
of the tympanum forwards and removing the adjacent muscle
that lowers the mandible. (See p. 59.)
n
CHAPTER IT.
THE VASCULAR SYSTEM OF THE FBOG.
The vascular system is a closed system of tubes or vessels
filled with blood, and ramifying through all parts of the body :
its main parts are : (1) the heart, which by its contractions is
continually driving the blood round and round the system of
vessels: (2) the arteries, which are the vessels taking the
blood from the heart to all parts of the body : (3) the Teins,
which carry the blood from those parts back to the heart : and
(4) the capillaries, a system of very small vessels connecting
the arteries and veins together.
A. The Heart.
Fin down the frog on its hack vmder water and open the body
cavity as before, taking special ca/re to preserve the anterior ah-
dominal vein. (See p. 18.) In freeing the pectoral girdhfronx
the wnderVyvng muscles take care not to injure ike neighbouring
bloodvessels.
Open the pericardial cavity and dissect the pericardiu/m from
ike heart amd the roots of the great vessels, eocamine and draw the
heart in situ, shotoing its several divisions.
1. The divisions of the heart.
i. The auricles form the anterior and dorsal division
of the heart: they are thin- walled and appear
dark in colour owing to the blood being seen
through their walls. On close examination the
division into right and left auricles can be seen,
ii. The ventricle is posterior to the auricles : it is paler
in colour owing to the greater thickness of its
walls; and is conical in shape, with the apex
pointing backwards,
iii. The truncns arteriosus is a cylindrical body arising
from the right anterior border of the ventricle,
and running obliquely forwards across the
auricles.
THE vznre 25
lAft up the ventride and turn itt apex fomxurdt to at to
expote t/te ainue venomg.
IT. TlteBinnsTenofliiB is a thin-walled eac, lying dorsal
to the Teutride and behind the auricles; it
receivea the three large veue cava.
2. Tlie pnlaation of the heart.
A. N^ote that the contractions of the heart continue some
time after the frog has been killed, or even after the
heart is completely removed from the body.
b Note the character of the heart's pulsations ; a regularly
alternating aeries of contractions and dilatations.
C. Note farther that in each contraction or systole of the
heart all four divisions of the heart contract, but not
simnltuieously. The sinus venoaus contracts first,
then the two auricles, then the ventricle, and finally
the truncus arteriosus.
B. The Veins.
Flc. 4-— raagrammatic figure of the venous syslem of Ihe fn^.
(rom Ihe right side.
a, stomach ; a.v, nnlerior abdominal ^in ; b, bladder ; b.v, brachial
Tein; e.l, cloaca] apenure ; e.v, cardiac vein; d, lai^e intestine;
t, liver; e.v. extPmaJ jugulai vein ;/i>, femoral vein ; ^, gall-bladder ;
k, spleen ; i.e. posterior vera cava ; i.v, innominate vein ; jv, internal
jugular veio ; l.f, Irfl pelvic vein ; tti.v, musculo-culaneous vein ;
e, kidney ; *.v, hepatic ponal vein ; r.f, light pelvic vein ; r.v, rifht
renal portal vein ; t. sinus venosus : i.e. sciatic vein ; s.v. aiibdaviaii
vein; (, tongue; t.a, tnincua arteriosus; k, right auricle; v, veq-
iriclc ; v,v, vesical veins.
26 THS YASCULAB ST8TEM OF THE FROG
The veins should be dissected before the arteries, because, as
a rule, they lie nearer the surface and are therefore met with
first. The veins are further distinguished from the arteries by
their larger size and darker colour, due to the blood being seen
more clearly through their thinner walls, and to the presence
of pigment on them.
Dissect from the ventral sturface. In cleaning a vein take
hold with (^forceps, not of the vein itself hut of the tissue sur-
rounding it ; and take especial ca/re not to prick the vein, as hy
doing so you aUow the blood to escape and obscv/re ike dissection^
and also render the vein itself difficuU to see oioing to the loss of
colour. Always dissect along cmd not across a bloodvessel^ and
pin out the parts so as to stretch it slightly,
I. Veins opening into the Sinus Venosus.
a. The right anterior vena cava is a large vein opening
into the right side of the sinus venosus, and returning
to it the blood from the right side of the head and
body, and from the right fore limb. It is formed by
the union of three veins.
1. The external jngnlar vein is formed by
L The lingual vein, from the floor of the mouth
and the tongue.
IL The mandibular vein, from the margin of the
lower jaw.
In close connection with the ventral surface of
each external jugular vein is a small round
vascular body, the thyroid gland.
2. The innominate vein is formed by
i. The internal jugular vein, returning blood
from the interior of the skull, which it leaves
by an aperture at the posterior border of
the orbit.
ii. The subscapular vein, a small vein from the
back of the arm and shoulder.
8. The subclavian vein, the largest of the three,
is formed by
i. The brachial vein, from the fore limb,
ii. The mnsculo-cutaneons vein: a very large
vein returning blood from the skin and
THX VEINS 27
muscles of the side and back of the body,
and of the head as far forwards as the nose.
b. The left anterior vena cava corresponds in its course
and branches to the right one.
c. The posterior vena cava is a median vein which, com-
mencing between the kidneys, runs forward, dorsallj
to the Uver, to open into the posterior end of the
sinus venosus. It returns to the heart the blood from
the liver and from the kidneys, and indirectly from
other viscera and from the hind limbs. It receives
the following veins :
i. The right and left hepatic veins, from the liver :
these open into the posterior vena cava just before
it joins the sinus venosus.
ii. The renal veins, from the kidneys : of these there
are four or five on each side, which open into, or
rather form by their union, the posterior vena
cava. The most anterior of these receive the
veins from the fat bodies.
iii. The ovarian veins (in the female), or spermatic
veins (in the male) ; returning blood from the
ovaries or testes. They are usually four or five
in number on each side, and open into the pos-
terior vena cava between the renal veins.
n. Vein opening into the Left Auricle.
a. The pulmonary vein is formed by the union of the
right and left pulmonary veins, returning to the
heart the blood from the right and left lungs respec-
tively. Each pulmonary vein runs along the inner
side of its lung.
m. The Portal Systems.
A portal vein is one which, returning blood from the capil-
laries of some part, breaks up before reaching the heart into a
second set of capillaries within some other organ ; these again
unite to form a vein which carries the blood to the heart. In
the frog there are two portal systems, one supplying the
kidneys, and the other the liver.
28 THE YABOULAB SYSTEM OF THE FROG
a. The renal portal system.
Trace back the anterior abdominal vein to the hinder end of the
body, tohere it toiU be seen to be formed by the tmion of the two
pelvic veins. Follow back the pelvic vein of one side to the base
of the hind Umh ; here it vnll be seen to be one of two branches into
which the femoral vein, the large vein returning blood from the
hind limb, divides. The other branch of thefem>oral vein is the
renal portal vein, which is to be followed to dhe outer side of the
hdney,
1. The right renal portal vein is the dorsal branch of
the right femoral vein : it runs forwards along the
outer side of the kidney and ends in numerous
branches in its substance. It receives the follow-
ing branches :
i. The right sciatic vein, from the muscles and
skin of the back of the thigh, joins the
renal portal vein close to its commencement,
before it reaches the kidney.
ii. The right dorso-lumhar veins are small veins
from the dorsal wall of the body, and, in the
female, from the oviduct: they join the
renal portal vein opposite the kidney.
2. The left renal portal vein corresponds in its course
and branches to the right vein.
b. The hepatic portal system.
This is formed partly by the anterior abdominal vein, which
brings to the liver blood &om the hind limbs ; and partly by
veins returning blood from the alimentary canal.
1. The anterior abdominal vein is a median vein
formed by the union of the two pelvic veins, the
ventral branches of the femoral veins. It runs
forwards along the middle line of the ventral
body- wall to the level of the liver, where it leaves
the body-wall and divides into right and left
branches, which enter the right and left lobes of
the liver respectively. During its course it
receives the following veins :
i. Vesical veins, from the bladder.
ii. Parietal veins, from the ventral body-wall.
THE ABTEBIES
S. The liepatdc portal vein is a wide veiu which runs
in the mesentery and joins the anterior abdo-
minal vein at its point of division into right and
left branches ; giving off, before doing so, a branch
to the left lobe of the liver. It carries to the
liver the blood &om the walla of the alimentaiy
con&l, and is formed by the union of the follow*
ing veins:
i. The gutilc vein, from the stomach,
ii. InteBtinal veins, from the whole length c£ the
intestine, both small and large,
iii. The splenic vein, from the spleen : this usually
joins one of the intestinal veins.
0. Tlie Arteries.
Fic. 5. — Diagram mstlc Rguif of tlie arterlil ijntan of (he male
frog, from Ibe right aide.
a, sloniBcb; i, Dosliil ; c, sfibII intestioe; t.a, csrotid artery;
t.g, carotid gbnd ; en, cceliaco loesenleric ulery ; e.n, cutaneous
artery: d, laige intestine; it.i!, dorsal aorta; /, femur; A, spleen;
i.a, hepatic arleiy; t, righl lung; l.a, lingual artery; n, testis;
a, kidney; o.o, occipilo-vertebral anei^ ; p.a, pulmonary arlrry;
r. pelvic girdle : >, sternum ; i.a. subclavian artery ; i.f, sciatic artery;
I, tongue ; /.a, inincus artericsus : tm, nrinogeniial arteties ; v, ven-
tricle ; 1, caiolid arch ; 2, ^slemic aich ; S, pulmo-cutiuieous arch.
30 THE VASCULAR SYSTEM OF THE FROG
Dissect as for ths veins. Pass a roll of paper or plugs of cotton-
wool down the CBSophagus, so as to distend it and stretch the aortic
arches. Glean carefully tlie aortic arches j commencing at the trun-
cits arteriosus; and follow the several arteries to their distribution,
removing the veins and other structures which overlie them. Note
the division of the tnmcus arteriosus in front into right and left
branches, each of which again divides into three aortic arches — t)ie
carotid arch, the ssrstemic arch, and the pulmo-cntaneons arch.
I. The Carotid Arch is the most anterior of the three
arches: it runs round the side of the oesophagus, and is
connected dorsally with the second or systemic arch; its
chief branches are as follows :
1. The eztemal carotid artery is a small artery supply-
ing the tongue. Immediately beyond the origin of
the lingual artery the carotid arch presents a small
spongy swelling, the carotid gland.
2. The internal carotid artery runs round the side of
the oesophagus to its dorsal surface : it is connected
with the systemic arch by a short branch, the ductus
Botalli, which in the adult frog is usually impervious ;
and then turns forwards beneath the base of the skull,
dividing in front into the two following vessels :
i. The palatine artery, supplying the roof and sides
of the buccal cavity, and the orbit,
ii. The cerebral artery, which enters the skull and
supplies the brain.
II. The Systemic Arch, the middle arch of the three, runs
somewhat obliquely round the oesophagus to the dorsal surface,
and unites with its fellow of the opposite side about the level of
the anterior ends of the kidneys. From this point the right
arch gives off the main artery to the viscera, the left arch
being the chief source of the dorsal aorta. Near the level of
the posterior ends of the kidneys the aorta divides into the two
iliac arteries. The branches of the systemic arch are as follows :
a. Branches given off before the union of the two arches.
1 . The laryngeal artery is a small branch arising from the
inner side of the systemic arch near its origin from
the truncus arteriosus, and supplying the larynx.
2. The oesophageal arteries are one or two branches arising
THE ARTERIES SI
from the upper part of the arch and entering the
dorsal wall of the oesophagus.
3. The ocdpito-vertebral artery is a short branch arising
from the dorsal part of the arch: it rons upwards
immediately in front of the transverse process of the
second vertebra, and divides into two :
i. The occipital artery : which runs forwards, supply-
ing the side of the head and jaws.
ii. The vertebral artery : a large artery which runs
back alongside of and above the vertebral column,
and gives branches to the muscles of the body-
wall and to the spinal cord.
4. The subclavian artery : arises from the arch immedi-
ately behind the occipito-vertebral artery, and runs
outwards, supplying the shoulder and fore-limb.
b. Branches given off after the union of the two arches to
form the dorsal aorta.
1. The coBliaco-mesenteiic artery is a large median artery
arisuig immediately beyond the point of union of the
two arches, or sometimes from the left arch just
before the union, and supplying the stomach and
lutestines. Its branches are as follows :
L The coeliac artery : which divides into
a. The gastric artery, supplying the stomach.
j3. The hepatic artery, supplying the liver and
gall-bladder.
ii. The mesenteric artery : which divides into
a. The anterior mesenteric artery, supplying the
proximal part of the intestine.
/3. The posterior mesenteric artery, supplying
the distal part of the intestine.
y. The spleiiic artery, supplying the spleen.
2. The nrinogenital arteries are four to six small arteries
which arise from the ventral surface of the aorta
between the kidneys, and immediately divide into
right and left branches, supplying the kidneys, the
reproductive organs and ducts, and the fat bodies.
8. The lumbar arteries are small paired lateral branches
supplying the body- walls.
32 THE YASCULAK STSTEH OF THE FBOQ
4. The hamorrlioidal artery is a small median artery
arising from the hinder end of the aorta, and supply-
ing the large intestine.
c. Branches formed by the division of the aorta.
1. The iliac arteries are the two large arteries formed by
the division of the aorta, and suppljdng the hind-
limbs. Each gives off a hypogastric artery, which
supplies the bladder, giving epigastric branches to
the ventral body-wall, and then continues as the
sciatic artery down the leg, giving off branches to
the muscles and skin of the thigh, and dividing at the
knee into peroneal and tibial arteries supplying the
leg and foot.
m. The Pnlmo-cutaneons Arch is the hindmost of the three
aortic arches : it divides about the level of the carotid gland
into the following branches :
1 . The cutaneous artery is a large artery which at first
runs forwards and upwards and then turns backwards,
supplying the skin of the back along the whole length
of the body, and sending smaller branches to the sides
of the head and to the skin of the ventral surface.
2. The pulmonary artery rims with somewhat sinuous
course along the outer side of the whole length of
the lui^, giving off branches into its walls.
D. The StructTire of the Heart.
Having completed the dissection of the Uoodveasels, out them
(U!ro88y about half cm inch from the heart; remove the hea/rt com-
pletely, amd dissect it carefully under water. It is well to cut the
vessels of unequal lengths on the two sides, as this wiU facUitaie
the recognition of the sides of the hea/rt during the dissection.
Flaee the heart ai first with the dorsal swrface upwards.
1. The Sinus Venosos (Fig. 4, p. 25) is a thin-walled sac on
the dorsal surface of the heart ; it is tnangular in shape, with
the apex directed backwards. Into its anterior angles the right
and left anterior venae cavae open, and into its posterior angle
or apex the posterior vena cava.
CtU away with scissors the dorsal waU of the sinus venosus so
as to eoopose its cavity : wash out any contained blood.
THB HEABT 33
The sinn-anxicnlar apertnre (Fig. 6, SY) leading from the
sinous venosua to the right .auriole, is a tranavereely oval opening,
guarded by imperfect anterior and posterior valves, in the
ventral wall of the sinus venosus, close to its anterior end, and
very nearly in the mediaa plane.
CSP
Fig. 6. - The frog's heart seen from the vendal surface, and dis-
sected so OS to show its slrtKlure. The ventral walls of the tnincus
arteriosus, and of the amides and ventricle bave been removed. (From
a drawing by Dr. Hurst.)
A, auricula -veatticular aperture and one of its valves ; B, aperture
leading from ventricle to iruncus arteriosus, witb one of its valves ;
C left carotid arch ; C, style passed down right carotid arch into Ihe
iTUncus arteriosus ; LA, left auricle ; P, left pulmo-culaneous arcb ;
PP. style, passed down rii;ht pulmo -cutaneous arch into the tnincus
arteriosus; PV, opening of pulmonary vein into left aiuicle : RA, right
auricle ; 8, left systemic arch ; 8', style passed down right systemic
arch into the truncus arteriosus ; 8V. opening from sinus venosus into
right auricle; V, ventricle.
34 THE VASCULAR SYSTEM OF THE FROG
2. The Auricles. Twrn the heart over, wUh its ventral swr-
face upwa/rds. Gut away the ventral wall of both auricles with
fine scissors, taking care not to damage the truncus arteriosus
which lies across the right auricle. Wash out the blood from the
auricles,
a. The right auricle (Fig. 6, KA) is the larger of the two.
It has thin walls, thickened by muscular strands
which form interlacing reticular ridges on its inner
surface. In the dorsal wall of the auricle, very near
the median plane of the heart, is the aperture from
the sinus venosus already described (Fig. 6, SV).
b. The left auricle (Fig. 6, LA) is smaller, sometimes
much smaller, than the right auricle, which it resem-
bles in the structure of its walls. In its dorsal wall,
very close to the sinu-auricular aperture, is the opening
of the pulmonary vein (Fig. 6, PV).
c. The interauricular septum is the thin partition between
the right and left auricles. It is much thinner than
the walls of the auricles, and is placed somewhat
obliquely, the left auricle lying rather more dorsaUy
than the right. The septum ends with a free posterior
edge, opposite the auriculo-ventricular aperture.
Cut away with scissors the ventral wall of the ventricle, teeing
care not to damage the truncus arteriosus,
8. The Ventricle (Fig. 6, V) is conical in shape with the
apex backwards, and has a small central cavity, with thick
spongy walls. lie spongy character is due to great develop-
ment of a reticulum of interlacing muscular strands similar to
those of the auricles : the true outer wall of the ventricle is no
thicker than that of the auricles, and the meshes of the sponge-
work are really part of the cavity of the ventricle, and are filled
with blood.
The auriculo-ventricular aperture lies at the base of the
ventricle, and rather to the left side. It is guarded by valves
(Fig. 6, A) which hang into the ventricle, and are tied down at
their edges by fine tendinous threads ; and it is divided by the
free lower edge of the interauricular septum into right and
left divisions, admitting blood from the right and left auricles
respectively.
THE HEART 35
Out away carefuMy^ mihfine scissors, the ventral wall of the
truncus arteriosus so as to expose its cavity and the contained
valves,
4. The Truncns Arteriosus consists of two parts ; a proximal
part or pylangium, which is a single vessel arising from the
ventricle ; and a distal part or synangium, which consists of the
basal parts of the aortic arches closely united together.
a. The pylangium (Fig. 6) is a short tube arising from
the right-hand ventral corner of the anterior end of
the ventricle : it has thick muscular walls and is
widest about the middle of its length.
The opening from the ventricle to the pylangium
(Fig. 6, B) is guarded by three semilunar pocket
valves.
The opening from the pylangium to the synangium
is also guarded by three semilunar valves which are
of very unequal size, a large right one, a small left
one, and a still smaller dorsal valve.
The spiral valve is a longitudinal ridge, projecting
into the cavity of the pylangium : it commences at
the left side of the ventricular aperture and runs
forwards somewhat spirally along the dorsal wall of
the pylangium to its anterior end, where it fuses
with the large right valve of the three between the
pylangium and the synangium. The ventral edge of
the spiral valve is free and rounded, and the valve is
much wider at its anterior than at its posterior end.
b. The synangium is the distal part of the truncus
arteriosus. In its dorsal wall immediately beyond
the valves separating it from the pylangium, is an
aperture (Fig. 6, P'), leading to the right and left
pulmo- cutaneous arches, P,P'. Beyond this the
synangium contains a wide cavity continued right
and left into the two systemic arches — S,S'. The
cavity is partially divided by a vertical tongue-like
projection from its dorsal wall : on the ventral sur-
face of this tongue are two small openings, very close
together, which lead into the right and left carotid
arches, 0,0', both of which arise from the right side.
Old across the aortic arches, just beyond tfie division of the
36 THE VASOULAB SYSTEM OF THE FROG
tru/rKMS into right cmd left hrcmches, cmd note that though each
branch is appcMr&nth/ a single vessel its cavity is really divided
into three vessels corresponding to the three aortic arches. Pass
hrisUes down these aortic a/rches, amd note the points at which they
severally open into the trunctcs arteriosus,
E. The Lympliatic System.
The lymphatic system forms an accessory part of the vascular
system. Its main divisions are as follows :
1. The lymphatic vessels are a series of thin-walled tubes,
very variable in diameter and irregular in shape, which
traverse all the parts and organs of the body and are
in free communication with the veins. They are of
small size, and can only be recognised with the
microscope.
2. The lymph sacs are large irregular spaces communi-
cating with the lymphatic vessels. The most
important are the following :
a. The snbcutaneons lymph sacs are the large
cavities between the skin and the muscles,
which have already been seen when remov-
ing the skin. They are separated from one
another by narrow septa of connective tissue,
which bind the skin to the underlying body-
wall.
b. The abdominal lymph sacs are the large
spaces along the dorsal surface of the body-
cavity, ventral to the kidneys, and between
the peritoneum and the body-walls. (See
Fig. 2, p. 19.) The body-cavity itself also
communicates with the lymphatic system
through small openings or stomata in the
peritoneum.
8. The lymph hearts are two pairs of small globular con-
tractile sacs placed at points where the lymphatic
vessels communicate with the veins. They are quite
transparent.
a. The anterior lymph hearts lie immediately
behind the transverse processes of the third
BLOOD 87
vertebra, and beneath the shonlder girdle :
thej open into the subscapular veins.
b. The posterior lymph hearts lie at the sides of
the urostyle, close to its hinder end. They
communicate by short vessels with the
femoral veins. Their pulsations can easily
be seen in a pithed frog.
4. The spleen has been already referred to (p. 28).
F. Microscopic Examination of Blood.
I. Frog's Blood.
1. Normal.
Place on a slide a small drop of blood from the heart of a frog ;
dikUe it with a d^rop of normal salt sohitum {0'7b per cent.); ptU
on a thin cover-glass, and run a ring of oU round the edge to
pi'event evaporation : examine toith the high power.
Blood consists of a colourless fluid, the liquor sanguinis
or plasma, in which float the blood corpuscles. These
corpuscles are of two kinds.
L Bed corpuscles. These are very numerous, pale
red or yellowish red in colour, and of a flattened
oval shape, with rounded edges and a central
bulging, the nucleus. The flattened shape is best
seen when a corpuscle turns edgeways. They
measure 0*0285 mm. in length by 0*0145 mm. in
width ; or about y^nr ^ isbo ^^ ^'^ inch.
ii. White corpuscles. These are much fewer in number
and of smaller size : they are colourless, granular,
subspherical in shape, and exhibit '* amoeboid"
movements. Sketch one half a dozen times at inter-
vals of half a minute.
In order to find the white corpusdes, focus the red corpuscles
as sharply as possible ; then on turning the screw of the fine
adjuMment slightly, so as to throw them JTist out offoctis, the
white corpuscles stand out as blurred white points (owing to their
refracting Ught more strongly); when thus found, focus down-
wards again, and study them.
38 THE YASOULAB SYSTKM OF THE FROG
2. Action of acetic acid on blood.
Pkuse afresfh drop of blood on a dean dide : add a drop oj
a4!€tic acid : cover y cmd examine with the high power : note the
chemgea prodiiced,
i. Bed corpuscles: the nuclei become much more
apparent than before, and the red colour disappears.
ii. White corpuscles : become clearer, and show nuclei^
sometimes more than one in a single corpuscle. '
n. Human Blood.
1. Normal.
Prick the tip of yofwt finger j cmd place a small drop of the
blood on a slide : add a drop of normal salt soltUioUy cover, and
examine as before. Note the following points :
i. Bed corpuscles. These, which are much smaller
than in frog's blood, are in the form of circular
biconcave discs with rounded edges but no nuclei.
They have a tendency to run together into rou-
leauz, like piles of coina Their average diameter
is 0*008 mm., or about gg'bo ^^ ^^ inch.
ii. White corpuscles. These are very similar to those
of the frog : they are slightly larger than the red
corpuscles, averaging about 0*01 mm., or ^^^^ of an
inch in diameter: their amoeboid movements are
not well seen unless the slide is warmed.
2. Action of acetic acid.
Treat with acetic acid as before : note that, unlike the frog's
blood, no nuclei are visible in the red corpuscles.
G. Oirculation of the Blood in the Web of a Frog's Foot.
The web uniting the toes of the f rog*s foot is so thin and
transparent, that with the microscope the blood in it can readily
be seen coursing along the capillaries.
Exomiine a frog prepared to show the circulation in the web of
the foot. Note ifie following points :
1. With a low power.
a. The irregularly branched pigment cells to which the
colour of the frog's skin is due.
CIBCnJLATION OF BLOOD 39
b. The fine meshwork of bloodvessels along 'which the
blood can be seen flowing. These bloodvessels are of
three kinds.
i. The arteries, carrying blood to the web, are dis-
tinguished by the fact that when they divide, the
direction of flow of the blood is from the larger
trunk to its branches.
ii. The capillaries form a close network of very small
and thin-walled vessels, along which the blood
flows from the arteries to the veins.
iii. The veins, carrying the blood away from the web
back towards the heart, are distinguished from the
arteries by the fact that the blood in them flows
from smaller to larger vessels.
2. With a high power : Twie thefoUowvng pomts :
a. The walls of the arteries and veins are much thicker
than those of the capillaries, which latter are often
difficult to see.
b. The white corpuscles have a marked tendency to creep
along the sides of the vessels, while the red corpuscles
rush far more rapidly along the middle of the stream :
this is seen best in the small arteries.
a The variation in calibre of the small arteries and
capillaries: whilst under observation an artery or
capillary miay be seen to change its size to a con-
siderable extent.
d. The indefinite character of the capillary circulation.
Owing to changes of size in adjacent vessels, the
direction of flow of the blood in a given capillary may
become reversed.
e. The elasticity of the red corpuscles : seen best when they
are turning the comers of the capillary network.
f. The tendency of the white corpuscles to migrate through
the walls of the capillaries into the tissues outside.
This is much increased by the application of some
irritant substance, such as a drop of weak acid, to the
web.
CHAPTER III.
THE SKELETON OF THE FBOG.
Thb skeleton, which forms the hard internal parts of the frog,
is composed partly of cartilage and partly of bone. It forms a
framework giving definite shape to the body, and precision to
the movements ; and serves also to protect from injury some of
the more important and delicate organs, notably the central
nervous system, the sense organs and the heart. In the early
stages of its development the skeleton consists entirely of car-
tilage; in the adult this primary cartilaginous skeleton is replaced
to a greater or less extent by bone. Bone may also be developed
in places where there was no pre-existing cartilage, and is then
called memhrcme-bone, in contradistinction to the former kind,
or caxtilage-bone, which replaces cartilage. Membrane-bones
arise in the first instance as ossifications in the dermis or deeper
layer of the skin: in many fish they retain this primitive
position, but in the frog and most higher vertebrates they sink
below the skin and graft themselves on to the more deeply placed
cartilaginous skeleton. Cartilage may also become calcified,
i.6., have calcareous salts deposited in i^ matrix, without in any
way taking on the character of true bone.
The skeleton may conveniently be divided into (1) the axial
portion, including the skull and the vertebral column : and
(2) the appendicular portion, including the limbs, and the
Umb-girdles which attach them to the body.
Exomvine the prepcured skdeUms, a/nd maht caurefvX dra/mnga to
scale of the several parts. In yau/r drawings colour the cartiUige
blue, the cartilage-bones yellow, amd the membrane-bones white or
red. Prepare skeletons for yourself by dipping the parts in hot
wa4er, and carefully brushing away the soft tissues until the
skeleton is clean.
THS SEELXTOM OF THK FEOO
n from the derail surface ; the
a. astragalus: ', calcaneum ; d, suprascapular; t, exocci.
/, femur; /f, fronioparieial ; g, metacarpals ; A, humerus ; i, ilium ; i,
tnetalarsati : /, carpus; m, maiilla; a, nasal; o. pro-otic; ^.pterygoid;
fm, preitnxilla; j, "quadratolueal" ; r, radio-ulna; i, squamosal;
», ipbenethnund ; j.v, sacral venebra; I, tibio-fibula; n, urostjrle.
42 THB SKELETON OF TH£ FROG
A. The Axial Skeleton.
I. The Vertebral Column or "backbone."
This is a bony tube which surrounds and protects the spinal
cord ; it consists of an anterior part, which is divided trans-
versely into nine rings or vertebrse, and a posterior unsegmented
portion of about equal length — ^the nrostyle. At the sides of
the tube, between the successive vertebrae, are the intervertebral
foramina through which the nerves pass out from the spinal
cord to the various parts of the body.
a. Structnre of a vertebra. Examine one of the vertebrcBy
say the third, more cLoaely : d/raw i^, showing the
following points :
i. The vertebra is a bony ring ; the spinal cord lying
during life in the central neural canal.
ii. The centrum or body is the thickened ventral por-
tion of the ring : it articulates with the centra of
the vertebrae in front of and behind it ; and forms
the floor of the neural canal.
iii. The neural arch consists of the lateral and dorsal
portions of the ring; and forms the sides and
roof of the neural canal.
iv. The spinous process or neural spine is a small blunt
median process, projecting upwards and back-
wards from the top of the neural arch.
V. The transverse processes are a pair of large processes
projecting horizontally outwards from lie point
of union of centrum and neural arch.
vi. The articular processes or zygapophyses, on the
anterior and posterior borders of the neural arch,
articulate with corresponding processes on the
vertebrsB in front and behind, and so serve to
link the vertebrae together.
a. The anterior articular processes, or prezyg-
apophyses, face upwards and slightly in-
wards.
3. The posterior articular processes, or post-
zygapophyses, face downwards and slightly
outwards.
THS VEETEBRAL COLUMN 43
b. Special vertebrae.
i. The atlas or first vertebra articulates in front
with the posterior end of the skull: it has no
transverse processes. Note the large gap on the
dorsal surface between the skull and ihe neural
arch of the atlas: through this gap, which is
closed by the strong occipito-atlaiital membrane,
the central nervotis system is divided and de-
stroyed in the operation of pithing a frog.
ii. The sacrmn, or ninth vertebra, has very stout
backwardly directed transverse processes, which
support at their outer ends the pelvic arch.
c. The nrostyle is the unsegmented posterior portion of
the vertebral column. It articulates in front with the
body of the sacral vertebra by two surfaces. Along
its dorsal surface runs a prominent vertical ridge,
highest in front and gradually diminishing posteriorly:
the neural canal is continued down the anterior part
of this ridge. At the sides of the urostyle, and about
the length of a vertebra from its anterior end, are a
pair of small holes through which nerves pass out,
and which therefore correspond to intervertebral
foramina.
n. The Skull.
The skull consists of, (1) an axial portion, the cranitun,
enclosing the brain and forming an anterior continuation of the
vertebral colxmm ; (2) the olfactory capsules and the auditory
capsules, which are fused with the anterior and posterior ends
of the cranium respectively; (3) the bony framework of the
jaws ; and (4) the hyoid apparatus.
In the skull the original cartilage, or chondrocranium, is not
so largely replaced by bone as in the vertebral column, large
tracts of unossified cartilage persisting in the adult. Besides
the cartilage-bones the skull is further strengthened by the
addition of numerous membrome-honeB.
1. The Cranium is originally an unsegmented cartilaginous
tube, whose cavity forms the anterior part of the
neural canal, and lodges the brain. The roof of the
44 THS SKELETON 07 THE FROa
tnbe is imperfect, there being one Urge anterior fon-
tanelle, and two smaller posterior fontuielles, which
are dosed b^ membrane onlj'. In the cai-ti^ge are
developed cartilage-bones, and around it membrane-
bones.
To study the erontum aatisfactorib/, the 77temhrane-hone* should
be alripped from (me of the iktiUs lohieh you have prepared for
yowself.
a. Oartilage-bonea of cranium.
i. The exoccipitala are two irregular bony masses at
the sides of the posterior end of the skull. The^
almost completely surround the forauen magnnia
or entrance to the cruiial cavity ; and bear on
their posterior surfaces the occipital condyles,
two oval convex processes which articulate with
the first vertebra or atlas.
1, from the TentntI surbce.
; t, eiocdpital ; /.f, frontoparietal ;
.,., .,„.,_, .,, , ., 1^ — 'Otic; f, pterygoid ; M, palatine ;
i)m, premaiilla ; ;, " quadratojugal " ; Jt, sphenethnioid.
ii. The aphenethmoid or girdle-bone is a bony tube
which encircles the anterior end of the cranial
cavity, and extends forwards into the olfactory
region : in front it is divided by a vertical parti'
tion into right and left cavities, in which lie the
olfactory sacs.
THE SKULL 45
b Meffibrcme-banes of cranium,
L The JrontoparieUUs are two long fiat bones on the
top of the brain-case, covering the fontanelles, and
overlapping the hinder end of the sphenethmoid.
ii The pa/rasphenoid is a j^-shaped bone on the ventral
surface of the cranium ; its lateral processes
underlying the auditory capsules.
2. The Sense Capsules are cartilaginous and bony capsules
which surround and protect the olfactory, optic, and
auditory organs; two of them are fused with the
cranium so as to form parts of the skull,
a The auditory capsules are fused with the sides of the
posterior end of the cranium, to which they form
wing-like projections : they consist largely of cartilage.
1. Cartilage-bone of auditory capsules.
The pro-otics are a pair of irregular-shaped bones
in the anterior walls of the capsules, and forming
also parts of their roof and floor.
b. The optic capsules are not fused with the skulL They
remain permanently cartilaginous under the name of
the sclerotic cartilages of the eyes.
c. The olfactory capsules are fused with the anterior end
of the cranium, and also with each other. They
consist very largely of cartilage, which is produced in
front into the rhinal processes,
i. Cartilage-bone of olfactory capsules. The spheneth-
moid, as already noticed, extends forwards so as to
invade the olfactory region, but does not properly
belong to the olfactory capsules,
ii. Memhrane-honea of olfcutory capsvlea,
a. The viaaala are two triangular bones on the dorsal
surface of the anterior end of the head : the bases
of the triangles are turned towards the middle
line and meet each other in front, while their
posterior ends diverge and enclose, with the
anterior ends of the frontoparietals, a diamond-
shaped patch in which the sphenethmoid is visible
on the dorsal surface of the skull.
j3. The vomers are two triradiate bones on the ventral
surface of the fore part of the skull : each vomer
bears in its inner or posterior angle a small
16 THE SKELffTON OF THE FROQ
group of pointed teeth, and forms the inner
boundary cj the posterior narial opening of its
Bide.
3. The Jarva consist of two cartilaginous arches on each side,
maxillarj' and mandibular, in connection with which
caitilage-boiieB and mambraTte-bortas are developed.
Each arch meets its f eUow in the middle line in front ;
and the maxillary arches, forming the upper jaw, are
firmly connected with the cranium by anteiior and
posterior bony struts.
' a. The Maxillary Arch.
Fig. 9.— The frog's skull from (he right sida
A, parasphenoid ; AS, angulosplenial ; B, anlerior cornu of tiyoid ;
C, columella ; D, tientary ; E, eioccipital ; F, nostril ; FP. frtHilo-
parielal ; H, body of hyoid ; L, aperture for exit of optic nerve;
M, maxilla ; MM, meniomeclieliaa ; M', apenure for exit of fifth and
seventh nerves ; N. nasal i O, pro otic ; P. pterygoid : PM, premaxllla:
Q, "quadralojugal "; R, Bpenure for exit of ninih and tenth nerves;
8, tquamosal; 8E, sphenethmoid ; T, posterior comu of hyoid.
i. The pterygoid" is a large triradiate bone, the inner
limb of which is connected with the auditory
capsule ; while the posterior limb mns back to
the angle of the mouth, and the anterior limb
forwards along the upper jaw to the palatine
ji. The palatine* is a slender transverse bone, con-
THE SKULL 47
necting the upper jaw with the anterior end of
the sphenethmoid.
iii. A short bone forming the posterior part of the
outer margin of the upper jaw, is in the position
of the quadratqJTigal of other skulls.
iv. The maodUa is a long thin bone forming the greater
part of the outer margin of the upper jaw : it bears
teeth along its whole length which are anchylosed
with the bone. It is connected behind with the
" quadratojugal " ; about the middle of its length
with the anterior limb of the pterygoid and with
the palatine ; and in front with the premaxilla.
V. The premctxiUa is a small bone which meets its
fellow in the middle line in front, and so com-
pletes the outer margin of the upper jaw : like the
maxilla it bears teeth. It gives off on its dorsal
surface a backwardly projecting process which
forms part of the inner boundary of the nostril.
vi. The quadrate cartilage, which forms the suspen-
soriiun, t.c., serves to connect the lower jaw with
the skull, is a rod oi cartilage which is fused
above with the auditory capsule, and runs down-
wards and backwards to the angle of the mouth,
where it is connected with the hinder end of
the quadratojugal bone. In the adult frog, the
quadrate cartilage lies between the squamosal
and pterygoid bones, and is almost completely
conceieded by these.
vii. The aqucmioaal is a T-shaped bone, the stem of
which is closely applied to the outer surface of the
quadrate cartilage. The posterior limb of the T
is attached to the outer surface of the auditory
capsule, and with the body of the squamosal
helps to support the annulus tympanicus.
b. The Mandibular Arcb.. The arch persists in part un-
ossified as Meckel's cartilage, which forms the basis of
the lower jaw, and is ensheathed by cartilage-bones and
membrane-bones.
i. The cmgidosplenial ensheathes the inner and lower
surfaces of Meckel's cartiLage along the greater
THE BKBLETON OF THE PEOCt
part of its length ; near its hinder end it is pro-
duced upwards into the coronary process.
i. Ilie dentartf is a flat bone covering the outer sur-
face of the distal half of Meckel's cartilage, as far
forward as the mentomeckelian bone.
. The meutomecbeUan is a small oGsiflcation in
Meckel's cartilage at the symphysis, i.e., the
union of the ai'ches of the two sides at the chin.
Pig. io. — A Iransverae section across ihe poslcrior pa.Tl of the frog's
beftd, 10 show ibe posiiion and relations of the auditory organs.
Eustachian tubes, and hyoid apparatus. On the right side the seciion
passes tbrotigh the tympanic cavity and the columella.; on the left side
through the anterior coinu of the hyoid. The cartilage is dotted, and
the bones, except the columella, are represented black.
A, paras pheno id ; AG, angulosplenial ; B. buccal cavity; C. cola-
melia; O, tympanic metubrane ; E, Eustachian tube; F, anterior
corau of the hyoid; FP, frontoparietal; Q, glottis; H, arytenoid
cartilage ; I, posterior comu of hjroid ; K, auditory nerve ; L vesti-
bule; M, BQterioi vertical semicircular canal; N. horizontal semi-
circular canal; O, pro-otic; P, pterygoid; Q, quadrate cartilage;
R. " quadratojugal " ; 8. aquanmsal ; T, annulus tympanicus ; V, vocal
cord ; x, mid-team.
1. The Hyoid Appaxatus (Fig. 9). This consists of the
hyoidean arch and the remains of the branchial arches
of the two sides, together with a median ventral
plate, the body of the hyoid, which unites their
lower ends together, and lies in the flotn* of the mouth.
The hyoid apparatus oonoste almost entirely of
cartilage.
THE APPENDICULAB SKELETON 49
a. The hyoid arch.
i. The colnmella (Figs. 9 and 10) is formed from the
top of the hyoid arch : it is a small rod, partly
bone and partly cartilage, the inner end of which
is inserted into the fenestra ovalis, an aperture
in the outer wall of the auditory capsule ; while
the outer end is attached to the tympanic mem-
brane rather above its middle.
ii. The anterior comu of the hyoid (Fig. 9) is a long
slender curved rod of cartilage, attached above to
the auditory capsule just below the fenestra ovalis,
and curving at first backwards and then forwards
and downwards to be attached to the anterior
outer angle of the body of the hyoid.
b. The body of the hyoid is a fiat squarish plate of cartilage,
formed by the fused ventral ends of the hyoid and
branchial arches, and lying in the fioor of the mouth :
short processes are given off from its angles.
c. The posterior comua of the hyoid are a pair of stout
bony processes, diverging from the hinder border of
the body of the hyoid.
B. The Appendicular Skeleton.
This comprises the limbs and the limb-girdles. As in the case
of the axial skeleton it consists at first entirely of cartilage, which
becomes afterwards replaced to a greater or less extent by carti-
lage-bono. Membrane-bones, i.e., bones developed independently
of cartilage, are very rare, the clavicles being the only examples
met with in the frog. (Fig. 7, p. 41.)
1. The Pectoral Girdle.
This consists originally of two half rings of cartilage, one on
each side of the body, which they encircle a short way behind
the head: the dorsal ends of the half rings are attached by
ligaments and muscles to the vertebral column, while the ven-
tral ends are united together in the median plane by the sternum
or " breast bone.'*
Each half ring bears in the middle of its hinder surface a
cup-shaped cavity, which, mth the first bone of the fore-limb,
V
60 THB SKELETON OF THE FROO
forms the shoulder-joint. The part of the arch above the joint
is the scapular portion : and the part below, which is divided
into anterior and posterior divisions, the coracoid portion.
a. The scapular portion is divided into two parts.
i. The suprascapula, the upper portion, is a thin
expanded plate of cartilage overlapping the first
four vertebrae : it is partly calcified and partly
ossified, though very imperfectly.
ii. The scapula is an oblong bony plate, constricted in
the middle, and forming the upper half of the
glenoid cavity, or cavity of the shoulder- joint.
b. The coracoid portion forms the lower half of the glenoid
cavity : it is divided into anterior and posterior por-
tions, separated by the coracoid foramen.
i. The precoracoid is a slender horizontal bar of car
tilage connecting the anterior edge of the scapula
with the stemiun.
ii. The damcU is a slender bone, closely applied to the
anterior border of the precoracoid ; its outer or
scapular end is bent forwards almost at a right
angle.
iii. The coracoid is a stout bone, wider at its inner
than its outer end : it connects the posterior
edge of the scapula with the sternum.
iv. The epicoracoids are a pair of narrow strips of
cartilage closely applied to each other, and lying
between the ventred ends of the precoracoids and
coracoids.
c. The stemnm lies in the mid-ventral line : it consists
originally of two lateral halves, which fuse com-
pletely in front and behind, but remain distinct
in the median portion. It presents from before
backwards the following parts :
i. The presternum.
a. The epistemum is a flat circular plate of car-
tilage.
j8. The omostemum is a slender bony rod pro-
jecting forwards in front of the clavicles.
THE APPENBICULAIt SKELETON 61
ii. The mesostemnm is a rod of cartilage, ensheathed
in bone, projecting backwards bdind the cora-
coids.
iii. The metasteninm or xiphistemnm is a broad ex-
panded plate of cartilage at the hinder end of
the sternum.
2. The Fore limb.
The bones of the fore-limb are all cartilage-bones. With the
exception of the small bones of the wrist, they are elongated,
with enlarged ends capped with articular cartilage. The
enlarged ends or epiphyses ossify independently of the shaft of
the bone, with which tiiey do not unite until late in life. The
end of a bone which, when the limb is extended, is nearer to
the body, is called its proximal end, the opposite extremity the
distal end.
a. The arm. In the arm there is only a single bone.
i. The hnmems. The proximal end or head is en-
larged, and articulates with the glenoid cavity
of the pectoral girdle: below the head is the
strong deltoid ridge extending along the proximal
half of the anterior surface. At the distal end
is a spheroidal articular surface for the bone of
the forearm : and at either side of this a
prominent condylar ridge, the inner or postaxial
one being the larger of the two, especially in the
male frog.
b. The forearm.
i. The radio-ulna corresponds to two bones, radius
and ulna, in other animals : it is single at its
proximal end, but in its distal half is imperfectly
divided by a groove into anterior or radial, and
posterior or ulnar portions. Its proximal end is
hollowed out to articulate with the lower end of
the humerus, and so form the elbow-joint, be-
hind which it projects backwards as the olecranon
process.
c. The wrist consists of six small carpal bones arranged
in two rows, proximal and distal, each row having
three bones.
52 THE SKELETON OF THE FBOG
d. The hand has four complete digits and a rudimentary
pollex or thumb. Each digit consists of a proximal
metacarpal bone, beyond which are a variable number
of phalanges.
i. The pollez, the anterior or preaxial digit, consists
simply of a small metacarpal bone.
ii. The first complete digit, corresponding to the fore-
finger of man, consists of a metacarpal and two
phalanges.
iii. The second digit, corresponding to the middle
finger of man, consists of a metacarpal and two
phalanges.
iv. The third, corresponding to the ring-finger of man,
consists of a metacarpal and three phalanges.
v. The postaxiaJ digit, corresponding to the little finger
of man, consists of a metacarpal and three
phalanges.
3. The Pelvic Girdle.
This consists primitively, like the pectoral girdle, of a couple
of half -rings of cartilage, fused together below and attached
above to the tips of the transverse processes of the sacrum. In
the adult frog the girdle is placed very obliquely, so as to be
nearly parallel with the vertebral column instead of at right
angles to it.
Each half presents on its outer aspect a cup-shaped cavity —
the acetabulum — forming, with the thigh bone, the hip- joint : we
accordingly distinguish an iliac portion above the acetabulum
and an ischio-pubic portion below it, corresponding respectively
to the scapular and coracoid divisions of the pectoral girdle.
i. The ilium forms the anterior and upper half of
the acetabulum, and extends forwards as an
elongated laterally compressed bar, which is
attached in front to the transverse process of the
sacrum and bears along its dorsal surface a
prominent vertical ridge of bone, the iliac crest,
ending behind in an abrupt vertical border.
Posteriorly the two ilia meet each other and are
united together in the n^edian plane to form the
iliac symphysis.
THE APPENDICULAB SKSLETOK 68
ii. The pubes consists entirely of cartilage : it forms
the anterior portion of the ventral division of the
girdle, and therefore corresponds to the pre-
coraooid in the pectoral girdle. The two pubes
are completely fused together in the median
plane, and form only a very small portion, about
one-sixth, of the .acetabular cavities.
iii. The ischinm is the posterior portion of the ventral
division and corresponds therefore to the eoracoid
in the shoulder girdle. It forms the posterior
third of the acetabulum. The two ischia are
completely fused together in the median plane.
4 The nind-limb.
The bones have the same general characters as those of the
fore-limb, to which they correspond very closely.
a. The thigh.
i. The femur is a long slender bone, expanded at both
ends, and curved slightly in a sigmoid manner.
The proximal end or head is spheroidal, and fits
into the acetabulum to form the hip- joint : the
distal end is somewhat expanded laterally.
b. The leg.
i. The 08 crnxis or tibio-fibula is a single bone, rather
longer than the femur, slightly curved, and
expanded laterally at both ends. It presents
along the greater part of its length a groove
indicating its correspondence with two bones,
tibia and fibula, which in man and many other
animals remain distinct from each other.
0. The ankle, corresponding to the wrist in the fore-limb,
consists of two rows of tarsal bones.
a. The proximal row of tarsal bones consists of two
elongated bones united together at both ends, but
widely separated in the middle.
i. The astragalus is on the preaxial or tibial side.
ii. The calcaneum is on the postaxial or fibular
side, and is the larger of the two bones.
54 THE SKELETON OF THE FROG
/3. The distal row of tarsal bones consists of two very
small bones.
d. The foot has five complete digits, and a supernumerary
toe as well. Each digit consists of a proximal meta-
tarsal bone, beyond which are a variable number of
phalanges.
i. The hallux or preaxial digit, corresponding to the
great toe of man, is the smallest of the series.
It consists of a metatarsal and two phalanges.
On the inner side of the hallux is the calcar,
supposed to be an additional or supernumerary
toe : it may have one or two joints in addition to
a short metatarsal.
ii. The second toe consists of a metatarsal and two
phalanges.
iii. The third consists of a metatarsal and three
phalanges.
iv. The fourth, the longest of the five, consists of a
metatarsal and four phalanges.
V. The postaxial digit, corresponding to the little toe
in man, consists of a metatarsal and three
phalanges.
CHAPTER IV.
THE MUSOULAB STSTEM OF THE FBOG.
The muscles, or flesh, are the direct means by which the
various movements of the body and of its several parts are
brought about. A muscle consists of a fleshy belly, which is
usually attached at each end by means of tendons to some hard
part, very commonly to bone. Motion is effected by the muscle
conteactiiig, i,e., shortening, and so bringing its two ends, and
consequently the parts to which the ends are attached, nearer
together. Of the two attachments of a muscle one is usually
to a more fixed and central part, the other to a more movable
and peripheral part : the former attachment is called the origin
of the muscle, the latter its insertion.
Muscles are of two kinds : (1) voluntary muscles, t.e., those
which are under the control of the will, as the muscles of the
arm : and (2) involuntary muscles, i.e,, those over which the
will has no direct control, as the muscles of the heart and
bloodvessels, or of the alimentary canal.
Voluntary muscles, which are the only ones dealt with in this
chapter, are usually attached at both ends to bone ; but one or
other end, or both, may be attached to aponeuroses, strong
connective tissue membranes which ensheath the muscles and
other parts, and separate them from one another.
For the dissection of tlie muscles, take a frog that has been in
spirit for a day or more. When deaning a muscle he ca/refuL to
ptU it on the stretch, and to dissect along , and not across its
fibres : defme the origin a/nd insertion of the muscle very clearly,
amd test its action by putting it gently with the forceps in the
direction of its fibres. Always have the skeleton in front of you
80 as to see accu/rately the origins and insertions of the muscles.
In the foUovnng description soms of the smMer muscles, especially
in the head, a/re omitted.
56 THE MUSCULAB SYSTEM OF THE FROO
A. Muscles of the Trunk.
1. Muscles of the ventral body-wall.
Pin out the frog on its hack^ remove the skiriy <md dean ike
muscles,
i. The rectus abdominis runs longitudinally along
the mid- ventral wall, the muscles of the two sides
being separated from each other in the median
plane by the linea alba, a longitudinal band of
connective tissue, immediately dorsal to which
lies the anterior abdominal vein. Each rectus
muscle is divided into bellies by five transverse
tendinous intersections.
The muscle arises from the pubes, runs for-
wards and is inserted into the dorsal surface of
the sternum and coracoid.
ii. The pectoralis is a large fan-shaped muscle, con-
sisting of a thoracic portion, which arises from
the whole length of the ventral surface of the
sternum ; and an abdominal portion, arising from
the aponeurosis along the outer side of the
rectus abdominis almost as far back as the
pubes. From this extensive origin the fibres
converge to the deltoid ridge of the humerus,
into which they are inserted, the line of insertion
extending down almost to the elbow.
ili. The obliquus eztemus is a thin sheet of muscle
which arises from the aponeurosis of the back, a
short distance from the vertebral column, and
covers the whole of the side of the body, the
fibres running obliquely downwards and back-
wards to end in an aponeurosis which passes
dorsal to the rectus abdominis to be inseited
into the linea alba.
iv. The obliquus intemus lies beneath the obliquus
eztemus, which must be removed in order to
see it. It arises from the transverse processes of
the vertebrsB from the fourth backwards, and
from the ilium. The fibres run downwards and
forwards, and are inserted in front into the
coracoid and sternum: some of the fibres sur-
round, and are inserted into, the oesophagus and
MUSCLES OF THE TBUNK 67
pericardium. The hinder two-thirds of the
muscle pass dorsal to the tendon of the obliquiis
eztenms and are inserted, like it, into the linea
alba.
2. Muscles of the back.
Pin out the frog on its hdly : remove the skirif cmd dean the
muaclea in order,
i. The depressor mandibulsB is a broad triangular
muscle which arises from the fascia covering the
dorsal surface of the suprascapula : the fibres run
downwards behind the tympanic membrane, and
converge to be inserted into the angle of the
lower jaw. The muscle bj its contraction opens
the mouth.
ii. The cncnllaris is a small oblong muscle which,
arising from the exoccipital near the middle lix^,
runs backwards and outwards, and is inserted
into the dorsal border of the suprascapula.
iii. The latissimtis dorsi is a triangular muscle lying
behind the depressor mandibiilflB : it arises from
the fascia dorsalis just behind the shoulder girdle,
its origin being partly covered by the obliquus
extemus. The fibres run forwards and outwards,
converging to be inserted by a long tendon into
the deltoid ridge of the humerus.
Dissect (vway the depressor mandibulsB cmd latissimus dorsi
from their origins^ amd turn them doum,
iv. The infraspinatus arises from the dorsal surface
of the suprascapula, partly overlapped by the
latissimtis dorsi : it runs outwards to be inserted
into the deltoid ridge of the humerus : its action
is to elevate the arm.
Lift up the suprascapula amd note the muscles aitaching it to
the bodi/f viz.f
V. The retrahens scapulsB, behind,
vi. The levator anguli scapulae, in front.
Remove the suprascapula on one side, and dean the median
longitvdinal musdes of the hack.
58 THE HUSGTTLAtt STStEM OF tttt ITROO
vii. The extensor dorsi commnnis is a longitudinal
mass of muscle arising from the iux)style, and
running forwards and slightly outwards: it is
inserted into the ilium, into the transverse pro-
cesses of the vertebr», and in front into the
posterior end of the skiill. The anterior part is
divided by transverse tendinous intersections.
viii. The intertransversales are small muscles, running
between the transverse processes of the vertebrsBy
and lying beneath the extensor commnnis.
ix The glntfldns arises from the outer side of the pos-
terior two-thirds of the iliiun, and runs backwards
to be inserted into the trochanter of the femur.
B. Muscles of the Head.
1. Muscles of the ventral surface of the head.
i. The mylohyoid or submandibular muscle is a flat
sheet of muscle running across from one ramus
of the mandible to the other, and divided down
the middle line by a tendinous intersection. A
narrow strip along the posterior border is
commonly separated by a slight interval from the
major or anterior part of the muscle.
Remove the mylohyoid musde^ cmd the sternal portion of the
pectoralis.
ii. The geniohyoid is a narrow longitudinal band a
short distance from the middle line : it arises
from the lower jaw close to the chin, runs back
on the ventral surface of the body of the hyoid,
and divides posteriorly into two portions which
are inserted respectively into the bony and the
cartilaginous posterior processes of the hyoid.
iii. The sternohyoid is practically the anterior con-
tinuation of the rectus abdominis. It arises from
the dorsal siuf ace of the coracoid and clavicle,
and is inserted into the ventral surface of the
body of the hyoid, the tendon passing between
the two divisions of the geniohyoid.
MUSCLES OF THE HEAD 59
iv. The hyoglossus arises on either side from the
posterior bony horn of the hyoid : the two muscles
converge and meet each other in front of the
larynx. In front of the larynx the muscle runs
forward in the middle line as a stout band nearly
to the chin : it then enters the tongue, and runs
along it backwards to the tip.
V. The petrohyoid muscles are a set of five muscular
bands which arise close together from the outer
surface of the auditory capsule, and diverging in
a fan-like manner, pass round the floor of the
pharynx and oesophagus to be inserted in front
into the median ventral line of the pharynx, and
behind into the side of the hyoid. The first or
most anterior band is a wide thin sheet of mus-
cular tissue, while the four posterior portions
are very narrow slips.
2. Muscles of the side of the head.
R&itwve the skin oa/refidly from, the side of the head and jaws,
noticing how m/uch more closely it is attached to the underlying
parts than was the case in the body,
a. Depressors of the lower jaw ; opening the mouth.
i. The depressor mandibulsB has been already seen
and dissected. (See p. 57.)
b. Elevators of th^ lower jaw; shutting the mouth.
These lie in the space between the auditory capsule
and the eye.
ii. The temporalis arises from the upper surface of
the auditory capsule, and passes outwards and
downwards between the pterygoid and maxillary
bones, and in front of the cartilaginous ring
supporting the tympanic membrane, from which
some of its fibres arise : it is inserted into the
ooronoid process of the lower jaw.
iii. The pterygoideus is a slender muscle placed just in
front of the temporalis and partly covered by it :
it arises from the side wall of the skull, and is
inserted into the mandible further back than the
temporalis, and very close to the joint.
60 THE MUSCULAK SYSTBM OF THE FROO
£▼. The masseter is a small muscle placed behind the
temporalis : it arises from the quadratojugaJ and
runs downwards and slightly backwards to be
inserted into the outer surface of the mandible,
just in front of the joint.
To see the insertions of these last three muscles the motUhshotdd
be opened widely.
3. Muscles of the eyeball.
Remove the temporal and pterygoid muscles careJ^tUy^ dissecting
them away from thei/r origins, and then twming ^ mibsdes dotcn
and cutting them short dose to their insertions. Remove also the
lower jaw ; pin the frog out on its hack omd dissect away carefuG/y
the mucous msfmhrams of the roof of ike mouth.
i. The levator bulbi is a thin sheet of muscle lying
between the mucous membrane and the eye. Its
fibres arise from the side of the skull, run out'
wards underneath the eye, and are inserted into
the upper jaw. The muscle by its contraction
serves to lift up the eyeball and so make it more
prominent. Some of its fibres are inserted into
the lower eyelid, which they serve to depress,
acting as a depressor palpebro inferioris.
Remove the levator bnlbi and dean the remaining musdes^
dissecting them partly from the dorsal and partly from the
ventral surf ace.
a. The recti mnscles are a group of four small muscles
which arise dose together from the inner and posterior
angle of the orbit close to the optic foramen, and run
forwards and outwards, diverging from one another, to
be inserted into the bulb of iiie eye.
i. The rectos superior is inserted into the dorsal
surface of the eyeball : it is seen best from above.
ii The rectus extemns, the most posterior of the
four, is inserted into the posterior surface of the
eyeball : it is seen best from the side or from
below.
iii. The rectus intemns, the longest of the four, runs
forwards between the skuU-wall and the eyeball,
MUSCLES OF THE KTSBALL 61
and is inserted into the inner or median surface
of the eyeball : it is seen best from below.
iv. The rectos inferior is inserted into the under surface
of the eyeball : it is seen best from below.
b. The obliqiii muscles are two small muscles which
arise close together from the palatine bone at the
anterior end of the orbit, and run backwards to be
inserted into the eyeball.
i. The obliqnns superior is inserted into the dorsal
surface of the eyeball just in front of the rectos
superior : it is seen beist from above.
ii. The obliqoos inferior passes backwards beneath
the rectos intemos, and is inserted into the eye-
ball between it and the rectos inferior : it is seen
best from below.
c. The retractor bolbi, or choanoid muscle, is a funnel-
shaped muscle which lies within the four recti and
embraces the optic nerve: it arises from the para-
sphenoid, and is inserted into the eyeball. It is best
exposed from below by carefully removing the recti
muscles.
0. Muscles of the Hind-limb.
If the frog's leg be stretched back parallel to the longitudinal
axis of the body, as in the act of swimming, we distinguish in
it ventral and dorsal surfaces, an outer border in whidi is the
projection of the knee, and an inner border along which is the
bend of the knee. The outer border, which corresponds to the
front of the leg in man, is called the extensor surface, inas-
much as the muscles which extend or straighten the leg lie
along this edge : the inner border is the flexor surface. The
"ventral" and "dorsal" surfaces only appear to be such in
consequence of the extreme obliquity of the pelvic girdle : they
are really anterior and posterior, and are better called preaxial
and postaxial : they correspond respectively to the inner and
outer surfaces of the human leg. If the foot be examined care-
fully, it will be seen that the first digit or " big toe " is on the
preaxial side, and hence may be called the preaxial digit:
while the fifth or " little toe " is on the postaxial side, and is
therefore the postaxial digit.
1
62 THE MUSCULAR STSTEM OF THE FBOO
1. Mnscles of the thigh.
Remove the akin from one of the lege of the frog ^ and dean tJte
nmedeaf/rst of the preaadal and then of the poetaxicU eurfaces.
a. Superficial muscles of the preazial (apparent ventral)
surface of the thigh.
i. The sartorius is a long narrow muscular band which
crosses the thigh somewhat obliquely from the
outer to the inner side. It arises from the iliac
symphysis below the acetabulum, and is inserted
into the inner side of the head of the tibia.
ii. The adductor magnus is a large muscle lying along
the inner border of the sartorius, but passing
beneath it at its distal end. It arises from the
pubic and ischial symphyses, and passes under
the sartorius to be inserted into the distal third
of the femur.
iii. The adductor longus is a long narrow muscle lying
along the outer side of the adductor magnus,
and often completely hidden by the sartorius:
it arises from the iliac symphysis beneath the
sartorius, and unites a little way beyond the
middle of the thigh with the adductor magnus.
iv. The rectus intemus major is a large muscle lying
along the inner side of the adductor magnus
and of the sartorius. It arises from the ischial
symphysis and is inserted into the head of the
tibia.
V. The rectus intemus minor is a narrow flat band of
muscle running along the inner, or flexor margin
of the thigh : it arises from a tendinous expansion
connected with the ischial symphysis, and is
inserted into the inner side of the tibia, just
below its head.
b. Superficial muscles of the extensor surface of the thigh.
i. The triceps extensor femoris, the great extensor
muscle of the thigh, arises by three distinct
origins, which will be described separately, and
is inserted into the tibia just below its head.
UnSCLBS OF THS HIHD-LIMB
/^'
Fic tt. — The SDpnIicial muscles of the frog's led bind-Iimb.
A ; from (he preaxial surface ; B : from Ihe postaiial surface.
AB, adducloi brevis ; AM, adduclor maf^us ; B. biceps j C, cloacal
■pertm^ ; EC, extensor cruris ; F, distal end of femur ; FT, lendou of
flexor tarsi ; o.gastrocDeinias ; GL, glula^us ; P, peroneus ; PY, pyii-
formis ; RA, rectus amicus femoris ; Rl, rectus intemus major;
lilzrialis snticusi TF, ti
IS extcmus ; VI. nutui inter
64 THE MUSGULAB SYSTEM OF THE FROG
a. The rectus anticns femoris forms the middle
division of the triceps : it arises from the ven-
tral border of the posterior third of the ilium,
in front of the acetabulum : about half-way
down the thigh it joins the next division.
/3. The vastns intemus, the preaodal division of
the triceps, is a large muscle arising from
the ventral and anterior border of the
acetabulum, and lying in the thigh between
the sartorins and the rectos anticns.
y. The vastns extemns, the postaxial division of
the triceps, arises from the posterior edge
of the dorsal crest of the ilium, and joins
the other two divisions of the triceps about
the junction of the middle and distal thirds
of the thigh.
e. Superficial muscles of the postaxial (apparent dorsal)
surface of the thigh.
i. The glutaus has been already noticed : it lies in
the thigh between the rectus anticus and the
vastus extemus.
ii. The biceps is a long slender muscle which arises
from the crest of the ilium just above the
acetabulum : it lies in the thigh along the inner
border of the vastus extemus, and is inserted by
a flattened tendinous expansion into the distal
end of the femur and the head of the tibia.
iii. The semimembranosus is a stout musde lying along
the inner side of the biceps, between it and the
rectus intemus minor. It arises from the dorsal
angle of the ischial symphysis just beneath the
cloacal opening, and is inserted into the back of
the head of the tibia. It is divided about its
middle by an oblique tendinous intersection.
iv. The pyriformis is a slender muscle which arises from
the tip of the urostyle, passes backwards and
outwards between the biceps and the semi-
membranosus, and is inserted into the femur at
the junction of its proximal and middle thirds.
MUSOLBS OF THE HIND-LIMB 65
d. Deep muscles of the thigh.
La^ the frog on via hack cmd dissect the thigh from the pre-
axud surface. Separate the adductor magnus cmd the rectus
intemus major with Ihjuni instrmnents so as to expose thefoUow-
ing muscles :
i. The semitendinosus is a long thin muscle which
arises by two heads ; an anterior one from the
ischium dose to the ventral angle of the ischial
symphysis and the acetabulum ; and a posterior
one from the ischial symphysis. The anterior
head passes through a slit in the adductor
magnus and unites with the posterior head in
the distal third of the thigh. The tendon of
insertion is long and thin, and joins that of t'he
rectus intemus minor to be inserted into the
tibia just below its head.
Divide the adductor magnus om^ the sartorius in the middle
and turn the cut ends backwards and forwards, so as to expose
the f (Mowing musdes :
ii. The adductor brevis is a short wide muscle, lying
beneath the upper end of the adductor magnus.
It arises from the pubic and ischial symphyses,
and is inserted into the preaxial surface of the
proximal half of the femur.
iii. The pectineus is a rather smaller muscle, lying
along the outer (extensor) side of the adductor
brevis. It arises from the anterior half of the
pubic symphysis in front of the adductor brevis,
and is inserted like it into the proximal half of
the femur.
iv. The ilio-psoas arises by a wide origin from the
inner surface of the acetabular portion of the
ilium : it turns round the anterior border of the
ilium, and crosses in front of the hip- joint, where
for a short part of its coiu'se it is superficial
between the heads of the vastus intemus and of
the rectus anticus femoris : it then passes down
the thigh beneath these muscles, and is inserted
into the back of the proximal half of the femur.
66 THE MUSCULAR SYSTEM OF THE FROG
V. The quadratus femoris is a small muscle on the
hack of the upper part of the thigh : it arises
from the ilium ahove the acetahulum, and from
the- hase of the iliac crest : it lies heneath the
pyrif ormis and hehind the biceps, and is inserted
into the inner surface of the proximal third of
the femur between the pyriformis and the ilio-
psoas.
vi. The obturator is a deeply situated musde which
arises from the whole length of the ischial sjon-
physis and the adjacent parts of the iliac and
pubic symphyses, and is inserted into the head
of the femur dose to the glutaus.
% Muscles of the leg.
As in the thigh, we distinguish extensor and flexor surfaces,
corresponding to the front and back of the leg in man ; and also
preaxial and postaxial surfaces, corresponding to the inner and
outer sides of the human leg.
Lay the frog on its heUy amd cormnence the dissection from the
postaasial sv/rfo/ce,
i. The gastrocnemius is the large muscle forming the
calf of the leg : it has two heads of origin, of
which the larger arises by a strong, flattened
tendon from the flexor surface of the distal end
of the femur; while the smaller head, which
joins the main muscle about one-fourth of its
length below the knee, arises from the edge of
the tendon of the triceps extensor femoris where
it covers the knee. The muscle is thickest in
its upper third, and tapering posteriorly ends in
the strong tendo Acblllis, which passes under
the ankle-joint, being much thickened as it does
so, and ends in the strong plantar fascia of the
foot.
ii. The tibialis posticus arises from the whole length
of the flexor surface of the tibia : it ends in a
tendon which passes round the inner malleolus,
lying in a groove in the lower end of the tibia,
and is inserted into the dorsal surface of the
astragalus.
MUSCLES OF THE HIND-UMB 67
iii. The tibialis anticus lies on the extensor sorface
of the leg : it arises by a long thin tendon from
the lower end of the femur, and divides about
the middle of the leg into two bellies which are
inserted into the proximal ends of the astragalus
and calcaneum respectively.
iv. The extensor cruris lies along the preaxial side of
the tibialis anticus, partly covered by this and
partly by the strong fascia of the leg. It arises
by a long tendon from the preaxial condyle of
the femur, runs in a groove in the upper end of
the tibia, and is inserted into the extensor surface
of the tibia along nearly its whole length.
V. The peronens is a stout muscle which lies along
the postaxial surface of the leg, between the
tibialis anticus and the gastrocnemius. It
arises from the distal end of the femur, and is
inserted into the outer malleolus of the tibia and
the proximal end of the calcaneiun.
CHAPTER V.
THE NERVOUS SYSTEM OF THE FBOQ.
Thb nervous system consists of the following parts :
1. A central portion, the brain and spinal cord, which lies
in the cartilaginous and bony tube formed by the
cranium and vertebral column, and which is the centre
where sensations are felt, and whence motor impulses
causing the muscles to contract take their origin*
2. A peripheral portion, the nerves themselves, which
connect the central portion with the skin, sense
organs, muscles, viscera, etc., and serve to convey
sensory impulses from these parts to the brain and
cord, or motor impulses from the central organs to
the muscles. These two functions are fulfilled by
different nerves, which may accordingly be distin-
guished as (a) afferent or sensory nerves, conveying
impulses to the brain or cord and (6) efferent or
motor nerves, conveying impulses from the brain
or cord.
Fig. 12. — The nervous system of the edible frog [I^ana escuUnta), from the
ventnd surface. (From Ecker. )
F, facial nerve ; G, ganglion of pneumogastric nerve ; He, cerebral hemi-
sphere ; Lc, optic tract ; Lop, optic lobe ; M, boundary between medulla
oblongata and spinal cord; M 1-10, the spinal nerves; MS, connection
between fourth spinal nerve and sympathetic chain ; N, nasal sac ; Ni, sciatic
nerve; No, crural nerve; o, eyeball; S, trunk of sympathetic; S 1-10, the
sympathetic ganglia ; Sp, continuation of sympathetic into head.
I, olfactory nerve; II, optic nerve; III, motor oculi ; IV, fourth nerve;
V, trigeminal and facial nerves ; Va, ophthalmic branch of trigeminal ;
Vc, maxillary branch of trigeminal ; Vd, mandibular branch of trigeminal ;
Ve, hyomandibular branch of facial ; Vg, Gasserian gangUon ; Vs, upper end
of sympathetic trunk, in connection with Gasserian ganglion ; VI, abducens
nerve; VII, facial nerve; VIII, auditory nerve; X, glossoph- ryngeal and
pneumogastric nerves ; XI, ramus anterior of glossopharyngeal ; X2, ramus
posterior of glossopharyngeal ; X 8-4, branches of pneumogastric.
THE HEETOQB BYSTEM
70 THB NBRVOnS SYSTEM OF THE FROO
A special set of nerves in connection with the bloodvessels
and viscera forms the sympathetic nervous system.
For the dissection of the nervous system specimens should be
taken which ha/ve been in strong spirit for two or three days, and
in which the brain has been exposed to the action of the spirit by
removed of the roof of the skuM.
A. The Central Nervous SysteuL
This is divisible into an anterior portion — ^the brain — flying
in the cavity of the cranium; and a posterior portion — ^the
spinal cord — ^which lies in the neural canal of the vertebral
column. There is no sharp line of demarcation between the
two portions, which are directly continuous with each other.
If the brain and spinal cord ha/ve not ah^eady been exposed,
dear a/way the dorsal muscles from both sides of the spine : cut
through die ocdpito-atlantal membrane^ flexing the fro^s head
slightly to make the membra/ne tense, amd being carejul not to
injure the brain beneath it, Ini/roduce one blade of the scissors
into ths cranial cavity, with the fUU swrfa^ of the blade parallel
to tJie back of the frog, and keeping as dose to the roof of the skull
as possible. Cut carefully through the side walls of the skull,
first on one side and then on the other, Twm the roof of the
skull forwa/rds with forceps, a/nd remove it aUogelher.
Similarly cut through and remove the neural arches of the
vertebroi one by one, from before backtpards.
Examine and draw the central nervous system in situ, showing
its several pa/rts,
I. The Brain.
a. The dorsal surface of the brain : note from before
backwards the following parts, removing the pigm>ented
membrane (pia mater) covering the several parts as you
come to them.
i. The olfactory lobes, which form the most anterior
portion of the brain, are united together in the
median plane : they give off the olfactory nerves
from their anterior ends, and are separated behind
by slight constrictions from the hemispheres.
ii. The cerebral hemispheres are a pair of smooth
ovoid bodies which touch each other in the median
plane but are not fused together.
THE BEaIN 71
iij The thalamencephalon U a lozeogeahaped portion
lying immediately behind the hemispheres and
between their diverging posterior ends : it is
covered by a thick vasoular membrane — the
choroid ^exns — over which passes the stalk of
the ^neal bod?, a small body adherent to and
generally removed with the roof of the skull. On
removing the choroid pleXDs a elit-like hole is
Pig. 13. — The brain of the ttog : dorsal surface, x 4.
FiQ. 14,— The brain of Ihe froe ; wntral surface, x 4.
C. cerebellum; CH, cerebral hemisphere 1 CP.. choroid pkxus of
third venlricle ; F, fourth ventricle ; IN, tubu- dnereum; M. medulla
oblongata ; O, olfacioij lobe ; OG, optic cbiasma ; OL, optic lobe;
P, stalk of pineal body ; PB, piluilaiy body ; T. thalamenceptialun.
I, olfflclory nerve; II, optic nerve; III, third or motor oculi nerve;
IV, fourth nerve; V, fifth or trigeminal nerve; VI, siiith nerve;
VII and VIII, combined root of facial and auditory nerves ; IX and
X, combtned root of gtossophaiyngeal and pneumogastric nerves.
left in the roof of the thalamencephalon. The
vessels of the plexus, covered by a thin layer of
nervous matter, hang into the third ventricle, or
cavity of the thalamencephalon. The thickened
sidee of the thalamencephalon are the optic
tii^ami.
iv. 'Y\» optic lobes are a pair <rf prominent ovoid bodies
THR NERVOUS 8TSTEH OF THE FBOO
toaching each other in the median place and form-
ing the widest part of the brain : the pia mater
covering them is very strongly pigmented.
V, The cerebellom is a narrow truisverse band im-
mediately behind the optic lobes,
vi. The medulla oUongata ia the part of the brain
behind the cerebellum : it is widest in front and
gradoally tapera towards its posterior end, where
it is continuous with the spinal cord. It is
covered by a triangular and very vascular mem-
brane, beneath which lies the fourth ventricle.
Fig. 15.— a horiionul seclion through the brain of ite frog, to
show the inteinal cavities. (From Ek:ker.)
Aq, ventricles of the optic lobea ; Dt, third veatricle ; MF, fonunen
of Moiuti ; Sv, lateral ventricle ; Vv, fonrth ventricle.
t>. The cavittes of the brain.
SUee offtht wpper sairfaee of thx brain korizontaMy so at to
es^KuetAe several caoitiei or ventoidet, vnthout removijig it from
THE BRAIN 73
tJie akuU. These cavities care merelAf pa/rts ofy or (nUgrowths ofy the
original centfrdl canal of the neural tube of the embryo. (Gf
Chap. VIII.)
i. The lateral Tentrides extend through the whole
length of the cerebral hemispheres and a short
way into the olfactory lobes.
ii. The third ventricle is situated in the thalamen-
cephalon : it opens in front through the foramina
of Monro into the lateral ventricles : the stalk
of the pineal body opens into it above ; and in
the hinder part of its floor is a conical depression,
the infandibnlunL
iii. The aquadnctns Sylvii or itera tertio ad quartum
ventricnlum is a narrow passage leading from the
third to the fourth ventricle: it communicates
above with the cavities or ventricles of the optic
lobes, which are hollow.
iv. The f onrth ventricle is the large triangular cavity in
the medulla, already ezpos^ by removal of the
vascular membrane covering it.
c. The ventral surface of the brain.
CrU through the medidla at the level of the hinder end of the
skuU : carefully remove the brain from the cranial cavity, noting
the several nerves arising from it, a/nd cutting through these as far
from the brain as possible. Lay the brain on its dorsal surface,
examine and draw the ventral swrface, showing the following
parts :
i. The optic chiasma is formed by the decussation
of the roots of the optic nerves; the point of
crossing being opposite the hinder ends of the
hemispheres, and immediately in front of the
infundibulum.
Trace bach the optic nerves behind their point of crossing to
their origins from the optic lobes.
ii. The tnber cinereum is a small median swelling
immediately behind the optic chiasma, caused by
the depression of the floor of the third ventricle
to form the infundibulum. It is divided by a
median groove into right and left halves.
74 THE NERVOUS StStEM OP THE FROG
iii. The pitnitary body is a flattened ovoid sac, lying
behind, and continuous with, the tuber cinereum.
It is almost certain to be left behind in the skull
on removing the brain, in which case the infundi-
bulum will be seen torn across.
iv. The crura cerebri are two dense white columns of
nervous matter, lying at the base of the optic
lobes, and partly hidden by the pituitary body :
they serve to connect the hemispheres with the
medulla and spinal cord.
v. The ventral Assure of the brain is a median longi-
tudinal groove on the ventral surface of the hinder
part of the brain : it is continuous with a similar
groove on the ventral surface of the spinal cord.
n. The Spinal Cord.
The spinal cord is a somewhat flattened band, presenting
brachial and lumbar enlargements opposite the pointe of origin
of the nerves for the fore and hind limbs respectively, and
slightly constricted between these two points. About the level
of the sixth or seventh vertebra the cord narrows rapidly to
form a fine thread, the filum tenninale, which extends back into
the canal of the urostyle.
The tubular character of the spinal cord is best seen on
making transverse sections of it. (See p. 85.)
B. The Peripheral Nervous System.
1. The Spinal Nerves. Ten pairs of nerves arise from the
sides of the spinal cord; each nerve arising by two roots, a
ventral or " anterior," and a dorsal or " posterior," which unite
at their point of exit from the vertebral canal through the
intervertebral foramen : just before their union the dorsal root
bears a ganglionic swelling.
Within the vertebral canal the roots of the anterior spinal
nerves run nearly transversely outwards, so as to leave the canal
opposite their points of origin from the spinal cord. The roots
of the middle and posterior nerves, owing to the vertebral
column being of greater length than the part of the cord
belonging to it, pass obliquely backwards to their points of exit :
and in the case of the hindmost nerves, the roots run backwards
THE 8PIKAL KtRYlSS 76
within the vertebral canal some distance before reaching their
foramina of exit : the bundle formed by these roots, together
with the filum terminale, is spoken of as the cauda equina.
a. The spinal nerves outside the vertebral canal.
Lay the frag on its back : ciU through amd pin aiU the body-
walls, and remove the abdominal viscera. Note ike spinal nerves,
seen as white cords at the sides of the vertebral col/u/nm. Clean
the nerves on one side amd follow them to their distribution.
Each nerve divides, directly after the union of its two roots,
into a small dorsal branch, and a much larger ventral branch.
i. The ventral branches.
1. The hypoglossal, or first spinal nerve, leaves the
vertebral canal between the first and second ver-
tebrae, and then runs forwards on the under sur-
face of the head beneath the mylohyoid and in
the substance of the geniohyoid muscle to the
chin, where it enters the tongue, in which it
ends. It supplies the muscles of the tongue and
floor of the mouth, and also some of the muscles
of the ba«k and shoulder. (Fig. 16.) It fre-
quently sends one or two branches to the brachial
plexus.
2 and 3. The second and third spinal nerves leave
the canal between the second and third, and
third and fourth vertebrae respectively. The
brachial plexus is formed by the fusion of (a)
one or two branches from the hypoglossal nerve,
(b) the second spinal nerve, (c) a branch from
the third spinal nerve. These unite together to
form the brachial nerve, which gives off a large
coracoclavicular branch to the shoulder muscles
and then runs down the arm, supplying it with
muscular and cutaneous branches, and divides
just above the elbow into the radial and ulnar
nerves, supplying the forearm and hand.
4, 5, and 6. The fourth, fifth, and sixth spinal nerves
are small and supply the muscles and skin of
the body- wall. They leave the vertebral canal
76 THE NEBYOUS 8TSTEM OF THE FBOO
between the fourth and fifth, fifth and sixth, and
sixth and seventh vertebrse respectively.
7, 8, and 9. The seventh, eighth, and ninth spinal
nerves together form the sciatic plexus. The
roots of these three nerves within the vertebral
canal form the main part of the canda equina.
The seventh nerve leaves the canal between
the seventh and eighth vertebrae, the eighth
nerve between the eighth and ninth vertebrae,
and the ninth nerve between the ninth or
sacral vertebra and the urostyle. Outside the
vertebral canal the three nerves unite together
opposite the middle of the urostyle to form
the sciatic plexus, from which branches are
given to the large intestine, bladder, oviducts,
etc. Just before joining the plexus the seventh
nerve gives off the ileohypogastric and crural
nerves, supplying the muscles and skin of the
abdomen and thigh. Beyond the plexus is the
large sciatic nerve, which runs down the thigh,
giving branches to it, and dividing a short distance
above the knee into the tibial and peroneal nerves
supplying the leg and foot.
The relative sizes of the nerves forming the
sciatic plexus, and the mode of their union with
one another are subject to considerable individual
variation in different frogs.
10. The coccygeal or tenth spinal nerve emerges
through a small hole in the side of the urostyle
near its anterior end. It gives branches to the
bladder, cloaca, and other adjacent parts, and
contributes a branch to the sciatic nerve.
ii. The dorsal branches of spinal nerves 2 to 8 inclusive
are to be found traversing the dorsal lymph-sac in
their passage to the skin.
b. The spinal nerves within the vertebral canal.
Ctit away tvith scissors the centra of the vertebras one by one, to
expose the spincU cord and the roots of the spinal nerves from the
ventrai surface. Note the foUoicing points :
THE SYMPATHETIC NERVOUS SYSTEM 77
i. The roots of the nerves : dorsal and ventral,
ii. The obliquity of the middle and posterior roots,
iii. The cauda equina, formed by the roots of the
hinder nerves together with the filmn termixiale.
iv. The ganglia on the dorsal roots as they pass
through the intervertebral foramina: these are
covered on their ventral surfaces by whitish cal-
careous patches (the periganglionic glands or
"glands of Swammerdam") which form con-
spicuous objects on either side of the vertebral
column. Remove these patches carefuUy to see the
gcmgUa.
II. The Sympathetic Nervous System. This consists of a
longitudiaal nervous band on each side of the body, connected
by branches with the several spinal nerves. The two main
sympathetic trunks lie, in front, close to the dorsal surface and
alongside the vertebral column : further back they are in dose
relation with the dorsal aorta, alongside which they run.
Each S3nnpathetic trunk receives a branch from each of the
spinal nerves of its side, and at the junction of each of these
branches with the main trunk there is a ganglionic enlargement.
The coccygeal or tenth spinal nerve, unlike the others, is
connected with the sympathetic by more than one branch :
the actual number of these branches is not constant, but is
said to vary from two up to as many as twelve.
From the sympathetic ganglia nerves are given off to the
bloodvessels and viscera, the chief ones being the following :
i. The cardiac plexus is formed by nerves arising
from the first sympathetic ganglion : the plexus
is a meshwork of nerves on the auricles, and
around the great vessels at. their openings into
the heart.
ii. The solar plexus lies on the dorsal surface of the
stomach : the nerves are derived mainly from the
third, fourth, and fifth ganglia.
Hepatic, renal, genital, hsemorrhoidal, and vesical plexuses
also exist iq connection with the liver, kidney, reproductive
organs, large intestine, and bladder respectively.
m. The Oranial Nerves. There are ten pairs of cranial
nerves in the frog, which are numbered in order from before
backwards. (See Figs. 12, 14 and 16.^
78 THE NKRT0TJ3 SYSTEM OF THE FROQ
To dissect the oraniai nerves ea^ose the brain by remowng th6
roof of the skvU as already described, and then follow the special
inatrv^ioTis given in the case ofth« more important nerves.
1. The olfactory nerve, the special nerve of smell, arises
from the anterior end tmd outer side of the olfactoiy
lobe, and \a distributed to the membrane lining the
nasal cavity.
To see the com-ee and distr^mtion of the (Afadcfry nerve, dissect
from the dorsal swrface, removing the roof of the anterior part of
the skull, iatduding the nasai bone.
2. I'lie optic nerve, the nerve of sight, arises from the side
Fig. i6. — The ingemina], facial, glossopharyngeal, and pneumc-
ga^tric nerves at the frog, dissecied from the right side.
a. stomach ; i. Eustachian lube : k. hypoglossal neive ; i. lung ;
n, second spinal nerve ; s, sinus venosus ; I, squamosal ; t.a, tiuncus
arteriosus ; a, right auricle ; v, veniricle r v.c, right anterior vena
cava ; Va, ophthalmic bianch of trigeminal; Vb, tnaiillan branch of
trigeminal ; Vc, mandibular branch of the trigeminal; Vila, palatine
branch of facial ; Vllh, hyoidean branch of facial ; IX, glosso-
phaiyngea] ; Xa, laryngeal branch of pneumogostric ; Xb, pulmonary
branch of pneumogastiic ; Xc, cardiac branch of pneumc^astric ;
Xd. easlric branch of pneumogastric ; 3, transverse proceas oi second
THE CRANIAL NBRVBS 79
of the brain just below the optic lobe, partially crosses
over at the optic chiasma on the under surface of the
brain, and then runs outwards to the eyeball.
The ccywrae of the optic nerve has been fully seen in premous
dissections.
3. The motor oculi is a small nerve arising from the ventral
surface of the brain close to the median line and between
the cnu^ cerebri. It supplies four of the muscles
moving the eyeball ; viz., the rectus superior, rectus
intemus, rectus inferior, and obliquus inferior.
Oiving to its smaU size the third nerve is not easily made out
in the frog,
4. The fourth or pathetic nerve is a very slender nerve,
arising from the dorsal surface of the brain between the
optic lobe and the cerebellum, and supplying the obliquus
superior muscle of the eyeball.
The fourth nerve is too smaU to he dissected satisfactorily in
the frog,
5. The trigeminal is the largest of the cranial nerves in the
frog. It arises from the side of the anterior part of the
meduUa, and runs outwards and forwards to the skull
wall : just before reaching this it expands into a large
swelling — the Gasserian ganglion. It then passes
through the skull wall immediately in front of the
auditory capsule, and divides at once into two main
branches.
i. The ramus ophthalmicus runs forwards through the
orbit, lying close to its inner side, between the
skull wall and the eye. It passes beneath the
rectus superior, but above all the other muscles
of the eyeball and the optic nerve. At the
anterior end of the orbit it divides into two
branches, which pass through the walls of the
nasal capsule, and supply the skin of the fore
part of the head.
To trace this branch, dissect from the dorsal swrface : cut away
carefully unth scissors the side waM of the cranium, : cut through
and turn aside the rectus superior, cmd fmd the nerve running
80 THE NBRVOUS SYSTEM OP THE FROO
doae ahngside the shuU wallf between it tmd the eyebaU, Trace
it/arwarde to the noee,
ii. The ramus mazillo-iiiaiidilnilaris nms directly
outwards behind the eyeball, in front of the
auditory capsule and between the temporal and
pterygoid muscles. After a very short course it
divides into the maxillary and mandibular nerves.
To (reuse this nerve cmd its branches remove the squcmhosdl bone
ca/refuUy, and find 1M nerve lying on the pterygoid nrnsde cmd
invmediaU^ behind the eye. Follow the nerve bMnd the pterygoid
cmd temporal muscles to the shiU^ remxmng the vnuscles if neces-
sary ; cmd then trace the branches outwards to their distribution.
a. The ramus maziUarisruns forwards and out-
wards in the floor of the orbit, behind and
below the eyeball, to the margin of the upper
jaw, which it reaches about midway along
its length : it then ends in branches which
run along the jaw, some forwards and some
backwards, supplying the upper lip, the lower
eyeUd, and other neighbouring parts.
/3. The ramus mandibularis runs parallel to
and behind the ramus maxillaris as far as
the outer border of the eyeball, giving
branches to the temporal and pterygoid
muscles : it then turns backwards, outwards,
and downwards, and passing across the inner
side of the upper jaw, reaches the outer
surface of the mandible just behind the
insertion of the temporal muscle : it then
runs forwards along the outer side of the
lower jaw to the chin, supplying the lower
lip and the muscles of the floor of the mouth.
6. The abducens is a very slender nerve which arises from
the ventral surface of the medulla close to the median
line, and a short way behind the pituitary body. It
passes either through, or in very close contact with, the
Ckisserian ganglion, and entering the orbit supplies the
retractor bulbi and the rectus extemus muscles.
The ahdvcens nerve is too small to be made out satisfactorily
in the frog.
THE CBANIAL NERVES 81
7. The facial nerve arises from the side of the medulla
immediately behind the trigeminal nerve, and passes
forwards to the skull wall, where it is very closely con-
nected with the Gasserian ganglion. It passes through
the skull wall immediately behind and in close company
with the trigeminal nerve, and divides at once into its
two main branches!
i. The ramus palatinus runs forwards in the floor
of the orbit a short distance from the side wall
of the ^nill, and immediately upon the mucous
membrane of the roof of the mouth. Near the
anterior end of the orbit it divides into two
branches, one of which runs outwards and
anastomoses with the ramus maxillaris of the
trigeminal nerve, while the other runs forwards
to the anterior part of the roof of the mouth. It
supplies the mucous membrane of the roof of the
mouth.
Dissect this nerve from the ventral su/r/ace. CtU away the lower
jaw : carefully r&move the miicous me7nbra/ne of the roof of the
mouthy amdfmd ike nerve lying on the ventral surface of the eye-
ball amd its miiscles, and rwaning pa/rallel to and a short distance
from the sIcuU waU. Trace it backwards ami forwards.
ii. The ramns hyomandibularis runs outwards and
backwards round the front end of the auditory
capsule ; it then crosses over the inner end of the
columella and turns downwards in the posterior
wall of the Eustachian tube to the angle of the
mouth, giving branches to the tympanic mem-
brane and to the articulation of the mandible.
It then divides into two branches.
Tlie eocposure of the above nerve, which is not easy^ may be
performed thus : remove the sh&idder girdle of one side ; also the
depressor mamdihdce and temporalis muscles : open the cramial
cavity as before^ to eocpose the brain : remove the tympa/nic m,&m-
brane amd dean the outer end of the eolv/mella. Gut away care-
fully the roof of the a/uditory capsule by a horizontal cut, just
above the level of the columella : find the facial nerve nmning
round the front end of the auditory capsule and in close contact
82 THE NERVOUS SYSTEM OF THE FROG
tffiVi it J and trace it back over the eokumeUa amd daum to the angle
of the mouih,
a. The ramns mandibulaxis runs forwards in the
floor of the mouth, lying along the inner edge
of the lower jaw and between the mylohyoid
muscle and the skin, as far forward as the
chin.
Dissect from the ventral stirfaoe : remove the skin from the
under swrf(koe of the floor of the mouth, and fvnd the nerve rvmmmg
along the inner border of the mandible.
/3. The ramns hyoidens is the posterior and larger
of the two branches : it runs forwards in the
floor of the mouth along the anterior cornu
of the hyoid, supplying its muscles.
8. The anditory nerve, the nerve of hearing, arises from the
side of the medulla immediately behind and in dose
contact with the root of the facial nerve : it enters the
auditory capsule and ends in the internal ear.
9. The glossopliaryngeal nerve arises from the side of the
medulla behind the auditory nerve, by a root common
to it and the tenth nerve: it leaves the skull by an
aperture immediately behind the auditory capsule, and
divides behind the capsule into two branches.
i. The ramus anterior runs downwards and forwards
round the hinder border of the auditory capsule
and beneath the depressor mandibulse muscle to
join the facial nerve just after it has crossed over
the columella.
Tlie dissection already made for the raraus hyomuvndnJbulwris of
the facial nerve vnU fhow also the above branch of the glosso-
pharyngeal,
ii. The ramns posterior runs downwards and for-
wards to the ventral wall of the pharynx, passing
beneath the fourth division of the petrohyoid
muscle but superficial to the others; it runs
just behindhand parallel to the anterior cornu of
the hyoid. On reaching the floor of the mouth
it crosses the hypoglo^al nerve obliquely, lying
dorsal to it, and then runs forwards in a
THE CRANIAL NERVES 83
peculiarly sinuous course, close to the middle
line and between the geniohyoid and hyoglossus
muscles, to the base of the tongue, which it
enters and in which it ends. It supplies a petro-
hyoid muscle, and the mucous membrane of the
pharynx and tongue.
ne exposwre of the first part of the above nerve is best perform^
from the sidCy amd is miich facilitated by distending the cesophagibs
a/nd pharynx with a cork or roU of paper. Its course along the
floor of the mouth to the tongue should be dissected from the
ventral sv/rfa4X,
10. The pneumogastric or vagus nerve arises, as already
noticed, in common with the glossopharyngeal. It
leaves the skull by the same aperture as the ninth
nerve, and immediately outside the skull presents a
ganglionic enlargement : it gives off dorsal branches to
the muscles of the back, and then runs backwards and
downwards round the side wall of the pharynx, running
along the hinder border of the fourth division of the
petrohyoid muscle ; behind this muscle it divides into
its main branches, which are as follows :
i. The ramus larsmgens or recurrens loops round
the posterior comu of the hyoid and round the
pulmocutaneous artery close to its origin from
the aortic trunk : it then passes inwards, dorsal
to the artery, to the middle line, where it ends
in the larynx.
ii. The ramus cardiacns passes dorsal of the pul-
monary artery to the interauricular septum of
the heart, and to the sinus venosus.
iii. The rami pulmonales follow the course of the pul-
monary artery to the lung, in which they end.
iv. The rami gastrici, usually two in number, run
through the partial diaphragm formed by the
anterior fibres of the obliquus intemus muscle,
and end in the walls of the stomach.
7^ dorsal portions of the several branches of the pneumogast'f^
nerve are best eocposed from the side ; to see them properly , the
shoulder girdle and fore Hmb must be refmomdamd the cesophiigus
84 THE NERVOUS SYSTEM OF THE FBOG
weU distended : the terminal brcmchea nrnet be dissected from the
ventral surface,
rv. The Cranial Portion of the Sympathetic Nervous
System.
The main sympathetic trunk of each side extends forwards
in front of the first ganglion, and enters the skull at the
foramen in the exoccipital bone through which the glosso-
pharyngeal and pneumogastric nerves pass out : it is connected
with the pneumogastric nerve, and then runs forwards within
the skull to the Gasserian ganglion of the trigeminal nerve, in
which it ends.
0. Histology of Nerves.
Nervous matter consists histologically of elements of two
kinds, nerve cells and nerve fibres. The nerve cells are
branching nucleated cells, connected by their processes with one
another and with the nerve fibres. The nerve cells are the
centres whence impulses originate, while the nerve fibres serve
to convey these impulses from one part to another. A local
accumulation of nerve cells is called a ganglion.
1. Nerve Fibres are of two kinds, meduUated and non-
medullated.
a. Mednllated nerve fibres foi^m almost the whole of the
cranial and spinal nerves, and a large part of the brain
and spinal cord.
Take a small piece of the sciatic or some other nerve from a
freshly killed frog : spread it out and tease it in a drop of normal
salt solution : exmnine vnth low amd high powers : note
i. The nerve fibres : unbranched.
ii. The perineorinm, or connective tissue binding the
nerve fibres into bundles, or " nerves."
In each nerve fibre note
iii. The primitive sheath, or sheath of Schwann : a
very delicate external investment, seen with
difficulty, and only in certain places.
iv. The medullary sheath : a thick fatty layer within
the primitive sheath ; it is highly refraxstive, and
gives the nerve fibre its double contour.
mSTOLOGT OF NEEVES 86
Tease in glycerine a small piece ofinerve that has been treated
with osmic acid : examine with the high piywer a single nerve
fibre : note the following points :
i. The medullary sheath is stained darkly in conse-
quence of its fatty nature : it is interrupted at
intervals by the nodes of Ranvier.
ii. The nodes of Banvier are spots where the medul-
lary sheath is absent, and the primitive sheath
foi^ constrictions touching the <ms cylinder.
iii. The axis cylinder is the central cylindrical rod,
the essential part of the nerve fibre : it is clearly
visible at the nodes, and is much less deeply
stained than the medullary sheath.
iv. Nndel are seen projecting into the medullary sheath
about midway between the nodes.
Tease a small piece of fresh nerve in chloroform : this will
partially dissolve the fatty medullary sheath and so render the
prvmitive sheath am,d axis cylinder more dewrly visible : note
i. The primitive sheath, or sheath of Schwann,
ii. The axis cylinder.
b. Non-medullated nerve fibres occur chiefly in the sympa-
thetic nerves : they branch and anastomose ; and they
have no medullary sheath.
2. Nerve cells : Tease in glycerine a small fragmerU of the
ventral comu of the spinal cord of the ox (Iv/mbar region) : cover,
a/nd examtins with low and high powers : note
i. The nerve cells : large nucleated cells with many
branching processes.
ii. The nerve fibres,
iii. The fine connective tissue binding the several parts
together.
3. Structure of the Spinal Cord.
Take one of the prepa/red transverse sections of spinal cord of
frog ; moimt in balsam, and examine with low am,d high powers :
note thefollomng points :
a. With the low power.
i. The section is bilaterally symmetrical, and oval in
shape ; the transverse diameter considerably ez'
ceeding the vertical.
86 THE NEBYOUS SYSTEM OF THE FROG
ii. The ventral or " anterior " fissure is a broad and
shallow median cleft : there is in the frog no
distinct dorsal fissure.
iii. The white matter forms the outer part of the cord,
and is chiefly composed of medullated nerve fibres.
iv. The grey matter forms the central part of the cord,
and is composed of a dense network of non-
medullated nerve fibres, in which are imbedded
numerous nerve cells.
V. The comua are the processes, ventral or " anterior,"
and dorsal or " posterior," into which the grey
matter is produced on either side.
vi. The central canal of the cord lies in the median
plane, nearer the ventral than the dorsal surface.
vii. The nerve roots are only seen if the section happens
to pass through their points of origin.
a. The dorsal or " posterior " root is connected with
the dorsal comu of the grey matter; it is a
single thick band of nerve fibres.
fi. The ventral or '^ anterior" root is connected
with the ventral comu of the grey matter ; it
consists of a number of very [Render bands of
nerve fibres.
b. With the high power.
i. The ganglion cells are large nucleated branched
cells lying in groups in the grey matter : they
are largest and most numerous in the ventral
comua.
ii. The neuroglia is a delicate network of connective
tissue fibres and cells, penetrating and supporting
all parts of the cord, and continuous at the surface
with the pia mater.
iii. The pia mater is the delicate connective tissue
membrane closely ensheathing the cord.
iv. The bloodvessels of the cord are small and numerous:
they enter from the pia mater.
V. The central canal is lined by a ciliated epithelium.
CHAPTER VI.
THE ETE AND EAB.
A. The Eye of the Frog.
1. Bmwve the eye from a freshly kiUed frog : snip off with
scissors the muscles of the eyebaU : note the following points :
i. The shape. The eyeball is flattened on its outer
side, more convex on the inner or deeper side.
ii. The sclerotic is the firm outer wall of the eyeball,
formed of hyaline cartilage, and dense white
connective tissue.
iii. The cornea is the transparent patch on the outer
side of the eye through which the light enters :
it is continuous at its margin with the sclerotic
iv. The iris is a pigmented ring placed behind the
cornea and seen through it: it acts as a dia-
phragm, limiting the amount of light that enters
the eye.
V. The pnpil is the aperture surrounded by the iris,
which serves to admit the light to the interior
of the eye.
vi. The optic nerve is seen piercing the sclerotic to
enter the eyeball on its inner side.
2. Place the eye under water amd divide it with scissors into
two halves by a cut passing through the middle of the cornea omd
through the sclerotic dose to the optic nerve, so as to lay open com-
pletely the interior of the eye : note thefoUovnng points :
i. The lens is a firm, solid, transparent body, just
behind the iris and attached to its outer margin :
it is more convex on its inner than on its outer
surface.
8d THK lETS
ii. The anterior chamber of tlie eye is the space l^etweeii
the cornea and the lens : it is small and contains
the aqneous humonr, a wateiy fluid.
iii. The posterior chamber of the eye is a large space
behind the lens: it is filled by the vitreous
humonr, a gelatinous body.
iv. The choroid is the black pigmented layer lining
the sclerotic, and continuous in front with the
iris.
V. The retina is a delicate transparent membrane
lining the eyeball. It is readily detached from
the dhoroid, except at the point where the optic
nerve enters.
B. The Eye of the Sheep or Ox.
1. Dissect off the rmbscles of the eyeball^ amd the fat which
surrownds the optic nerve : note the foUovoing points :
i. The shape. The eyeball is more spherical than in
the frog.
ii. The sclerotic covers about five-sixths of the eyeball :
it is tough, white, and opaque.
iii. The cornea, which covers the outer sixth of the
eyeball, is circular, transparent, and continuous at
its margin with the sclerotic : it is more convex
than the sclerotic.
iv. The conjunctiva is a delicate epithelial layer, con-
tinuous with the skin, covering the front of the
cornea and part of the sclerotic.
V. The iris is the oval pigmented ring seen through
the cornea.
vi. The pupil is the central, oval, or dumb-bell shaped
aperture surrounded by the iris.
vii. The optic nerve is a thick white bundle of nerve
fibres piercing the sclerotic at the back of the
eye.
2. Cut all rov/nd the cornea, toith stout scissors^ about \ inch
fi-om its jundion •mih the ederotic : remove fhe cornea : take eare
not to ggueeze the eye, or the lens will he driven oitt iiatantly : note :
i. The ag.ueons bumonr : the transparent watery
fluid filling the anterior chambei of tbe oye
and escaping when the cornea is removed.
ii Thelena.
'2
Cli— 1
Fra. 17. — A diagrammatic section through the human eye passing
through the centres ol the cornea and lens, and throagb the yellow
spot and point of entrance of (he optic nerve.
A, anterior chamber; Ar, cetitral anery of retina; B, blind spot ;
C, cornea ; Ch, choroid; CP, ciliary processes; H, hyaloid membiime,
enclosing the vitreous humour; I, iris; L, lens; o. optic nerve;
p. posterior chamber ; R. retina ; 8, sclerotic ; SL, suspensory liga-
ment ; V, yellow spot. (Theyellovrspotisahsentfromabeasl'seye.)
3. Pass the handle of a tccdpel vmder ihe cut edge of the cornea,
between it and the iris, ajid carefully separate the sclerotic from
the choroid the whole teay round for a distance of dbmit half an
inch heyond the edge of the cornea. Makefow radial cuts, equi-
dUtantfrom om another, through the margin of the cornea and
90 THE EYE
the sclerotic, taking eaire not to injure the deeper parte; am.d
eoctend the cuts back totoards the optic nerve, GwrefvXLy peel off
Uie four fia/ps into which the sderotic is now divided from the
fjmderlt/ing black choroid coat : trim them downy and pin them to
tha dissecting board so as to fx the eye with the iris upwa/rds : note
t/iefollovnng points :
i. The ciliary muscle is a whitish ring of iinstriped
muscle connecting the outer margin of the iris
with the junction of the cornea and sclerotic.
ii. The choroid is the dense black coat exposed by the
removal of the sclerotic.
iii. The ciliary vessels pierce the sclerotic to convey
blood to and from the choroid, which is extremely
vascular.
iv. The ciliary nerves are seen passing through the
sclerotic to the choroid while the flaps are being
turned down.
4. Make d couple of radial incisions, cibovi \ inch <xpart, through
the iris a/nd ciliary muscle, amd twm back the portion of the iris
betioeen the two cuts, so as to expose its hinder surface.
i. The ciliary processes are a series of densely pig-
mented and close-set radial folds on the hinder
surface of the outer margin of the iris : they flt
into corresponding folds in the ligament which
surrounds and supports the lens.
5. With a large pair of scissors cut the eye into two halves by a
horisxmtal incision at its equator. Twm over the anterior or ottter
half amd examine it from behind : note the following parts :
i. The ciliary processes.
ii. The uvea is the layer of dense black pigment at
the back of the iris and ciliary processes.
iii. The ora serrata is the indented anterior boundary
of the part of the retina sensitive to light:
in front of this the retina becomes extremely
thin, but really extends forwards as far as the
free edge of the iris.
6. Turn the anterior half of the eye over, so that its outer or
corneal surface is uppermost : cut away the iris completely : note
the following points :
THE ETB 91
L The capsule of the lens is an elastic transparent
membrane holding the lens in its place.
ii. The suspensory ligament of the lens, or zonule of
2iinn, is the outer margin of the capsule of the
lens: it is marked with radiating folds into
which the ciliary processes fit.
iii. The cut edges of the retina and choroid should be
recognised.
7. Remove the lens Jram its capsule; note its shape, more
convex behind ihom in front : ha/rden it with spirit, or by boiling
for a few mintUes in water.
8. Remove the vitreous humour from the posterior half of the
eye : note the following points :
i. The retina is a delicate pulpy membrane between
the vitreous humour and the choroid.
ii. The blind spot is the point of entrance of the optic
nerve: the retina adheres firmly to this spot,
though it can be readily separated from the
choroid at all other parts.
iii. The retinal vessels enter with the optic nerve, and
radiate from the blind spot.
0. Histology of the Eye.
1. The Choroid. Spread a smaU piece of fresh choroid on a
slide in normal salt solution ; examine with low and high powers,
i. The choroid is a network of bloodvessels thickly
invested by pigment cells.
ii. The pigment cells are irregularly branched, with
clear nuclei.
2. The Lens. Tease in glycerins a small piece of lens,
hardened by boiling ; examine mth low and high powers : note :
i. The laminated character of the lens as a whole.
ii. The elongated epithelial cells of which the lens is
composed.
iii. The serrated edges of many of the cells.
S. TIieBetina.
Mmmt in bahavt one of the prepared HCliong of Uie poslerioi
part of the frog's eye : esmmine wvth low and high powers.
Fig. i8. — Vertical section Ihrough Ihe poslerior wall of the ejeof a
frog; Ihe section passes througb the sclerotic, the choroid, ami the
entire Ibickncss of Ihe retina, x 300.
BC, red blood corpuscle ; C, cone ; Q, ganglion cell ; IL, inner
limiting membrane; IM. inoer molecular layer; IN, inner nuclear
layer ; NF, layet of netre fibres ; OL. ouler limiting membraoe ;
OM, outer molecular layer; ON, Outer nuclear layer; P, pigment
cell of retina; PC, pigment of choroid ; R, rod; RF, cadial or MUller'l
fibre; S, the cartili^nous scleiotic
THE ETE 93
a. The sclerotic consists chiefly of hyaliiie cartilage.
b. The choroid is a vascular plexus, with much pigment.
c. The retina is composed of the following layers from
without inwards :
i. A layer of pigment cells, sending processes between
the rods and cones.
ii. The rods and cones are a layer of columnar bodies
placed vertically to the surface : the rods are
far more numerous, and much larger than the
cones: each consists of an inner and an outer
segment.
iii. The onter limiting membrane is seen in sections
as an exceedingly thin line separating the rods
and cones from the outer nuclear layer.
iv. The onter nuclear layer is a moderately thick layer,
well stained, and very closely connected with the
rods and cones.
v. The outer molecular layer is a very thin layer,
not stained : it is finely granular or reticular in
structure.
vi. The inner nuclear layer is thick and well stained.
It consists of several layers of large nuclei, sur-
rounded by thin layers of protoplasm, produced
into radial processes.
vii. The inner molecular layer is a thick, jGoiely granular
layer, not stained.
viii. The layer of nucleated nerve cells (ganglion cells)
is a single layer of large branched cells.
ix. The layer of nerve fibres is formed by the branches
of the optic nerve.
X. The inner limiting membrane, formed by the cone-
shaped ends of Miiller's fibres.
xi. The radial fibres, or Miiller's fibres, conmience with
expanded ends in the layer of nerve fibres and
stretch outwards: they can easily be traced
through the inner molecular layer. They consist
of connective tissue and serve to strengthen and
support the retina.
D. The Ear of tbe Tng.
The frog's auditory organ is too small to diaaect aatisfactorily,
and is best studied by making transrerae sections of the entire
head in the following manner ;
Kill a frog 'with chloroform; cut off tAe head vnth atovt
aeitsora, and decalcify it by placing in a b to 10 per cent, solution
of nitme acid, or in a mixture of chromic acid wiUi a few drops of
ntfrtc acid. When the hone* are ih/<roughly soft, which will taie
Jrom a few howra to 3 or 4 days or ti,ore according to the strength
of add employed, remove the head from the decaidfying sokuion
and transfer to weak alcohol and theitce to strong alcohol. Then
stain u>ttA borax carmine and iinhed in paraffin, and cut into
transverse sections with a m,icrotome. Mov/nt the sections in
series ; easamine and draw them, showiTig the following points :
1. The periotic capsnle consists mainly of cartilage, and is
firmly fused with the hinder part of the cranium.
2. The Teatilrale is a membranous sac lying in the cavity of
the periotic capsule, and filled with a watery fluid, the
eodolymph : it ia partly divided by a constriction into
i. The atricnliu is the upper and larger division.
ii. The Baccnlos ia the inferior and smaller division :
from it arise three small saccular dilatations,
supposed to represent the cochlea of higher
animala.
iii. Tlie ductus endolymphaticus i-isea from the inner
.1. the antenot vertical semicircular canal; i, it
bonzoDlal canal ; t, its ampulla ; f, the posterior n
ampulla ; >, tbe laccultu ; », the ucriculua.
THE EAR 95
and upper border of the sacculns as a tube which
terminates in a large thin- walled sac.
8. The semicircular canals are three tubular of&ets of the
vestibule, into which they open at both ends. They lie
in canals in the periotic cartilage, and are placed in planes
at right angles to one another ; each has at one end, close
to its opening into the vestibule, a dilatation or ampulla.
i. The anterior vertical casial has its ampulla at its
anterior end.
ii. The posterior vertical casial has its ampulla at its
outer end, while its inner end joins the posterior
end of the anterior vertical canal to open into
the vestibule by a common orifice.
iii. The horizontal or external canal has its ampulla
at the anterior end.
4. The auditory nerve leaves the cranial cavity through a
hole in the inner wall of the periotic capsule, and divides
into branches distributed to the sacculus and its diverti-
cula, and to the ampullae of the semicircular canals.
5. The accessory auditory apparatus. The essential organs
of hearing — Le,, the vestibule and its o£^ts, and the
auditory nerve — are enclosed in the periotic cartilage,
which is deeply placed in the side of the head : the com-
munication with the surface is brought about by the
accessory apparatus, which consists of the following parts
(Fig. 10, p. 48).
i. The Eustachian passage and tympanic cavity are
formed in connection with the hyomandibular gill
deft of the tadpole. (See Chap. VIII., p. 126.)
ii. The tympanic membrane closes the tympanic
cavity on its outer side.
iii. The columella is a rod of bone and cartilage, the
outer end of which is attached to the tympanic
membrane, while its inner end is inserted into a
hole in the outer wall of the periotic capsule (the
fossa fenestrse ovalis) so as to lie in close contact
with the vestibule. It serves to communicate
the auditory vibrations of the tympanic mem-
brane to the vestibule.
CHAPTER VII.
THE BEPBODUGTIVE ORGANS AND THE CLOACA OP
THE FROG.
A. The Male Frog.
1. The Reproductive Organs. (Fig. 3, p. 20.)
Fin the frog on its back vrnder water ; open the body cavity from
the ventral siMrfaoe ; turn aside the cUimentary canal amd liver.
i. The testes are a pair of yellow ovoid bodies about
half an inch long, lying on the ventral surface of
the kidneys. Within the testes are developed
the essential male elements or spermatozoa.
ii. The vasa efferentia are a number — usually 10 to
12 — of slender ducts, connecting the testis of
each side with the inner or median border of the
corresponding kidney ; they serve to convey the
spermatozoa from the testis into the tubules of
the kidney, whence they escape by the ureter,
which acts as vas deferens.
iii. The vas deferens or ureter runs along the outer
side of the posterior part of the kidney, and then
backwards to the cloaca.
iv. The vesicula seminalis is a Jarge pouch-like dila-
tation on the outer side of the vas deferens, just
behind the kidney and before reaching the cloaca.
2. The Cloaca. (Gf Fig. 3, p. 20.)
Lay the frog on its back ; with a sUmt scalpel split the pelvic
symphysis in the msdia/n plane ; gently separate the two halves^
amd pin them out right and left, so as to expose the cloaca from
Hie veorUral surface.
THE REPRODUCTIVE ORGANS. 97
L Into the cloaca open the large intestine, the ureters,
and the bladder.
ii. The bladder is a thin-waUed bilobed muscular sac,
lying on the ventral surface of the large intestine
and cloaca, its two lobes communicating freely
with each other. It is invested by peritoneum
and attached to the sides of the body by special
peritoneal folds.
Inflate the bladder with a blowpipe inserted through the doaccU
aperture: pass a seeker up the cloaca to determine the exact
position qf the opening from the bladder to the cloaca, Cvi up
the cloaca along one side ; wa^sh out its contents and examine the
opening into the bladder,
iii. ^he ureter or vas deferens is continued behind
ttie vesicula seminalis as a very short tube, open-
ing into the dorsal wall of the cloaca almost
exactly opposite the opening of the bladder on
the ventral surface. The openings of the two
ureters are close together on the apices of two
small papillse, overhung by a slight valvular pro-
jection of the mucous membrane of the cloaca.
iv. Remove a small piece of the testis ; plaice it on a slide
in a drop of salt solution ; press it slightly ; cover
and examine with a high power to see the sperm^a-
tozoa,
B. The Female Frog.
1. The Reproductive organs.
Dissect as in thb male,
i. The ovaries are a pair of black masses lying in
folds of the peritoneum ventral to the kidneys,
in very much the same position as the testes in
the male. Their shape, colour and size vary much
at different seasons of the year. On their surfaces
are numerous rounded projections, like small
shot ; these are ova in various stages of develop-
ment ; the smaller and younger ones are white ;
the larger and more mature ones black in one
half and white or yellowish in the other. Each
Q
98 REPRODUCTIVB ORGANS AND C3L0ACA
ovary consists of a couple of folds united along
their ventral edges ; tlie space between the folds
is divided by partitions into about fifteen
pouches.
ii. The oviducts are a pair of white, twisted tubes,
with thick gelatinous walls. They commence
with open mouths at the extreme front end of
the body cavity, close to the outer side of the
roots of the lungs ; and run back, increasing in
size, and becoming much convoluted, llieir
hinder ends, the ovisacs, are greatly dilated, but
have thinner walls ; in these tiie eggs are stored
just before laying commences. Unlike the male,
the female has genital ducts distinct from the
ureters.
2. The Oloaca.
Dissect as in the male.
i. The cloaca is very similar to that of the male,
except that the urinary and genital products are
discharged into it by separate ducts.
ii. The bladder is like that of the male.
iii. The oviducts open separately into the dorsal wall
of the cloaca, just opposite the bladder, by two
wide apertures separated by a narrow median
partition.
iv. The ureters open by two small apertures, placed
dose together, into the dorsal wall of the cloana
iust behind the oviducts.
Fig. ;
. Embno
E Det:
from ihe right sirie. showing lh» if niral Eland, Ihe hyo
cleft and ol ihrep Bill cltfts : a rounded swelling anletiorly merks (he >.
.•honing ihe rudimenls ofihe exlernal gills. 3. Larva from Ihe right
below, showing the mouih and horny jaws, the ventral rland split ini
5. l-rom below : Ihe operculum has covered the gills except on the left
opercular cavily. and rudiment of hind limb just above the rectum,
anui. B. From Ihe left side at tt>e commencement of metamorphosis
uarent operculum, the elbow projeodng, 9. Young tailed frog.
(TAe a
n ty tie a
all outline drtr
mg!)
[dean and two branchial arches, and indicalions of the byomandibular
iitP of the eye. a. Embryo 31 Ihe lime of hatching, from ihe right side,
side, with well developed external gills, nose, and eye 4. Larva from
o two; the transverse fold indicates the nidinient of the operculum.
side. 6. From the left side : showing the beak, nose, eye, opening of
7. From the right side, showing the develo).ing hind limb, and Ihe
. The tail beginning to shorten : the fote limb seen Ihrough the irans-
Bi/wan pages g3 and 99.
CHAPTER VIII.
THE DEVELOPMENT OP THE PBOQ.
I. General Acconnt.
The (Tog's eggs are laid in water, usually during March or
the early part of April.
During the act of oviposition, which may last several days,
the male frog clasps the female firmly, embracing her with his
arms ; and as the eggs pass out from the cloaca of the female
they are fertilised by spermatozoa discharged over them by the
male.
The eggs, which are very numerous, are small spherical bodies
about 1*75 mm. in diameter ; they are invested by thin coatings
of an albuminous substance, which swell up very greatly in
the water, and stick together to form the bulky masses we call
frog's spawn. Such spawn consists of a transparent gelatinous
mass, formed by the swollen albuminous matter, in which are
embedded the eggs : these latter appear as small round bodies,
each presenting a black half and a white half.
If a number of hen's eggs were broken into a basin, care
being taken not to rupture the yolks, amass would be produced
similar to frog's spawn : the yellow yolks corresponding to the
frog's eggs, and the whites or albuminous investments of the
yolks to the gelatinous matrix of the spawn. And just as the
chicken is formed from the yolk, and not from the white of a
hen's eggy so also is the frog developed from the egg and not
from the gelatinous investment.
The frog's eggs, laid in this way and fertilised by the sperma-
tozoa shed over them by the male, begin to develop at once.
Each egg is at first spherical, but in about a week becomes ovoid
in shape, and ther rapidly increases in length. By the tenth
day it is divided by slight constrictions into head, body, nnd
tail. The whole animal becomes fish-like in appearance, the
tail growing rapidly : two pairs of branching tufts, the external
100 DEVELOPMENT OF THE FROG
gills, followed shortly by a third pair, grow out from the
sides of the neck, and in about a fortnight from the time
of laying of the eggs the young tadpoles make their way out
of the gelatinous mass of the spawn, and swim freely in the
water.
The egg is surrounded by a tough membrane through
which the tadpole has to find a way. The method of hatch-
ing appears to be as follows. The tadpole possesses a frontal
gland immediately in front of the horseshoe-shaped cement-
organ on the under side of its head. Fixing this cement-
organ to one point of the egg-membrane, it performs
movements around this fixed point by the aid of the cilia
which cover the greater part of its body, and rubs the
frontal gland over this part of the vitelline membrane.
Ultimately the membrane is softened and gives way, allowing
the tadpole to creep out and adhere to weeds.
At the time of hatching the tadpole has no mouth, and
is dependent for food on granules of food-yolk which are
contained in large numbers in the egg, and at the ex-
pense of which all the earlier processes of development are
effected.
A few days after hatching the mouth appears, bordered
by a pair of horny jaws, and fringed with fleshy lips prO'
vided with horny papillae. The alimentary canal which has
hitherto been wide and short, now rapidly increases in length,
becoming tubular and convoluted in the form of a reversed
spiral; the liver and pancreas are formed; the anus is de-
veloped even before the mouth, and the tadpole now feeds
eagerly on confervse and other vegetable matter.
About the time of appearance of the mouth, i.e., shortly
after hatching, a series of four slit-like openings, the gill clefts,
appear on each side of the neck, leading from the pharynx to
the exterior. The ectoderm covering them becomes tasselled
and forms a series of opercular gills. At the same time the
earlier formed more dorsally placed external gills degenerate.
While external gills bordering the gill-slits are developing,
a fold of skin, the operculum, appears on each side of the
head in front of the gills, and grows backwards over these,
so as to enclose them in a gill chamber. Towards the end of
the fourth week the hinder edges of the opercular folds fuse
FiO. aoA.— Side view of a ladpole at Ihe time of hatching,
B. Ventral view of Ihe fame tadpole.
en. I, external gill of first branchial arch; =■> • •««-«»
second branchial arch; os, slomatodseal pit ; i
seen through the skin ; oc, olfactory pit; Q, o
u, proclodxal or cloacal apolure.
DEVELOPMENT OF THE FROG 101
with the body-wall on the ventral surface and along the right
side. On the left side a spout-like opening remains which
communicates with the gill-chambers of both sides, and
through which the water taken in at the mouth for respira-
tion, and passed through the gill-slits, makes its escape to
the exterior, Fig. 20, 5, g, g. It is the inclusion of the
gills by this opercular fold that has given rise to the notion
of opercular or "internal" giUs. True internal gills, how-
ever, are not present in the frog or its allies at any stage of
development.
During this time the tadpole has been feeding freely, and
has increased greatly in size. The body. Fig. 20, 5, is broad
and round ; the tail is much larger than before, and forms
a powerful swimming organ ; while the cement-organ on the
under surface of the head, though still present, is small and
but little used for adhesion.
Yery shortly afterwards rudiments of the hind limbs can be
seen as a pair of small papillae at the root of the tail, one on
each side of the anus, Fig. 20, ^ ; these steadily increase in
size : stbout the seventh week they become divided into joints ;
and a week later the toes appear. Fig. 20, ^.
The fore-limbs arise about the same time as the hind ones,
but are covered by the opercular folds, and hence do not
become visible till a later stage, Fig. 20, g.
Towards the end of the second month the lungs come into
use, and the tadpoles which now have the form shown in
Fig. 20, g, frequently come to the surface of the water to
breathe. The gills now begin to degenerate, but for a
time respiration is effected both by the gills and the
lungs.
A fortnight or three weeks later a distinct metamorphosis
occurs, whereby the tadpole becomes transformed from the
fish-like condition in which it has hitherto remained to the
purely air-breathing stage characteristic of the adult. The
tadpole ceases to feed ; a casting, or ecdysis, of the outer
layer of the skin takes place ; the gills are gradually absorbed ;
the horny jaws are thrown off ; the large frilled lips shrink
up ; the mouth loses its rounded suctorial form and becomes
much wider ; the tongue, previously small, increases consider-
ably in size; the eyes become larger and more prominent;
102 DEVELOPMENT OF THE FEOG
the fore-limbs appear, the left one being pushed through the
spout-like opening of the branchial chamber, and the right
one forcing its way through the opercular fold, in which it
leaves a ragged hole. The abdomen shrinks; the stomach
and liver enlarge, but the intestine becomes considerably
shorter than before, and of smaller diameter; the animal,
previously a vegetable feeder, now becomes carnivorous.
The gill-clefts close up ; and important modifications accom-
panying the change in breathing, occur in the blood-
vessels.
The tail, which is still of great length, Fig. 20, g, now
begins to shorten, and is soon completely absorbed ; the hind-
limbs lengthen considerably, and the animal leaves the water
as a Frog, Fig. 20.
Explanation of the Metamorphosis.
The structure and mode of life of the tadpole (opercular
gills; gill slits; a tail; and lateral sense organs) give it a
fish-like character. But there is no known fish which at
all closely resembles the tadpole. The mud-fish of Africa
and America approach it. Still more close is the agreement
between the tadpole and the young stage of these fish.
Hence we may say that the most probable explanation of
the problem, "Why the frog first becomes a tadpole?" is
that its fish-like ancestors passed through a similar stage
in their career. The frog, in brief, climbs up its genealogical
tree in its own life-history.
SimpMcation of the Frog's Structiire.
Before following the development of the frog in detail it
will be useful if we imagine its structure to be reduced to
the most essential features ; and anticipating one or two of
the developmental steps, trace in thought the simplification
of these essential organs to the earlier stages and ultimately
to the egg.
From this point of view we may look upon the frog as
simply a trunk, disregarding the head and limbs. This step
is justified by the structure of the lowest vertebrates in
DEVELOPMENT Of THK PllOG lOS
which no limbs are present and only a very indistinct head
is developed. Essentially, then, the frog consists of a muscular
body -wall forming a tube thick above, thin below, and bound-
ing the body-cavity. On the dorsal surface is a tube, the
central nervous system, imbedded in the body-wall. In the
body-cavity is a second tube, the alimentary canal. Around
the nervous tube is a bony sheath divisible into skull and
vertebrae. The most essential part of this skeleton is
a rod of cells running down the centra. In the lowest
vertebrates this alone persists and is called the notochord.
Thus reduced to its essential anatomical characters, a frog
consists of two tubes lying one over the other, separated
by a rod, the whole enclosed in a flexible muscular body-
walL
The wall of the alimentary tube and that of the body are
made up of three complementary layers. Counting from
the body-cavity inwards to the gut-cavity, tlie coats are
peritoneum, muscle, and mucous lining : from the body
cavity outwards they are peritoneum, muscle, and mucous
skin.
The development of the frog is mainly the history of the
formation of these two tubes, one nervous the other digestive ;
of the intervening rod or notochord ; and of the coats which
compose the body-wall and the gut- wall.
We may anticipate the first history. The nervous tube
is derived from the skin. It is folded oflf along the back,
surrounded by nerve-cells and sunk inwards. We are thus
left with a double tube : the alimentary canal and the body-
wall with its notochordal rod.
The complementary nature of the inner and outer coats of
the body-cavity suggest that they have a common origin in
a common tissue. This would reduce the frog to a tube
composed of a central digestive layer surrounded by a
middle coat of indifferent tissue and covered with the outer
skin. These three coats have received names. The outer
one, being potentially both nervous system and skin, is called
by a name signifying surface-growth or epiblast ; the tniddle
one, mesoblast ; the inner, hsrpoblast.
There are two ways in either of which this triple-layered
organism could arise from a simple beginning. Imagine a
104 DEVELOPMENT OF THE FROG
hollow ball pushed in at one pole while the space between
the inner and outer cavities was filled up by growth of
tissue round their junction. Or, starting with a solid ball,
imagine a liquefaction to occur at the centre, an inner layer
of tissue condensing round this central cavity and round the
outer surface. We must now anticipate that the latter
process is the one chiefly followed by the frog, and that it
starts at the lower pole of the ball. Thus we come to the
conception of the triple-layered embryo arising from a solid
or nearly solid mass of cells whose upper and lower poles
differ : and from this it is an easy step to the next, a single
cell with different poles. This cell is the egg, whose fortunes
we shall now follow in the reverse direction.
II. Detailed Account.
A. Formation of the Egg.
The early stages in the formation of the egg cannot be
seen in the adult frog, but must be studied in tadpoles. In
tadpoles of about 10 mm. length, shortly after the opening of
the mouth, a pair of longitudinal ridge-like thickenings of
peritoneum appear along the dorsal surface of the body cavity,
close to the mesentery, and along the inner borders of the
developing kidneys. These genital ridges are found in all
tadpoles of this age, no distinction of sex appearing until a
much later period.
Each genital ridge is at first due merely to slight modifica-
tion in the shape of the peritoneal epithelial cells which, else-
where flattened, become here cubical or slightly columnar.
The ridges soon become more prominent, especially at their
anterior ends ; their growth being due partly to the epithelial
cells increasing by division so as to form a layer several cells
thick, and partly to the ingrowth of an axial core of connective
tissue from the basal membrane of the peritoneum along which
the bloodvessels gain access to the ridge.
From the posterior two-thirds of the genital ridge the ovary,
or in the male the testis, is developed ; while the anterior
third undergoes degenerative changes, and becomes converted
into the fat body.
FORMATION OF THE EGG 105
The Primitive Ova. At an eariy stage certain of the
epithelial cells of the genital ridge become conspicuous by
their larger size and more spherical shape ; and around these
larger cells, or primitive ova, as they are called, the smaller
epithelial cells become arranged so as to form capsules or
follicles; the follicles, with their contained primitive ova,
form small knob-like projections on the surface of the
genital ridge. New primitive ova arise either directly from
the surface epithelium, or by division of the already existing
ones.
The Permanent Ova. Up to this time there has been no
distinction between male and female, the processes described
occurring in all tadpoles alike. Sexual differentiation appears
about the time of the metamorphosis. In the female the
change consists essentially in a great increase in the size of
the genital ridge, which now becomes the ovary, and in the
formation of the permanent ova, or eggs. The permanent
ova are derived from the primitive ova ; in some cases each
primitive ovum is directly converted into a permanent ovum,
but it has been suggested that in others two or more primitive
ova are concerned in the formation of a single permanent
ovum.
A permanent ovum is enclosed in a follicle or capsule like
the primitive ovum, and differs from this latter in the following
points : (1) it is of larger size ; (2) it contains within its sub-
stance a number of small sharply defined yellowish granules of
food yolk, which are elaborated by the follicle cells and passed
on from them to the ovum; these yolk granules increase
rapidly in number, and to them the greater size and opacity of
the permanent ovum are chiefly due ; (3) important changes
have occurred in the nucleus; in the primitive ovum the
nucleus is small, granular in appearance, and apparently solid ;
in the permanent ovum the nucleus, or germinal vesicle, is of
very large size, up to half the diameter of the entire ovum,
and consists of an elastic capsule or nuclear membrane,
filled with fluid and traversed by a protoplasmic reticulum
enlarged at its nodes to form the nucleoli, or germinal
spots.
When the permanent ovum has reached a diameter of about
0*5 mm., an exceedingly thin structureless investment, the
106 DEVBLOPMBNT OP THE FROG
vitelline membrane, is formed immediately around it, within
the follicle. The mode of origin of the vitelline membrane is
not clearly made out, but it seems to be formed from the ovum
itself rather than from the follicular epithelium.
A little later still a layer of black pigment appears on the
surface of the ovum ; it is at first irregularly distributed over
the whole surface, but as the ovum ripens it becomes restricted
to one half or hemisphere. The pigment is contained, and
apparently formed, within the ovum itself, but it is not clear
how it is formed or what purpose it fulfils.
B. Mattiiation of the Egg.
The eggs have now reached their full size, and project from
the surface of the ovary like small shot ; but they have still to
pass through the process of maturation, or ripening, before
they are ready to be fertilised. This process of n^aturation
concerns the nucleus almost exclusively.
The nucleus, which at its full size we have seen to be quite
half the diameter of the egg itself, begins to shrink; the
nuclear membrane becomes wrinkled, its surface presenting a
number of small wart-like projections, so that the whole nucleus
has a blackberry-like appearance. Part of the nuclear fluid
exudes through the nuclear membrane into the substance of
the egg ; a great part of the nuclear reticulum disappears, or
becomes broken up into isolated globules or nucleoli, but a very
small part remains in the centre as a slender intricately coiled
thread, the nnclear skein.
About this time the eggs are discharged from the ovary, the
follicles rupturing, and the eggs falling into the body cavity of
the frog ; along this they pass forwards, directed partly by
contraction of the muscular body-walls, partly by the action
of the cilia of the peritoneum, to the mouths of the oviducts,
which are situated at the anterior end of the body cavity
opposite the roots of the lungs. In the first, or thick-walled
part of the oviduct the eggs acquire gelatinous investments,
secreted by glands in its walLs. The terminal, or hinder, part
of the oviduct forms a thin-walled sac capable of great disten-
sion, within which the eggs accumulate in large numbers.
Finally, the eggs are passed out through the cloaca into
MATURATION OF THE EGG 107
water, in which the albuminous inyestments of the eggs
speedily swell up to form the gelatinous mass of the frog's
spawn.
During the discharge of the egg from the ovary, and its
passage down the oviduct, further changes occur in its nucleus.
The nuclear membrane still further collapses, and finally dis-
appears completely; the nuclear fluid and nucleoli become
distributed through the substance of the eggj and of the
original large nucleus the exceedingly minute nuclear skein
alone remains.
This nuclear skein moves from the centre of the egg to its
surface, which it reaches opposite the centre of the black
hemisphere. The skein, previously an irregularly tangled
thread, now assumes the definite form and arrangement of
a nnclear spindle, such as may be seen in the nucleus of an
epithelial or other cell immediately before division of the cell
occurs.
The First Polar Body. About the time the egg is laid,
but before it is fertilised, the egg becomes slightly flattened
at its upper or black pole, a certain amount of fluid being
exuded between the egg and the vitelline membrane. The
nuclear spindle now divides into two equal parts, one of
which remains within the egg, while the other is extruded
from it as the first polar body, a minute ovoidal white
globule, which lies on the surface of the egg in the exuded
peri-vitelline fluid.
The Second Polar Body. The half of the nuclear spindle
that remains within the egg retreats from the surface a
little distance, and then divides into two equal parts, one
of which remains within the egg as the female pronnclens,
while the other is extruded as the second polar body, a
minute white globule very similar to the first polar body,
and like this lying freely in the perivitelline fluid on the top
of the egg.
In the case of most animals in which the formation of polar
bodies has been observed, both the first and second polar
bodies are extruded before fertilisation is effected. In the
frog the extrusion of the second polar body does not occur
until after the spermatozoon has entered the egg, though
before the completion of the act of fertilisation.
108 DEVELOPMENT OF THE FEOG
0. Fertilisation of the Egg.
Fertilisation, or impregnation, consists in fusion of the
spermatozoon with the egg ; or, more strictly speaking, fusion
of the nuclei of these two bodies.
The spermatozoa, after being shed over the spawn by the
male, swim actively by means of their long tails, penetrate
the gelatinous investment of the eggs, bore their way through
the vitelline membrane, and so penetrate into the eggs them-
selves, which they enter at or close to the upper or black
pole.
A single spermatozoon is sufficient to fertilise an egg, and it
is doubtful whether more than one is ever normally concerned
in the process.
In about an hour after the spermatozoon has entered, a
pigmented process may be seen projecting inwards from the
surface of the egg, with a clear spot in its centre. This spot
is the nucleus of the spermatozoon, and is spoken of as the
male pronucleus : it penefautes farther into the egg, carrying
the pigment with it, so that it appears surrounded by a
pigmented capsule connected with the surface of the egg by
a pigmented stalk.
By this time the second polar body has been formed and
extruded, and the female pronucleus is the only part of the
original egg nucleus still remaining. The inale and female
pronuclei, which are at first some little distance apart,
rapidly approach each other, come into close contact^ and
after having increased considerably in size, fuse together,
about two and a half hours after fertilisation has commenced,
to form the segmentation nucleus.
The segmentation nucleus is a large spherical vesicle im-
bedded in finely granular protoplasm, and surrounded by
an ill-defined capsule of pigment : its formation by the
fusion of the male and female pronuclei completes the act
of fertilisation.
The female pronucleus may be regarded as an imperfect
nucleus, and the upshot of the process of fertilisation is the
completion of this nucleus ; the nucleus of the spermatozoon,
or male pronucleus, replacing the part of the egg-nucleus
which has been lost as the polar bodies. The further
SEGMENTATION OF THE EGG 109
explanation of the sexual process is probably to be found
in the great advantage, as regards vigour of offspring,
that is known to result in both animal and vegetable
kingdoms from cross fertilisation, i.e., from combining the
protoplasm of two distinct individuals in the act of repro-
duction.
D. Segmentation of the Egg.
The earliest stages of development consist in repeated
division of the egg, whereby it becomes converted from the
unicellular condition, which is permanent only in the lowest
animals, to the multicellular state characteristic of all higher
animals. To these early processes of development the name
segmentation is given.
Very shortly after the completion of the act of fertilisation
and formation of the segmentation nucleus this latter loses
its spherical form and becomes spindle-shaped, the yolk
granules at the same time showing a tendency to become
arranged in lines radiating outwards from the ends of the
spindle. The nucleus now divides into two halves, which
move away from each other; the yolk granules tend to
aggregate themselves around the two nuclei, and a thin
vertical plate of finely granular substance is left, bisecting
the egg.
At the upper or black pole of the egg a depression
now appears, at first as a small pit an4 then as a groove,
which soon extends all round, and, rapidly deepening,
divides the egg into two completely separate halves along
a plane corresponding with the vertical plate mentioned
above.
Each of the two nuclei soon divides again into two, and a
second cleft is formed in the same manner as before : it also
is vertical, but in a plane at right angles to the first one, and
on its completion the egg consists of four precisely similar
segments or cells, each with a nucleus.
The third cleft is horizontal, but not equatorial, lying
nearer the upper than the lower pole : it divides each of the
four cells into two, an upper smaller and a lower larger
one.
(
no
DETKLOPHEKT OF THE FBOG
\
Two more vertical clefts next appear simultaneously at
tho^upper pole, midway between the two primary clefts, and
extending downwards divide first the smaller and then the
larger cells, giving sixteen cells ^ all, eight smaller upper
ones, and eight larger lower ones. Two more horizontal clefts
then appear, which again double the number of segments,
giving thirty-two in all.
From this stage segmentation proceeds in a less regular
manner, the upper and smaller cells dividing more rapidly than
the lower and larger ones. By means of radial and concentric
io
Fig. 21.— Segmental 'on of the frogs egg. (From Haddon, after
Ecker.)
The numbers above the figures indicate the number of segments at
the several stages. The dotted lines mark the positions of the clefts
that will next appear.
clefts, the number of cells is rapidly increased, division of the
cells being in all cases, as from the first, preceded by division
of their nuclei.
At the stage when only eight cells are present^ i.e., on the
completion of the third cleft, a small cavity appears in the
centre of the egg, round which the cells are grouped : during
the later phases of segmentation this segmentation cavity
or blastocoele, as it is called, increases considerably in size :
it is from the first situated nearer the upper than the lower
pole of the egg, and is filled with fluid.
At the close of segmentation the egg has the structure
shown in section in Fig. 22.
BEGMRNTATION OF THK EGG 111
It is a bdlow ball with its walls composed oi three or four
layers of cells, and of very unequal thickness, owing to Uie
segmentation cavity lying in the upper half of the egg. The
celb of the upper half are smalt, fairly uniform in size, and
regularly arranged, while those of the lower half are larger,
and more irregular both in shape and size. The superficial
cells of the upper half are deeply pigmented, while the cells
of the lower half are almost colourless.
FiC aa. — Ve'Iical section through a frog's egg at rhe close <rf s^-
mcntalion. >: 28.
B, epiblast ; 8C. segmeotatioa cavity ; V. lower layer or yolk cells.
The distinction between upper and lower cells is however
not an absolute one, a ring rf numerous cells more or less
intermediate in size, shape, and depth of pigmentation,
occurring round the equator of the egg at the junction of
its upper and lower halves. These intermediate cells (Fig.
22a, I C) take an important part in the formation of the frog's
tissues.
The process of segmentation is, as mentioned above, simply
one of cell-division; and the unequal rates ftt which the
112 DEVELOPMENT OF THE TROG
different parts of the egg segment, are to be regarded as
due to the retarding influence of the granules of food-yolk,
which, being themselves inert, must hinder the activity of
the protoplasm in which they are imbedded. These granules
of food-yolk are more abundant in the lower than the upper
half of the egg, and this unequal distribution of food-yolk
is the direct cause of the unequal segmentation of the egg.
The purpose of food-yolk is to afford a supply of nutiiment
at the expense of which the earlier developmental processes
Fic. 23.— Longiludinal vertical section of a frog embryo, shoviog
commencing bypoblasi al B. x sS.
B, blastopore ; EE, ouler or epidermic layer of epiblasl ; EN, inner
or nervoua layer of epiblasl ; 6C, segmentaiion cavity ; V, yolk-cells.
may be accomplished, until the young animal is sufficiently
advanced to obtain food for itself; and the direct influence
<rf this food-yolk will be to hinder rather than to help these
jHTocesses.
E. The QemUnal Layen.
At the close of segmentation we have seen that the egg
consists of cells of two kinds ; firstly, those of the upper haS
of the egg, which are smaller, pigmented, more regularly
arranged, and comparatively free from food-yolk ; secondly,
Figs. 3a»-as'-— The four small figures represent ihesem.
the formalion and shifting of the blastopore (ap). Thefoii
through its horizonlal axis iz, and represent four stages fatlig
of tbe egg through more than 90°.
DL. dorsal lip of blastopore ; ic. intermediate cells : m\ ^
VLi ventral lip of blastopore ; vi%, axis of embryOp whicbt 9
Yz. axis of embryo, which is .it first horizontal and subsuqj^^?
meriting egg. seen from its lower pole, and illuslraie
r large figures are seclions of the egg taken vertically
be formation of tbe mesenleron. Notice the rotation
3C, segmenlalion cavil; or blasloccel ;
FORMATION OF THE OERUINAL LATEllS 113
tboee of tbe lower half of the egg which are larger, less
regular, and almost free from pigment, but much distended
bj food-yolk, which is present in such quantity as to render
them comparatively inert.
The former are the epiblast cells; the latter may con-
venienUy be spoken of as the lower layer cells or jrolk-
cells.
Tbe epiblast ubows almost from the first a distinotdon into
Fig. 24.— Longimdinal vertical section Ihrough a frog embrjti at
a later stage in the fannation of the mesenteron.
H, inTOeinate hypoblast ; M, mesoblast ; MN, mesenleron ; H, noto-
cbord ; 8C, s^menlatioa caTitf ; YP, yolk plug, fillinf; up Ihe blasto-
pore.
two layers ; the most superficial cells being somewhat cubical
in shape and closely applied side by side so as to form a
continuous and deeply pigmented layer ; while the deeper
cells are more spherical, less strongly pigmented, and loosely
arranged in a layer two or more cells deep.
Hie epiblast cells continue to increase by division, and very
early, owing apparently to multiplication of the cells at the
margin of the layer, seem to spread over the lower or yolk-
cells. Owing to tbe difference in colour of the two halves of
the egg, the various stages o£ this process can be readily
114 DKVBLOFMEKT OF TEE FSOCF
followed, the black epiblast cells seeming to spread over and
gradually almost encloae the white yolk-cells.
This apparent spreading of the epiblast does not take
place equtJlj fast all round ita margin, and at one place the
epiblast, instead of extending over the yolk-cells, bends
inwards towards the interior of the egg. This place is
visible externally as a sharply defined horizontal or slightly
creacentio groove, concave downwards, bounded above by the
small black epiblast cells, and below by the large white yolk-
FlG. 35. — Longitudinal vertical section through a frog embryo show-
ing the completion of the mesenleroo.
B, blastopore : EE, (^idermic layer of epiblast ; EN, nervous layer
of epiblast ; H, in vaginate hypoblast ; M, mesoblast; MN, mesenteron ;
N, notochord.
cells. As the epiblast continues spreading over the rest of
the yolk this groove becomes horseshoe-shaped, and a little
later circular.
If we examine the egg over a mirror, it will be seen that
this groove does not remain stationary but shifts gradually
from near the equator (OL, Fig. 22a) towards the lower
pole, and when it has reached this point it has assumed
a circular form (Fig. 34a). In other words the dorsal.
FORMATION OF THE GERMINAL LAYERS 115
lateral, and ventral lips of the blastopore appear successively
during its downward movement. From this point its progress
is again shifted. The egg rotates about its horizontal axis
through about 100' owing to the development within it of
an eccentric cavity which causes it to topple over. This
brings the original vertical axis of the egg downwards and
the blastopore up to the equator again.
The egg has now the appearance shown in Fig. 24, the
epiblast covering the whole surface except a circular patch,
where alone the yolk-cells are visible from the surface. This
circular aperture in the epiblast is called the blastopore ; it is
situated at what will become the posterior end of the embryo ;
and it is bordered by a distinct rim or lip, round which the
epiblast turns inwards into the interior of the egg. The
circular plug of yolk-cells filling up the blastopore is spoken
of as the yolk plug.
The structure of the egg, or rather of the embryo at a
slightly later stage, is shown in Fig. 25, which represents
a vertical section passing through the middle of the blasto-
pore.
The epiblast covers the whole surface except at the blasto-
pore. From the lip of the blastopore a layer of cells appears
to grow into the egg concentrically with its surface.
This layer is called the hypoblast : it appears to be formed
partly by differentiation of the subequatorial intermediate
cells during overgrowth of the lip of the blastopore, partly
by differentiation of yolk-cells ; it extends much more rapidly
dorsally than ventrally, so that while on the upper or dorsal
surface it extends to the anterior end of the embryo, at
the sides and below it only extends a very short way as
yet.
Between this ingrowing layer of hypoblast and the yolk-
cells there is a space. This is a very narrow chink near the
blastopore, but farther forwards it dilates on the dorsal
surface to form a cavity of some size (Fig. 25, MN),
wider from side to side than it is dorso-ventrally. (Cf.
Fig. 26).
This cavity, which is named the mesenteron, is the
future alimentary canal: it communicates with the ex-
terior through the blastopore, though the aperture is
116 DBVBLOPMENT OF THE FROG
reduced to a narrow chink and is almost stopped up by
the yolk plug. The permanent mouth and anus are not
yet formed.
During the process of formation of the mesenteron, the
segmentation cavity gets pushed out of place and be-
comes reduced to a chink between the epiblast and hypo-
blast. (Fig. 25a, SC.)
The Mesoblast. Between the epiblast and hypoblast a
third or intermediate layer of cells, the mesoblast, is soon
established.
It is a separate layer formed by differentiation in part of
the subequatorial intermediate cells, and in part from the
yolk-cells lying immediately beneath the epiblast, but quite
distinct from it. It extends all round the embryo except
along the mid-dorsal line, where the space between the
epiblast and hypoblast is occupied by the notochord. It is,
for a time, incomplete in fronts opposite the segmentation
cavity, but soon grows in from the sides so as to fill up the
deficiency.
The cells of the mesoblast become early arranged in two
parallel layers or sheets, which separate lightly from each
other, so as to leave between them a narrow space, which
later on becomes the body cavity or coelom. {Of, Fig, 26.)
In many specimens the mesoblast cells are from the first
arranged in two layers.
The Notochord. Along the roof of the mesenteron there
is a dorsal cell-mass where the roof of the mesenteron and
the lateral sheets of mesoblast are folded together. This is
the notochord, and is derived from both the intermediate
and yolk-cells. (Figs. 24 to 26, N.) It is formed at a very
early stage, and serves to slightly stiffen the back of the
embryo, and is for some time the only skeleton which it
possesses.
It very early splits off from the roof of the mesenteron,
except at its hinder end, where it remains for some time in
continuity with both hypoblast and epiblast at the lip of the
blastopore.
Growth of the hypoblast. The hypoblast is formed in the
first instance from the inner sheet of intermediate cells at
the lip of the blastopore : its aftergrowth is effected, how-
>.
THE NBRVOUS SYSTEM 117
ever, mainly at the expense of the yolk-cells, with which it
is in contact. By growth at its margin it gradually creeps
round until it extends all round the embryo, the yolk-cells
forming part of its ventral wall.
Fate of the germinal layers. From one or other of the
three germinal layers — epiblast, mesoblast, and hsrpoblast —
all parts of the embryo are formed.
The epiblast, or outer layer, gives rise to the epidermis
covering the body generally, and to the various glandular and
other structures derived from the epidermis ; to the nervous
system, both central and peripheral ; to the olfactory and
auditory epithelium, to the retina and lens of the eye, and to
the other sensory organs ; to the epithelial lining of the mouth
and anus (stomodseum and proctodseum) ; and to the pineal
and pituitary bodies.
The hypoblast, or inner layer, gives rise to the epithelium
lining the alimentary canal and its various diverticula, includ-
ing the glands of the cesophagus, stomach, and intestine, the
lungs, the bladder^ the bile ducts, gall-bladder, pancreatic
ducts, and the hepatic cells of the liver and the secreting
cells of the pancreas; the notochord is also formed from
hypoblast.
From the mesoblast, or middle layer, are derived all struc-
tures between the epiblast and hypoblast; t.e., the connective
tissue, muscles, skeleton (except the notochord), bloodvessels
and lymphatics ; and also the peritoneum, and the urinary
and reproductive organs.
F. Development of the Nervous System.
It is convenient from the point we have now reached to
deal with the several systems one by one. The nervous
system is a suitable one to commence with, as it appears at a
very early stage of development, and plays an important part,
especially in the younger stages, in determining the shape
and proportions of the embryo.
The epiblast consists almost from the first of two layers, the
distinction between which is already established at the close of
segmentation. (Fig. 23.) Of these the upper or epidermic
layer is a single stratum of closely fitting cubical cells ; while
118 DETELOPHKNT OP TBE TBOQ
the lower or nervooa layer consiste of ovoid or spherical cells,
more loosely compacted, and two or three deep. It ia from the
latter that the nervous s}rstem is developed.
The first trace of the nervous system is seen aboat a week
after fertilisation, when the embryo is still spherical and the
blastopore has become much reduced in size and difficult to see.
[Of. Fig. 26.)
The dorsal surface of the embryo now flattens slightly, and
along the flattened area the nervous layer of the epiblast
thickens to form the neural plate, which is wide in front but
Fig. a6.— Transverse seciion through a frog embryo during Ihe foi-
C. ccelom ; EE, epidennic 1a3^r of epiblnsl ; EN, nervous layer of
epiblast: H, hypoblast; M, mrsoblast ; ME, somatopleuric layer of
raesoHast; MH.splanchnopleuriclayerof mesoblasi; MN,inesenteroni
N, notochoid; NF, neural fold ; NQ, neural groove ; V, yolk cells.
narrows posteriorly towards the blastopore. Slightly raised
ridges, the neural folds, soon appear, bordering the sides of the
neural plate ; and a longitudinal neural grooTe ia formed along
its dor^ surface in the median line, extending forwards bom
the blastopore.
A transverse fold connects the anterior ends of the neural
folds tofi^ether, slightly raising up the anterior end of the neural
plate. The neuiul folds now grow rapidly : the gi-oove between
THB NKRVOUS SYSTEM 119
them deepens, and the folds beooming more and more prominent
bend in towards each other (Fig. 26)and finally meet and fuse,
thereby converting the neniul groove into a tube.
The neural folds first meet about the junction of the head
and trunk of the future tadpole, from which point the fusion
extends rapidly in both directions, forwards and backwards.
The last point at which fusion occurs is a little distance behind
the anterior end of the tube, at the place where the pineal body
will appear later.
In front, the neural tube ends blindly ; at its posterior end
B, blastopore; BF, fore-brain; BH, hind-brain; BM, mid-braio ;
H hypoblasl; L, liver ; M, mesoblasi ; MN, meseoteron; N, noto-
chord; HO. neurenieric canal ; p.in^wth of ejablasl to form pituitary
body ; PD, proctodaum ; R, reclal diyetticnlum of mesenteron ; S. cen-
tral canal of spinal cord ; Y, yolk-cells
it opens to the exterior at the blastopore, and ia in free com-
munication with the mesenteron. (C/. Pig. 27.) The short
channel of communication between the neural tube and the
mesenteron, i.e., between the nervous system and the alimen-
tary canal, is spoken of as the aenrenteric canal ; it is only
120 DEVBIX)PMENT OF TEE FROG)
present for s short time, and oloeea up before the tadpole
batches.
The neural tube, formed in this way, soon separates from the
surface epiblast, and by thickening of its walls and other
changes becomes converted into the central nervous Bysfcem ;
the anterior part forming the brain, and the posterior part the
A, Huricle of beajt ; BF, fore-biain ; BH. hind-brain ; BM, mid-br^D ;
C*. pericardial cavity; CV, vesicle of cerebra! bemispliercs ; I, infundi-
bulum ; L, liver ; N. notochord ; O, depression of floor of foce-brain
from whicb ibe optic vesicles arise ; OE, lesophagua ; P, [Htuilary
body ; PN, pineal body : 8, cenlral canal ot spinal cord : SD. stomo-
dffium ; T, tiuneus arteriosus ; V. ventricle; Y, s-olk-cells.
spinal cord. The lumen or cavity of the tube persists as the
central canal of the spinal cord and the ventricles of the brain.
The Brain. At the time of its first appearance tbe brain is
bent at right angles about the middle of its length ; the axis of
THE BKAIK 121
the anterior portion being vertical, and that of the posterioi
portion horizontal. (Fig. 27.) The posterior portion, or hind-
brain, BH, is wide from side to side, and has moderately thick
sides and floor, but a thin roof ; it is continuous behind with
the spinal cord.
The anterior or vertical portion has walls of nearly uniform
thickness in all parts. It is divided by a slight constriction,
FlC. 39. — Longiludinal verlical section through (be head and
anterior |Mn or the body of a tadpole about the lime of appearance of
the hind legs. Length of tadpole, 13 niin. x 14-
A, auricle of heart ; AD, dorsal aortu ; BB, basi-branchial cartilage;
BF, fbre-biain; BH, hind-brain ; BM, mid-brain; C, ccelom ot hod^
lity; CH, cerebral hemisphere ; CB, nidi-
mentaiT cerebellum ; CP, choroid plexus of fourth ventricle ; CP",
choroid plexus ol third veniricle ; P, pharynx; O, stomach ; H, lune ;
., inrundibulum ; J, horny jaws; K, lip; L, liver; N, notocliord ;
O, depression of floor of fore-brain from which the optic nerves arise ;
OE> oesophagus; P, pituitary body; PN, pineal body; 6, central
caiul of ^inal cord : T, ttuncus arteriosus ; V, ventricle.
best marked at the sides, into an upper or posterior part, the
mid-brain, BM, which forms the angle of the bend and lies
opposite the anterior end of the notochord ; and a lower and
122 DEVELOPMENT OF THE FROO
larger portion, the fore-brain, BF, which is produced laterally
into a pair of hollow outgrowths, the optic vesicles.
The further development of the brain is illustrated by Figs.
28 and 29. It will be seen that the rectangular bending of the
brain, which is known as the cranial flexure, and which was so
prominent a feature in the earlier stage, is no longer obvious ;
a closer comparison of the figures wiU show, however, that this
straightening of the brain, or rectification of the cranial flexure,
is apparent rather than real, and is brought about partly by the
development of the cerebral hemispheres, which grow upwards
and forwards from the fore-brain, and still more largely by the
formation of the mouth and the growth forwards of the face
and lips, which cause the brain to take a much less prominent
share in determining the shape of the head.
The hind-brain, BH, has undergone but little change in
Fig. 28, except an increase in thickness of its floor and sides.
At the stage represented in Fig. 29 it is separated from the
mid-brain on the dorsal surface by a well-marked groove, im-
mediately behind which the roof of the hind-brain is thickened
transversely to form the cerebellum, CB. The cavity of the
hind-brain remains as the fourth ventricle, the roof of which
is very thin and thrown into numerous transverse folds, CP|
which hang down into the ventricle, and between the layers of
which lie Ijie bloodvessels of the choroid plexus of the ventricle.
The mid-brain, BM, thickens on its floor to form the crura
cerebri. Its roof grows out laterally into a pair of hollow ovoid
processes, the optic lobes ; and its cavity persists as the
aqueductns Sylvii, or iter a tertio ad quartum ventriculum.
The fore- brain, BF, becomes the thalamencephalon of the
adult ; its cavity becomes the third ventricle, which by thick-
ening of its walls to form the optic thalami is reduced to a
vertical cleft, very narrow from side to side. Its floor is pro-
duced downwards and backwards into a hollow sac-like diverti-
culum, the infundibulum, I, in connection with which is the
pituitary body. In front of the infundibulum is a transverse
ridge projecting into the ventricle, and formed by the roots of
the optic nerves.
The roof of the fore-brain remains thin ; a little behind the
middle of its length the pineal body, PN, arises as a median
hollow diverticulum. Figs. 28 and 29 ; this is formed at the spot
where the final closure of the neural tube took place, ^ind is at
THE SIIKSE ORGANS 123
first directed baxskwards ; in the later stages it grows forwards
and forms a rounded vesicle connected with the brain by a long
pigmented stalk ; when the skull develops it cuts off the vesicle
from the stalk, the former remaining as a small rounded body
outside the skull, while the staJk persists as a slender pigmented
tract within the cranial cavity.
In front of the pineal body, and at the anterior end of the
fore-brain, the roof is thrown into folds which hang down into
the ventricle forming a choroid plexus, CP', similar to that in
the medulla.
The anterior end of the fore-brain gi'ows forwards as a median
thin-walled cerebral vesicle, from which at a slightly later stage
the cerebral hemispheres, CH, arise as a pair of hoUow out-
growths ; the foramina of Monro being the apertures of com-
munication between the lateral ventricles or cavities of the
hemispheres, and the third ventricle. The anterior ends of the
hemispheres grow forwards as the olfactory lobes, which become
fused together in the median plane.
The peripheral nervous system. The cranial nerves and the
dorsal roots of the spinal nerves are formed from the deeper or
nervous layer of the epidermis. They appear to arise as lateral
outgrowths from the edges of the neural plate, and may be
recognised at a very early stage, while the neural groove is still
shallow and open ; they are, therefore, at their first appearance
continuous with the brain or spinal cord.
The ventral roots of the spinal nerves arise later than the
dorsal ones, as outgrowths from the cord near its ventral sur-
face. They are at first independent of the dorsal roots, but
soon become connected with these.
G. Development of the Sense Organs.
The organs of special sensation are developed from the deeper
or nervous layer of the epiblast, and become connected with
their respective nerves at a very early stage of their formation.
The derivation of the sense organs from the epiblast is
explained by the fact that they are concerned with the appre-
ciation of the presence and nature of external objects, and are
therefore necessarily formed on the surface of the body. They
may be regarded as specially modified portions of the epidermis.
The Nose. The olfactory organs appear at a very early stage
as paii'ed thickenings of the nervous layer of the epiblast at
124 DEVELOPMENT OF THE FEOa
the anterior end ot the head, in the angles between the tat»-
bcain and the optic vesicles. A pitting-in of the surface,
involving both layen of the epiblast, soon appears in each of
these thickenings, and the pits so formed become the tibmI
sacs ; the moutha of the pita forming the nostrils or anterior
najes, and the epiblaet lining the pits giving rise to the olfac-
tory epithelium.
From the inner or deeper end of each olfactory pit a diverti-
oolum, at first solid, but bood becoming hollow, grows down-
wards to the roof of the pharynx, into which it opens, as the
posterior narss, very shortly after the formation of the mouth
opening.
Fig go, — Half sections in the iransverse plane ot a tadpole lo mm.
long (left half) and of a tadpole is mm. long (righi half]. X 35-
BF, fore-brain ; OD, outer wall of optic cup (pigment layer of adult
lelina); OC, inner wall of oplio cup (remainder of adult retina);
OL, lens, attached lo epiblast in younger tadpole, but forming a
hollow vesicle at the later stage ; TP, pharynx ; Q, sucker, [G. H. T.]
The Eye. The eye differs from the. other sense organs, inas-
much as the lens alone is formed directly from the surface epi-
blast, while the sensitive part of the eye, or retina, arises as an
outgrowth from the brain. The optic vesicles have already been
described ae arising at a veiy early period as lateral outgi-owths
from the fore-brain ; these soon become constricted at their
necks so as to be omnected with the brain by narrow stalks,
which ultimately become the optic nerves.
The outer surface of each optic vesicle, which is at first in
close contact with the suriace epiblast, soon becomes flattened
(Fig, 30, left half), and then thickens so greatly as almost to
THE SENSE ORGANS 125
obliterate the cavity of the vesicle. At the same time a thick-
ening of the deeper or nervous layer of the surface epiblast
takes place opposite the optic vesicle ; this grows rapidly and
forms a spherical body, projecting inwards from the surface ;
this is at first solid, but soon becomes hollow and breaks away
completely from the surface epiblast ; it becomes later on the
lens of the eye, and may be spoken of as the lens vesicle.
Partly in consequence of the ingrowth of the lens vesicle,
and partly through growth of the optic vesicle itself, this latter
becomes pitted on its outer surface, and so converted into a cup
— ^the optic cup — with double walls ; the inner wall being the
thickened and originally outer wall of the optic vesicle, and the
outer wall of the cup being the original inner or deeper part of
the wall of the vesicle. The lip of the cup is incomplete below,
owing to the presence of a slit, the choroidal fissure, through
which mesoblastic elements penetrate into the interior of the
eye.
From the optic cup and lens vesicle the adult eye is derived
in the following way : The lens becomes solid, owing to thicken-
ing of its inner wall, which proceeds so far as to finally oblite-
rate the cavity. The optic cup enlarges considerably ; it remains
in contact with the lens at its edge or lip, but elsewhere is
separated from it by a space which becomes the posterior
chamber of the eye, and in which the vitreous humour is
formed. The inner wall of the optic cup gives rise to the
retina, the rods and cones growing out from its outer surface ;
while the outer and thinner wall of the optic cup forms the
layer of pigment cells in which the rods and cones are imbedded.
The choroid and sclerotic coats are formed from the mesoblast
surroimding the optic cup.
The eye develops very slowly, and throughout the tadpole
stage of existence is in a very rudimentary and imped:ect
condition.
The Ear. The ears are developed as a pair of pit-like in-
vaginations of the nervous layer of the epiblast at the sides of
the hind-brain. The invaginations do not involve the epidermic
or siu*face layer of the epiblast, so that the auditory pits do not
open to the exterior.
The mouths of the pits very early narrow and close ; and the
auditory vesicles so formed separate from the epiblast and lie
in the mesoblast at the sides of the head. The vesicle becomes
126 DEVELOPMENT OF TUB FROG
the vestibule of the adult ear ; the Bemicircular canals arising
as outgrowths from it.
Throughout the tadpole stage of existence there is no further
modification ; but shortly after the metamorphosis the hyoman-
dibular cleft, which has at no period opened to the exterior,
is stated to widen somewhat and form the Eustaduan passage,
while the layer of integument closing its outer end becomes the
tympanic membrane. There is some reason, however, for
thinking that the Eustachian passage develops independently
in the frog, and not from the hyomandibular cleft. The colu-
mella, which has been described with the skull, is formed still
later. ((7/. Fig. 10, p. 48.)
Special Sense Organs. During the tadpole stage, while the
animal is leading an aquatic life, special sense organs in the
form of small epidermal papillse, supplied by branches of the
trigeminal and pneumoga^c n^, are found arnmged in
rows along the body, and round the eyes, and in other parts of
the head. These are lost at the time of the metamorphosis.
The mouth of the tadpole is also provided with small rounded
papillse, which are probably organs of taste. (See Fig. 29.)
H. Development of the Alimentaiy Oanal.
The alimentary canal is developed in three lengths : (1) the
mesenteron, which is formed by splitting apart of the yolk-cells
as described above ; this gives rise to nearly the whole length
of the alimentary canal ; and from it are developed the gill slits,
the lungs, the thyroid, the liver, the pancreas, and the bladder;
as well as the notochord; (2) the stomodsBum, which is a
pitting-in at the anterior end of the body, from which the
mouth and pituitary body are formed ; and (3) the proctodseum,
which is a similar pitting-in at the hinder end of the body to
form the anal or cloacal opening.
From the mode of their formation it follows that the mesen-
teron is lined by hypoblast, and the stomodseum and proctodseum
by epiblast.
1. The mesenteron. The early development of the mesen-
teron has already been described.
The anterior end of the mesenteron, in the head region, is
considerably dilated from the first : and at the hinder end of the
embryo a similar, though much smaller, expansion takes place.
In this way (c/*. Fig. 27), the mass of the food-yolk becomes
THE ALIMENTARY CANAL 127
confined to the ventral portion of the body region, not extend-
ing into either the head or the tail.
The hsrpoblast, which is a definite layer of cells, at first con-
fined to the roof of the mesenteron, gradually spreads round its
sides until it encloses the whole of the food-yolk, and the
alimentary canal is completed as a tube, which from the first is
slightly convoluted. When the tadpole begins to feed, the
alimentary canal lengthens rapidly, and becomes coiled in a
spiral manner. Except at the anterior end, in the gill-bearing
region, it is of approximately uniform diameter throughout.
During the metamorphosis, the alimentary canal shortens con-
siderably, and the distinction between stomach, small intestine,
and large intestine, is definitely established.
The liver is recognisable at a very early stage (Fig. 27) as a
ventral and backwardly directed diverticulum of the anterior
part of the mesenteron, forming the anterior boundary of the
mass of food-yolk. In the later stages the walls of the diver-
ticulum thicken, and become thrown into folds between which
the vascular mesoblast makes its way : the diverticulum itself
persists as the bile duct, and the gall bladder arises as an out-
growth from this.
The pancreas is developed as a pair of hollow outgrowths
from the mesenteron, behind the liver : in the later stages the
ducts shift so as to open into the bile duct instead of directly
into the intestine.
The bladder arises shortly before the metamorphosis as a
ventral outgrowth from the hinder end of the mesenteron, which
soon becomes bifid at its distal blind end.
The post-anal gut is an extension of the hinder end of the
mesenteron into the base of the tail, which appears as this latter
is developed : it becomes solid after a short time, and later on
disappears altogether. It is perhaps to be regarded as formed
by a mechanical drawing out of the intestine by the outgrowing
tail.
The lungs. Immediately behind the gill-bearing region or
pharynx, the alimentary canal narrows very considerably ; its
sides become folded inwards, and the two folds meeting each
other divide the canal into a dorsal tube or oesophagus, and a
ventral one which forms the laryngeal chamber: from this
latter the lungs arise as thin-walled lateral outgrowths. They
appear first in young tadpoles of about 8 mm. length, i.e., some
128 DKVBLOPMENT OF THE FROO
time after hatching, but shortly before the opening of the
mouth. About the time that the lungs first appear, in tadpoles
of about 8 mm. length, the oesophagus, which up to this time
has been tubular, becomes solid, and remains so until a short
time after the formation of the mouth. The meaning of this
curious point has not been ascertained.
2. The stomodflBHin. At the stage represented in Fig. 27,
shortly after closure of the neural caiial, a conical ingrowth, P,
of the nervous layer of the epidermis is formed at the anterior
end of the body immediately below the fore-brain : from this
ingrowth the pituitary body is developed, and a slight depres-
sion of the surface epiblast opposite its base, marks the position
of the stomodseum.
At the time of hatching, this depression is a small shallow
pit, separated from the anterior end of the mesenteron by a thin
septum. Soon after hatching, in tadpoles of about 9 mm.
length, this septum becomes perforated, and the alimentary
canal communicates with the exterior through the stomodaeal
pit. After the perforation is effected, the lips with the whole
anterior part of the face grow forwards rapidly ; the homy jaws
are formed, and the tadpole begins to feed vigorously, {fif. Figs.
28 and 29.)
The pituitary body (Figs. 27 to 29, P) is formed from the
ingrowing stalk of epibla^ described above: this rapidly
elongates, growing backwards between the brain and the roof
of the mesenteron imtil it reaches the inf undibulum ; its hinder
end now becomes tubular, gives off a few lateral diverticula ;
separates from the stalk, which soon disappears, and becomes
applied to the ventral surface of the hinder end of the inf undi-
bulum to form the pituitary body.
3. The proctodseum or anal invagination appears before the
stomodseum. Shortly before the neural folds have met to
form the neural tube, the proctodseum is visible as a small
median depression of the epiblast at the hinder end of the
embryo, a little way below the blastopore. The cells lining it
are rather strongly pigmented, and slightly larger than the
surroimding epiblast cells.
From the hinder end of the mesenteron a rectal diverticulum
(Fig. 27, R) extends downwards towards the proctodaeum; a
little later, and some time before the tadpole hatches, the two
structiu*es meet ; perforation occurs ; and the definitive anal or
_ _ ,_ . . , ^ ._. and fourth branchial
arcbes ; cH, nolochord; c P, peiiau'diil cavity ; Eit, anterior vertical
semidrcijar canal ; E F, efferent blood-vessel of second branchial arch ;
IH, boriiontal, semicircular canal ; er, recessm vestibuU ; EV, vesti-
bule <rf car ; qi, opercular gills ; HO.a, second branchial cleA ; lo,
iaryageal cbiambeT ; LT, gkniis ; LY, Ipnphatic space ; op, opercular
cavity; ra, auricleof heart; RV,venliKlei tp, pharynx; v. 4, fourth
ventricle; r' ' '"" -' " — """ — "~"'" -" —
THE GILL CLEFTS AND ARCHES 129
cloacal opening is formed. For a short time the blastopore
and the proctodseum are both open; but very shortly after
completion of the proctodseum the blastopore closes finally.
I. The Gill Olefts and Arches.
Some little time before the tadpole is hatched a series of
vertical ridge-like thickenings appear on the sides of the head
and neck. These are the viscersd arches, and are six in number
on each side.
The most anterior is the mandibular arch, and gives rise
later on to the lower jaw; the second is the hyoid arch;
and the succeeding four are the first, second, third, and fourth
branchial arches respectively.
About the time of hatching the external gills grow out as
branching and richly ciliated processes from the outer surfaces
of the first and second branchial arches, and a little later from
the third branchial arches as well (Fig. 31).
At the same time, the hypoblastic epithelium at each side of
the buccal cavity becomes thrown into folds, which extend out-
wards towards the surface of the neck as paired outgrowths, lying
between the visceral arches. Of these outgrowths or pouches,
which are known as visceral clefts, there are five on each side.
The most anterior one is the hyomandibular cleft, and lies
between the mandibular and hyoid arches : its outer end lies
very close to the surface of the neck, though it does not actually
open to the exterior.
The four hinder visceral clefts perforate the skin about the
time of formation of the mouth opening, t.e., in tadpoles of about
9 mm. length, and open to the exterior as the gill defts. These
are slit-like openings lying between the hyoid and first branchial,
first and second branchial, second and third branchial, and third
and fourth branchial arches respectively ; and are known as the
first, second, third, and fourth branchial clefts.
From the hyoid arches a pair of opercular folds arise, which
grow back over the external gills, and the branchial arches and
clefts. The two opercular folds meet below the neck in the
mid-ventral line, and enclose the gills in a branchial chamber.
The hinder borders of the opercular folds fuse with the body-
wall except at one place on the left side, where a spout-like
opening remains, through which the branchial chamber con-
municates with the exterior.
130 DEYELOPMKNT OF THE FROG
As the opercular folds develop, the first formed external
gills gradiiaily shrivel up, and are replaced by a second set
enveloped by the ectoderm on the lower and outer side of
the arches. These latter are delicate thin-walled vascular
tufts, arranged in a double row along the ventral half of
each of the first three branchial arches, and in a single row
on the fourth branchial arch.
The inner borders of the branchial arches are thickened,
and produced into processes which unite to form a kind of
filtering apparatus, or sieve, through which the water, taken
in through the mouth or nose, is strained before being passed
over the gills into the branchial cavity and so out.
E. The Vascular System.
The heart is at first a straight tube developed in the meso-
blast of the ventral wall of the pharynx. This soon lengthens,
becomes twisted into an S shape, and divided by transverse
constrictions into chambers. (Of, Figs. 28, 29, and 32.) The
auricle is at first single, but later becomes divided by the
downgrowth of a septum from its dorsal wall.
While the tadpole is breathing by means of gills, its circula-
tion is in all essential respects that of a fish. The venous blood,
returned from the body generally, enters the posterior end of
the heart, or sinus venosus : from this it passes into the second
or auricular chamber, thence to the ventricle, and from that to
the truncus arteriosus. From this latter arise on each side the
aortic arches, which carry the venous blood to the gills to be
aerated : from the gills the blood is collected by efferent
vessels, which unite above the alimentary canal to form the
dorsal aorta, which by its branches distributes the arterialised
blood to all parts of the body.
1. The Oirculation during the time the tadpole is breathing
by its external gills.
The arrangement of the bloodvessels, and the course of the
circulation in a 6j^ mm. tadpole, at a time when the external
gills are in full activity, is shown in Figs. 31 and 32.
The truncus arteriosus, on reaching the anterior end of the
pericardial cavity, divides at once into right and left branches.
Each of these again divides into two, the afferent vessels for the
first and second branchial arches, AF^ and AFgy which carry
THE TASOUUB STSTEM 131
blood into tbe eztomal gills and their bmnches : horn these the
blood passes through short wide capillaiy loops into the efferent
branchial vessels, EFj and EF^ which cany it, now aerated, to
F1G.31.— DiagnimniBticfipireof Itw'iea'land fore pari of the body
of a 6^ mnL udpole, thowing ihe Biraageintiil of (be branchial vessels
u leeu from the ventral snrlace. Tbe heart bas been removed, x 33.
A, donol Boita ; AF|, AFi, AFg, aflrmt branchial vessels of the
fiisl, second, and Ibird brancblal aiches; AP, pulmooary artery;
commissuraJ artery ',
J ciicle formed by these
le carotid arteries suirounds the infundi-
.. . .. . _. „ jFi, EFii EF,, eflfcrent bcouchial vessels of
the first, second, tbird, and fourth braiichlBl arches ; EH, efferent
byoldean vessel; EM. efferent mandibular vessel | OE, external gilii
GM, glomerulus; KA, seg:mental or arcbinephric duct; KP, head
kidney or pronephros ; K81. K81, first and third nephinstomes of
proDephroa ; RT, tnincus arteriosus.
the dorsal aorta in the roof of the pharynx. The dorsal aort»
of tbe two sides run forwards as the carotid arteries, AC, to
supply the head and brain, and abo run backwards in the roof
(rfthe pharynx, the aorte of the two sides meeting and uniting
132
DBVELOPMENT OF THE FROO
about the junction of head and body to form the single
systemic aorta which supplies arterial blood to all parts of the
body.
Besides the complete sets of afferent and efferent branchial
vessels in the first and second branchial arches, similar vessels,
EF4 EF3 EF.^ ^^' ^" AB
AF.3 AF.2 /^p.
• VK
Fkj. 32. — Diagrammatic figure of the head and fore-part of the body
of a 6f mm. tadpole, showing the heart, aorta, and vessels of the
branchial arches from the right side. The external giUs have been
removed, x 40.
A, dorsal aorta ; AB, basilar artery ; AC* carotid artery ; AFi, AF3,
AF3. afferent branchial vessels of first, second, and third branchial
arches; AP, pulmonary artery; AR, anterior cerebral artery; AT,
anterior palatine artery; EFi, EF2. EF3, EF4, efferent branchial vessels
of first, second, third, and fouith branchial arches; EH, efferent
hyoidean vessel ; EM, efferent mandibular vessel ; GM, glomerulus ;
LV4, lacunar afferent vessel of fourth branchial arch ; RA, auricle ;
RV, ventricle ; VD. Cuvierian vein ; VH, hepatic veins ; VK, vein of
sudcer ; VY, hyoidean vein ; YMi mandibular vein.
as yet incompletely developed, are present in the hinder arches
as well.
In the third branchial arch, there is a short afferent branch,
THE VASCULAR SYSTEM 133
AFg, from the afferent vessel of the second branchial arch,
which as yet ends blindly. There is also a well-developed
efferent vessel, EFg, which opens into the dorsal aorta.
In the fourth branchial arch there is no afferent vessel, but
an efferent vessel, EF^, is present, opening into the dorsal
aorta. From this efferent vessel, just before it reaches the
aorta, a backwardly directed branch arises, which will become
later the pulmonary artery, A P.
In front of the first branchial arch, vessels are present in the
hyoid and mandibular arches, which clearly belong to the same
category as the branchial vessels, but which never attain full
development, probably owing to the fact that no gills are
formed on these arches. Efferent branches, EH, EM, open-
ing into the dorsal aorta, are present in both hyoid and
mandibular arches ; but these have no connection with the
heart, as there are no afferent vessels corresponding to them.
The condition of the bloodvessels, while the tadpole is
breathing by external gills, may be summarised thus : — Com-
plete systems of afferent and efferent vessels, connecting the
heart with the aorta through the gill capillaries, are present
in the first and second branchial arches, and at a stage slightly
later than that shown in Fig. 32 in the third branchial arch as
well. A similar set of vessels, but incomplete, is present in the
fourth branchial arch : and vessels formed on the same plan,
but still less complete, and showing signs of degenerative
changes, are present in the hyoid and mandibular arches.
There are thus six sets of branchial vessels on each side of
the pharynx : of these, three, in the first, second, and third
branchial arches, are complete ; one, in the fourth branchial
arch, is incomplete ; and two, in the hyoid and mandibular
arches, are rudimentary.
2. The Oirculation daring the time the tadpole is breathing
by its opercular gills.
On the formation of the gill -slits, additional loops of com-
munication are formed in the gill tufts between the afferent
and efferent vessels of the first, second, and third branchial
arches, and also a series of similar loops between the afferent
and efferent vessels of the fourth branchial arch. The vessels
in the hyoid and mandibular arches undergo further retrograde
changes, and need not be described in detail.
DEVELOPMENT Or THE FBOQ
n the ventral mriacc
ihial vessels, and the
ia about double tbe
A, doisal aoita ; AFi, AFi, alTerctit branchial vessels of first knd
third broDchial arches ; AL, lingual aitei; ; CQ, carotid gland ; EA,
ninctioa between afferent aud efferent branchial vesiets of first
Irancbial arch ; EFi, EFii eRerent branchial vessels of first and third
branchial arches; OM, glomerulus; KA, archinepbric or segniental
duet; KM, Wolffian tubules; KP, pronephrta or head kidney;
K6ii K8i, GisC and third nephrostomes of bead kidney; LI, upper
lip; LJ. lover Up ; LP, hind limb ; OA, aperture of opercular cavitjp ;
CH>i opercular cavity; r8, sinus venosus; RT, tnuicus arlerioatis ;
— . . — , — .. jt short ;TB teclal»pout.
THE VASCULAE SYSTEM
135
In tadpoles of 12 mm. length, in which the opercular gills
are fully established, and the external gills shrivelling up, the
condition of the bloodvessels is shown in Figs. 33 and 34
The truncus arteriosus divides at once intx) right and left
branches, which run straight outwards in the floor of the
A GM AB AU EPj Kz CP eEi pa
MM// / r^
VD AF.+ R6 R'^ ^"^
AEi RT
Fig. 34. — ^A diagrammatic figure of the head and neck of a la mm.
tadpole from the ri(;ht side to show the heart and branchial vessels.
The gills and the gill capillaries are not represented, x 35.
A, dorsal aorta; AB, basilar artery; AFi, AFs> AF4, afferent
branchial vessels of first, second, and fourth branchial arches ; AL,
lingml artery ; AP, pulmonary artery ; AR, anterior cerebral artery;
A8, posterior palatme artery; AT, anterior palatine artery; AUi
cutaneous artery ; AY, pharyngeal artery ; CA, anterior commissural
vessel ; OQ, carotid gUmd ; CP. posterior commissural vessel ; EFi,
EF3, EF3, EF4, efferent branchial vessels of first, second, third, and
fourth Inranchial arches ; QM, glomerulus ; RA, right auricle ; RB,
left auricle; RT, truncus arteriosus; RV, ventricle; VO, Cuvierian
vein ; VH, hepatic vein ; VI, posterior vena cava ; VP, pulmonary vein.
pharynx. Each of these branches divides, after a short course,
into three vessels, and the hindmost vessel again into two. In
this way the four afferent branchial vessels, A Fj, A Fjf A Fg, A F^,
136 ■ DBVELOPMENT OF THE FROG
of the first, second, third, and fourth branchial arches respectively
are formed.
Each afferent vessel runs outwards and upwards in its own
arch. The efferent branchial vessels lie immediately in front
of the corresponding afferent vessels, with which they are
connected by very numerous capillary loops in the substance of
the internal gills, and not shown in the figures. At their upper
ends the efferent vessels open, as before, into the dorsal aorta,
Fig. 34.
The venoTis blood in the heart is driven by the contraction of the
ventricle into the truncus arteriosus, and then along the afferent
branchial vessels, through the capillary loops of the gills, in
which it gets aerated, to the efferent branchial vessels; and
thence to the dorsal aorta, and so all over the body.
The limgs are by this time of considerable size : they receive
blood by the pulmonary arteries, AP, which, as already noticed,
are branches from the efferent vessels of the fourth branchial
arches, and therefore contain blood which has already passed
through the gill capillaries. The blood from the lungs is
returned direct to the heart by two pulmonary veins which
unite and open into the left auricle, the single auricular cavity
of the earlier stage being by this time divided by a vertical
septum into right and left auricles.
One other point of great importance remains to be noticed
in the arrangement of the branchial vessels of the tadpole.
The afferent and efferent vessels of each arch at first com-
municate only through the gill capillaries : but in tadpoles of
about 12 mm. length each efferent vessel becomes directly
connected at its ventral end with the corresponding afferent
vessel. Fig. 34. These direct connections are situated ven-
traUy to the gills, so that the blood in any one of the
afferent branchial vessels has two paths open to it: it may
either (1) continue along the afferent vessel, and then reach
the efferent vessel by passing through the connecting loops
afforded by the gill capillaries ; or (2) it may pass at once
into the efferent vessel through the direct communication,
and so reach the dorsal aorta without having passed through
the gill at all.
So long as the tadpole is breathing by gills, these direct com-
munications between afferent and efferent vessels, though
present in all four branchial arches, are so small that practi-
THE VASOULAB SYSTEM 137
cally no blood passes through them, and all the blood is com-
pelled to pass through the gills to reach the aorta.
3. The Ohanges in the Oirculation at the time of the Meta-
morphosis.
-At the time of the metamorphosis, however, when the
anterior limbs are protruded, and the tail begins to shorten,
these direct communications enlarge, so that an increasing
amount of blood takes the direct short passage, and reaches the
aorta without having passed through the gills. Additional
work is thus thrown on the lungs and skin, which consequently
receive a larger supply of blood: the gills rapidly atrophy,
though remnants of them usually persist, in a f unctionless con-
dition, until the end of the first year ; and the change from the
gill-breathing to the air-breathing condition is completed.
The further changes necessary to convert the circulation into
that of the adult are slight. Of the four aortic arches present
at the metamorphosis (Fig. 34), the first, in the first branchial
arch, persists as the carotid arch of the adult frog ; the lingual
artery is a branch from the ventral end of the efierent vessel of
the arch, and is present from an early stage of development
(Fig. 33) ; and the external and internal carotid arteries are
already present. The carotid gland, CG, is not, as sometimes
stated, a persistent portion of a gill, but is formed by further
elaboration of the direct communication between the afferent
and efierent branchial vessels of the first branchial arch.
The second aortic arch, in the second branchial arch, becomes
the systemic arch of the frog. Its dorsal end remains connected
with the carotid arch, though the connection may in the adult
become closed and ligamentous. {Of. Fig. 5, p. 29.)
The third aortic arch, in the third branchial arch, loses its
connection with the aorta, and finally disappears altogether.*
The fourth aortic arch, in the fourth branchial arch, also
* According to another view, of fairly general acceptance the follow-
ing is the scheme of arterial arches in the frog :
Arch.
Embryonic.
Adult.
I.
Mandibular
1
II.
Hyoidean
?
III.
First branchial
Carotid arch.
IV.
Second branchial
Systemic arch.
V.
Third branchial
Cataneous artery.
VI.
Fourth branchial
Pulmonary artery.
138 DSYELOPMENT OF THE FBOG
loses its connection with the aorta, but persists as the pulmo-
cutaneous arch of the adult, from which both pulmonaiy and
cutaneous arteries arise.*
L. Development of the Moscnlar System and the Ckslom.
The splitting of the mesoblast into outer or somatopleuric,
and inner or splanchnoplenric layers has already been described.
(C/.Fig. 26,p. 118.)
In the body the mesoblast becomes veiy early divided on each
side into (1) a vertebral plate, which is dorsally situated, and
lies alongside of the spinal cord and notochord; and (2) a
lateral plate, which surrounds the side of the body.
The vertebral plate very early becomes divided transversely
into muscle-segments or myotomes, which form a row of hollow
and somewhat cubical bodies, lying along each side of the spinal
cord, and separated from each other by connective tissue septa.
Later on, the walls of the myotomes thicken considerably,
especially the inner walls, and become converted very largely
into muscles ; while the cavities become obliterated.
The myotomes may be well seen in the tail of the tadpole,
where they form the great lateral sheets of muscle on each side
of the tail, by which the swimming movements are effected.
Owing to the transparency of the tiol, their arrangement can
be very readily made out ; the septa dividing the successive
myotomes from each other are not transverse, but > shaped,
with the angles directed forward towards the head.
The lateral plates are also in part converted into muscle ;
the two layers, somatopleuric and splanchnoplenric, remain
comparatively thin, but the space between them widens out
considerably, and becomes the body cavity or coelom. This at
first consists of two separate halves, right and left ; but, owing
to the splitting of the mesoblast extending down to the mid-
ventral line, the cavities of the two sides soon became con-
tinuous. The anterior portion of the coelom is very early shut
off from the hinder part as the pericardial cavity. {Of, Figs.
28 and 29.)
The outer or somatopleuric layer of mesoblast, with the epi-
blast, forms the body-wall of the adult ; the inner or splanchno-
plenric layer, with the hypoblast, forms the wall of the
alimentary canal and its diverticula. The cells covering the
* See note, p. 137.
THS SKSLfiTON 139
free sorfaoes of both layers, t.e., the cells lining the body cavity,
become the peritoneum, or coelomic epithelium, from which, 6a
we have already seen, the ovaries and testes are formed.
M. Development of the Skeleton.
1. The Vertebral Ooliimn.
The earliest skeletal structure, and for a time the only one,
is the notochord, the development of which from the hypoblast
of the mid-dorisal wall of the mesenteron has already been
described. It forms a cellular rod extending from the blasto-
pore to the pituitary body ; and as the tail is formed, it extends
back into it. The notodiord consists of vacuolated cells, filled
with fluid, and is invested by a delicate structureless sheath.
About the time of appearance of the hind legs, a delicate
skeletal tube, at first sort but soon becoming cartilaginous, is
formed round the notochord from the mesoblast. This tube
grows upwards at the sides of the spinal cord, as a pair of longi-
tudinal ridges, with which a series of cartilaginous arches, which
appeared at the sides of the spinal cord at a slightly earlier
stage, very soon become continuous.
By the appearance of transverse lines of demarcation, the
cartilaginous sheath of the notochord becomes cut up into a
series of nine vertebrse, followed by a posterior unsegmented
portion, which later becomes the urostyle. This transverse
division does not affect the notochord, which remains as a
continuous structure until the complete absorption of the tail
at the end of the metamorphosis.
Shortly after the metamorphosis thin rings of bone, slightly
constricted in their centres, so as to be hourglass-shaped in
section, are developed in the membrane investing the cartila-
ginous sheath of the notochord : these correspond with the nine
vertebrse already present, and form the first rudiments of the
vertebral centra.
In the intervertebral regions, between the successive bony
rings, annular thickenings of the cartilaginous sheath occur,
which grow inwards so as to constrict and ultioiately obliterate
the notochord. Each of these intervertebral rings becomes,
after the metamorphosis, divided into an anterior and a posterior
portion, which fuse with the bony centra of adjacent vertebrae,
and ossify to form their articular ends.
From the circumference, and from the articular ends of each
140 DEVELOPMENT OF THE FROG
vertebra, ossification gradually spreads inwards; but a small
portion of notochord persists in the middle of each centrum
for a long time, or even throughout life.
The vertebrae are not placed opposite the myotomes, but
alternate with these ; so that each vertebra is acted on by two
myotomes on each side, one pulling it forwards, and the other
backwards.
The transverse processes are at first independent of the
corresponding vertebrae, but very early fuse with them. They
extend into the septa between the myotomes, and probably
correspond to the ribs of other vertebrates.
The urostyle is the part of the axial skeleton behind the
vertebras; it is not divided into vertebrae at any stage in
development.
The anterior end of the notochord, imbedded in the base of
the skull, is gradually encroached on by the cartilage and bone
around it, and ultimately completely absorbed.
2. The SknU.
The skull of the tadpole consists almost entirely of cartilage ;
none of the bones of the skull, with the exception of the para-
sphenoid, appearing until nearly the time of the metamorphosis.
In the adult frog, this cartilaginous skull is replaced to a con-
siderable extent by cartilage-bone; while other bones primitively
distinct, and probably of dermal origin — ^the membrane-bones —
graft themselves on to it.
The three morphologically distinct elements of which the skull
consists {cf, p. 48) may with advantage be described separately.
a. The Oranimn or brain case. This in its fully formed con-
dition is an unsegmented cartilaginous tube, enclosing the brain:
it is developed as follows :
In the front part of the head a pair of longitudinal cartilagi-
nous bars, the trabeculaB cranii, appear in tadpoles of about 10
mm. length : these grow back alongside of the notochord as a
pair of horizontal parachordal rods.
The hinder ends of the trabeculse are some little distance
apart, and between them is a space in which the pituitary body
lies. In front of this pituitary fossa, the trabeculae unite to
form a plate of cartilage, which underlies the anterior end of the
brain, and is produced into blunt processes at its outer angles.
The parachordals grow rapidly : they extend inwards M) as to
FlC. 34 BIS.
A. — The skull of a 12 mm. tadpole, seen from the right side. X30,
B.— The saiae skull from the dorsal surface. The lower jaw and the
hyoidean and branchial ban are omilled. x 30,
C. — Tbe same skull from the ventral surface, x 30.
BB, basibrancbial ; bh, roof of hind train; bm, roof of mid-brain ; en.i,
BH.a, BR>3. BFt.4, fint, second, third, and fourth branchial ban ; b9, cerebral
hemisphere; lh, notochord ; ec, audiloif capsule; He, Insihral; ho,
urohyal ; HQ, articulation of ceralotayaJ with quadrate; hb, ceralohral;
JL, lower jaw; ju, upper jaw ; Li, upper lip; lJ. lower lip; ll, lower labial
cartilage; lu, upper labial cartilage; MC, Meckel's cartilage; pn, pineal
body; Oi quadrate ; qo, orbital process of quadiate ; qp, palalo-pterygoid
process; gn, connection of quadrate with irabecula ; rc, parachordal
cartilage; Sl, trabeeula cranii ; s*. membranous patch in the outer wall of
the auditory capsule, in which the stapes is developed at a slightly later stage:
X, choroid plexus of third veulricle.
To /a« A 141.
THE SKULL 141
meet each other hoth ahove and below the notochord, which
they now completely surround. The two parachordals soon fuse
together to form the basilar plate, which, with the trabeculse,
forms a firm cartilaginous floor to the brain case. At their
hinder ends the parachordals grow upwards to form the side
walls of the cranium, and a little later bend inwards so as to
meet each other above the brain, and complete the occipital
part of the cranium. Further forwards the pituitary foramen
becomes closed by a thin plate of cartilage, and the lateral
margins of the parachordals and trabeculse grow upwards so as
to form the side walls of the skull, the roof remaining im-
perfect in this region.
The first bone to be developed is the pai*asphenoid. The
exoocipitals, the frontals and parietals, .which are at first
separate, and other bones soon follow ; and by the time the
metamorphosis is complete and the tail absorbed, all the bones
of the adult cranium are present, except the sphenethmoid,
which does not appear till some months £tter.
b. The Sense Capsules. The cartilaginous auditory capsules
appear in tadpoles of about 12 mm. length as thin shells of
cartilage investing the auditory vesicles. They are at first
quite independent of the cranium, but before the completion of
the opercular folds they fuse with the upgrowing parachordals
to form part of the side walls of the skull. The pro-otic appears
about the time of completion of the metamorphosis.
The optic capsules are thin shells of cartilage, forming part
of the sclerotic coats of the eyes. They arise about the same
time as the auditory capsules; and, unlike the other sense
capsules, they remain distinct from the cranium throughout life,
in order to secure mobility of the eyeballs.
The olfactory capsules are from their first appearance very
closely connected with the anterior ends of the trabeculse, which
grow up between them to form the median vertical intemasal
septum. They develop later than the auditory and optic capsules.
c. The Visceral Skeleton. This consists of a series of carti-
laginous hoops developed within the visceral arches, and forming
a framework which surrounds and stiffens the walls of the
pharynx. Each hoop consists of right and left halves, which
ai-e independent at their dorsal ends, but fused or closely con-
nected ventrally. There are in all six of these hoops or bars
forming the oral (mandibular) arch, hyoidean arch, and the
142 DEVELOPMENT OF THE FROG
four branchial arches respectivelj ; and they develop in order
from before backwards.
i. The oral (mandibular) bar, which is the largest of the
series, lies at first parallel to the others, «.«., perpendicular
to the long axis of the body. It very early, however, under-
goes important changes, and by the time that the external gills
are developed, and before the appearance of the opercular folds,
it has altered its direction, and now runs almost horizontaUy
forwards, parallel to and below the trabecuLe.
It soon unites with the trabecule, both behind and in front
of the eyeball, the latter union being effected by a short trans-
verse bar of cartilage — ^the iialato-pterygoid. Id. front of the
palato-pterygoid, the most anterior part of the oral bar becomes
segmented off as a short rod of cartilage, which is directed
upwards and forwards in the lower lip ; it ii9 known as Aleckel's
cartilage, and forms the basis of the lower jaw or mandible.
That part of the oral bar with which this segment articulates
will give rise to the quadrate of the adult. In connection with
the lips two pairs of small labial cartilages appear, serving to
support the homy jaws of the tadpole.
In the later stages the subocular or quadrate portion of the
oral bar acquires a very close connection at its hmder end with
the auditory capsule, and changes its direction, so that in place
of running horizontally forwards, it now runs downwards and
forwards. This change, which may be described as a rotation
backwards of the bar, causes lengthening of the palato-pterygoid
bar and of MeckeFs cartilage ; these latter become respectively
the basis of the upper and lower jaws of the tadpole, which
are completed later on by the development of the membranous
pterygoid, squamosal, maxilla and other bones.
This rotation backwards of the distal end of the quadrate,
with corresponding lengthening of the upper and lower jaws,
proceeds rapidly during and after the metamorphosis, so that
the quadrate, instead of being directed downwards and forwards,
soon runs vertically downwards, and later on downwards and
backwards as in the adult. {Of. Fig. 10, p. 48.)
ii. The hyoid bar also undergoes important changes. At
first it is a wide band of cartilage placed nearly vertically in
the side wall of the pharynx, immediately behind the oral bar.
When the mandibular arch becomes horizontal the hyoid forms
a broad stout bar of cartilage, articulating at its upper end with
BRA BB BR^ HR 1 MC -
Fig. 34A.-'SkuU of a tailed Frog towards iha end of meta.'
oioipho^s. X13.
BB, baaibnncblal; br.o, br.4, second and fourth branchial bars;
CL, columella ; eo, auditory capsule; h r, ceralobyiil ; mc, Meckel's
cartilage; oc, outline of the ej;e ; 00, olfactory capsule; OL, outline
-• ' ON, foramen for optic nerve ; q, quadrate cartilage ; qE,
Ml of quadrate with auditory capsule ; go, orbital process M
: ; qp, palato-pterygoid process ; ba, stapes.
To fact f. T4a.
THE URINARY SYSTEM 143
the subocular part of the oral har, and connected at its ventral
end with the hyoid bar of the other side by a small median
basi-hyal plate in the floor of the mouth.
At the commencement of the metamorphosis the hyoid bar
becomes narrower, and begins to extend upwards towards the
auditory capsule ; and by the end of the metamorphosis this
upper part of the hyoid has become the long slender anterior
comu of the hyoid, which acquires a loose connection at its
upper end with the cranium and with the quadrate cartilage.
The development of the columella is imperfectly known. It
consists of two elements, one of which — the stapes — is a small
plate of cartilage partially filling a hole, the fenestra ovalis,
which appears in the lower and outer wall of the auditory cap-
sule about the time that the opercular folds are growing back
over the gills. The other portion of the columella is a small
rod, partly cartilage, partly bone, which does not appear till
some months after the completion of the metamorphosis, and
which fuses with the stapes at its inner end, while its outer
end becomes connected with the tympanic membrane (c/1 Fig.
10, p. 48); this outer element of the columella is commonly
regfu:tled as formed from the uppermost part of the hyoid arch,
but appears to be really quite independent of it in the frog.
iii. The branchial bars are at first simple fbittened rods of
cartilage, independent of one another, but becoming early con-
nected with a median basi-branchial cartOage, which appears in
the floor of the mouth between the ventral ends of the first two
pairs of bars.
As the hind-legs appear,the branchial bars of each side coalesce
with one another both at their dorsal and their ventral ends :
they also become strongly curved, and together form a complex
basket-work supporting the gills. Later on, as the gills begin to
shrink, the branchial bars become more slender : their dorsal ends
disappear, while their ventral ends fuse with the basi-hyal and
basi-branchial cartilages, and together give rise to the body of
the hyoid and its posterior comua.
N. The Develoiiment of the Urinary System.
1. Qeneral Account.
The excretory organs of the tadpole, during the early stages
of its existence, are the head kidxkeys or pronephra. These
are a pair of globular organs imbedded in the dorsal wall of the
DEV£LOPMKMT OF THE FROG
body at ita anterior end, immediately behind the constricted
neck region (Figs. 83 and 35 KP). Each head kidney is a
FiO. 35. — A 40 ni n. tadpole dissecled from the venlrat surfice 10
sbow the heart, tbe brancbial vessels, and Ibe head kidneys and
Wolffian bodies. The tail has been cut off. x 5.
A, dorsal aona; AFi, AFj, aiferent branchial vessels of Gisl ajid
third branchial arches; AL, lingual aitery; CQ, carotid gland: EF),
EFj, efferenl branchial vessels of first aod third branchial arches ; F, fat
body; QM, glomerulus; KA, archinephrie or segmental duct; KM,
WoUGati body ; KP, pionephios or head kidney, nnw degeneraling;
LA, fore-Umb, still within opercular cavity ; lI, upper lip ; LJ, lower
lip; LP, hind-limb ; OR. genital ridge; RT, truncus arteriosus; BV,
ventricle ; TC, cloaca ; TO, ccsophagus, cut short ; TR, cloacal spoilt.
THE URINAKY SYSTEM 145
convoluted tube with glandular walls, opening into the body
cavity by three ciliated mouths or nephrostomes (Fig. 88, KS),
and continued back along the dorsal wall as the archinephric
or segmental duct, KA, to the hinder end of the body, where
it joins with the corresponding duct of the opposite side, and
opens into the cloaca.
The head kidneys and their ducts are well developed in the
tadpole at the time of hatching: they subsequently increase
considerably in size, and are the sole excretory organs of the
tadpole during its early stages. In tadpoles of about 12 mm.
length the adult kidneys or Wolffian bodies (Fig. 33, KM),
begin to form in the hinder part of the body as a series of paired
tubules, which grow towards and open into the segmental duct.
These Wolfi&an tubules rapidly increase in number, as well as
in size and complexity, and become bound together by connec-
tive tissue to form the compact Wolffian bodies or kidneys of
the fully formed tadpole (Fig. 85, KM). At the same time the
head kidneys diminidi in size, and undergo degenerative changes,
and by the time of the metamorphosis (Fig. 36) have almost
completely disappeared. The Wolffian bodies persist as the
kidneys of the frog ; and by a series of further changes the
ureters and generative ducts of the adult become established.
2. The Head Kidney and its duct.
In tadpoles of about d^ mm. length, i,e,, some time before
hatching, a pair of longitudinal grooves appear along the inner
surface of the somatopleure, extending from the neck to the
hinder end of the body, and lying a little distance to the right
and left of the notochord. The lips of each groove soon meet
and fuse so as to convert the groove into a tube or duct. The
closure of the tube takes place from behind forwards, and at the
anterior end is effected imperfectly, three holes or nephrostomes,
one behind another, being left, through which the tube opens
into the body cavity. As the embryo grows, the anterior end
of the duct becomes convoluted and twisted on itself to form a
ball, the three nephrostomes becoming at the same time
lengthened out into short tubes. This convoluted mass is the
head kidney or pronephros. The hinder part of the duct is
the archinephric or segmental duct ; it remains straight, or
nearly so, and shortly before the tadpole hatches acquires an
opening into the cloaca. I^
146 DETELOPUEMT OT THE rSOQ
At the time of h&tcltiiig, the excretory orgaiiE thus consist on
each side of (1) r head kidney, whiob ia a convoluted tube
lined by a glandular epithelium, and opening into the anterior
end of the body cavity by three ciliated openings, the nephio-
FlG. 96, — A tailed frog, near the close of Ihe melamorphosis,
dissected from tbe ventral surface to show ibe kidnejrs and repro-
ductive OTgajis. X 4.
A, dona.] aorta: F, fat body; QM, glotnemlus: KA, arcbiaepfaric
or s^mental duel; KM. Wolflian body; KP, head Iddnejr, dis-
appeariiiB; KU. oreter; O, moutbi OR. genital ridge; RV. tip of
ventricle; TO, cesophagm. cut short.
TOE DniNART SYSTEM 147
etomes; and (2) the archinephric or segmental dnct, which is
the posterior part of the tube, and runs back along the dorsal
body-wall nearly straight to the cloaca, into which it opens.
The head kidney Is closely surrounded by, indeed almost
imbedded in, the posterior cardinal vein (Fig. 37, VC), and it
is from the blood of this vein that the epithelial cells of the
Pio. 37. — Transverse sectloD through the body of a tadpole at ihe
time of hatching ; tbe section passing tliroiigh the second pair oT the
nephrostomes, and the ihird pair of myotonies. X 50. (From
Marshall's "Verlebrate Embtyology.")
A. aoita ; C. ccelom or body-cavity ; CH, notcxhord ; CJ, sub-
notochordal rod ; QM, glomerulus: KP, segmental 01 archinephtic
duct ; Ka, second nephroslome of left side ; ME, somatopleuric laytzr
of mesoblast ; MH, splanchnopleuiic layer □( mesoblast ; ML, myo-
tome; NL, lateral line branch af pneumogastric nerve; NS, spinal
cord; T, intestinal region of mesenleroa ; VC, posterior cardinal
vein ; VH, hrpatic vein ; w, liver diveiticulum.
head kidney tubules separate the excretoiy matters, which are
then passed down the duct to the exterior.
The head kidney continues to increase in size, the tubnies
becoming still more oonvolated, and lateral diverticula arising
148 DSVBLOPMBMT Of THS FROQ
from their sides, until the tadpole is about 12 mm. in length,
and the hind-limbs are just commencing to appear. It remains
stationary for a time and then, in tadpoles of about 20 mm.
length, begins to degenerate : the tubules become obstructed ;
some of them become collapsed, others for a time irregularly
dilated : the whole organ steadily diminishes in size, and in
tadpoles of 40 mm. (Fig. 85, KP) is not more than half its
former size. It now shrinks rapidly, and at the time of the
metamorphosis (Fig. 36, KP) has almost disappeared, all three
nephrostomes having closed up, and the organ being reduced to a
few small pigmented and irregularly twisted tubules, which have
separated from the duct, and whidi soon disappear completely.
Opposite the head kidney an irregular sacculated outgix)wth,
the f^omemlns, arises from the aorta on each side (Figs. 31
to 37, GM) : this appears first about the time of hatching, and
its development keeps pace with that of the head kidney. It
lies immediately opposite the nephrostomes, and very dose to
these, though not touching them. It begins to diminish in size
about the same time as the head kidney. At the time of the
metamorphosis (Fig. 36, GM) it is very small, and after the first
year it can no longer be recognised. Its close relation to the
head kidney, and the fact that its growth and subsequent
degeneration keep pace with those of the head kidney, point
to a dose physiological connection between the two organs,
though it is not easy to imagine what precise function the
glomerulus subserves.
3. The Wolffian Body.
The Wolffian body, or kidney, first appears in tadpoles of
from 10 to 12 mm. in length. It arises on each side as a series
of small solid masses of mesoblast cells lying along the inner
side of the segmental duct, between this and the aorta (Figs.
33 and 35). They develop from behind forwards, the hindmost
pair being a short distance in front of the cloaca, and the most
anterior ones about three segments behind the head kidney.
These solid masses soon become elongated into twisted rods,
which then become tubular, and growing towards the segmental
duct meet and open into it. At their opposite ends these
Wolffian tubules, as they axe termed, dilate into bulb-like
expansions, which become doubled up by ingrowth of little
knots of bloodvessels, derived from the dorsal aorta, and so
THE UBINABT 8T8TSM 149
form Malpighian bodies. From the necks of the Malpighian
bodies, short solid rods of cells grow towards the peritoneal
epithelium and fuse with it. These rods soon become hollow,
and open into the body cavity by ciliated funnel-shaped mouths
or nephrostomes : their opposite ends break away from the
Wolffian tubules and open directly into the renal veins on the
ventral surface of the kidney. The Wolffian tubules rapidly
increase in number ; they also branch freely, and so give rise to
a complicated system of glandular tubules, which, when bound
together by bloodvessels and connective tissue, form the
Wolffian body or kidney of the frog. The nephrostomes persist :
and in the adult frog as many as 200 or more are present on
the ventral surface of the kidney, as minute funnel-like ciliated
openings, leading by short tubes into the renal veins.
4. The Wolffian and Miillenan ducts.
So far we have only described one duct on each side, the
segmental duct, which acts as the excretory duct first of the
head kidney, and then of the Wolffian body as well. We have
now to consider in what way the ureters and generative ducts
of the adult frog are formed.
About the time of the metamorphosis the head kidney,
which has become rudimentary, separates completely from the
duct, which now ends blindly a short distance in front of the
Wolffian body.
A little later, after completion of the metamorphosis and
entire disappearance of the tail, this anterior end of the
segmental duct, in front of the Wolffian body, becomes divided
somewhat obliquely into two ; an anterior part, which is now
isolated from the Wolffian body, and will be called the
Miillerian duct ; and a posterior part, the Wolffian duct, which
is simply the posterior part of the original segmental duct, and
receives the Wolffian tubules of the kidney.
The Miillerian duct becomes connected in front with the
peritoneal epithelium, and acquires an opening into the anterior
end of the body cavity. At its hinder end it grows back along
the outer side of the Wolffian duct to the cloaca, into which it
opens. So far the changes are the same in both sexes. In the
male ttog the Miillerian duct persists in this condition through-
out life, and may be recognised as a slender longitudinal streEik
lying in the thickness of the peritoneum a short distance to the
ISO DETELOPMIENT OF THX FROO
outer side of the kidney, and extending some distance in front
of it. In the female frog the MiLllerian duct becomes the
oviduct, the anterior opening being carried forward first as a
groove, and then by closure of the lips as a tube, to the position
chariicteristic of the peritoneal opening of the adult oviduct ;
while the posterior part becomes greatly convoluted and acquires
thick glandular walls : the hindmost pe^ of the oviduct remains
thinner walled, but of much greater capacity.
The Wolffian duct becomes in both sexes the meter. In
the female frog it undergoes no further change of importance.
In the male frog the hinder end of the Wolffian duct becomes
dilated into a much-branched glandular enlargement, the
▼esicnla seminalis.
5. The Vasa Efferentia.
In both sexes at an early stage, as the Malpighian bodies
are forming in the Wolffian body, those nearest to the genital
ridges give off tubular branches from their capsules into the
ridges.
In the female frog these tubules are said to expand very
greatly, and to give rise to the chambers or cavities present
in the adult ovary : but the point is not established with
certainty.
In the male frog these tubules become the vasa efferentia :
they become connected with the spermatic tubules, and, as at
their other ends they open into the Wolffian tubules, they form
passages along which the spermatozoa can get from the testis
to the Wolffian duct or ureter, and so out.
CHAPTER IX.
ELEMEITTABT mSTOLOQT.
When examined under the microscope, all the different tissues
and organs of the body are found to consist of elementaiy bodies
called cells and of an intercellular substance, connecting the
several cells together ; in much the same way as a wall is built
of bricks cemented together with mortar. These cells, of which d
white blood corpuscle is a typical example, vary mudi in shape,
size, and structure in different tissues, but are to be considered
as fundamentally equivalent to one another. The intercellular
substance varies very much in quantity; it may be almost
absent, so that the several cells are practiodly in contact with
one another ; or it may be so abundant as to separate them
widely ; it is to be viewed as formed by the cells, and, therefore,
as secondary in importance to these.
When dratmng hiatologiecd preparoMona, it is toeU to look out
for, amd draw, a few red blood corpuscles^ to the same scale as the
rest of the drawing. The blood corpuscles form m^ost useful
standards of msasursTnent, as tlteir dimensions are already known
(p. 87).
A. Epithelium.
Epithelium consists of cells placed side by side so as to form
layers, which form the surface covering, or epidermis, of the
body, and line the alimentary canal, the blood vessels, and the
various internal cavities of l^e body. It may be defined as a
continuous sheet of cells lining a free surface. At the exteinal
apertures of the body, the epidermis is directly continuous with
the epithelial lining of the internal cavities.
The layers may be one or more cells in thickness ; in the
former case the epithelium is said to be simple, in the latter
stratified.
Epithelium is of different kinds, according to the shape and
structure of its component cells.
152 ELEMKNtABT H18T0L0GT
I. Squamons Epithelium. In this the oomponent cells are
flattened parallel to the surface they cover ; if the epithelium
is stratified, the flattening is most marked in the superficial
cells.
a. Isolated Oells.
Scrape gently the inside of yowr cheek trith the handle of a
scalpel, wad put the scrapings on a slide ; cover, and examine
toith a high power ; draw, shiywing the following points :
i. The cells are large, flattened and scale-like in shape,
often slightly curled up at their edges.
ii. The nnclens is oval and granular, and lies near the
middle of the cell ; it may be rendered more dis-
tinct by acetic acid or magenta.
b. Cells in situ ; cast skin of newt.
Take a smaU piece of the prepared specimen, which has been
stained in hoimatoxylin, and then, after treatmeni with ailoohoL,
cleared with oil of cloves. Moimt the specimen in haisam^
cover, and examine with the high power,
L The cells are flattened, and fitted together at their
edges, like a mosaic, to form a continuous layer.
Each cell has a large nucleus near its centre.
II. Columnar Epithelium. This consists of elongated rod-
like cells, placed vertically to the surface on which they rest.
If a columnar epithelium is stratified the columnar character is
most marked in the superficial cells.
a. Isolated cells: from the small intestine of the frog;
isolated by maceration for 24 hours in Banvier's
alcohol, and stained with picro-carmine.
Mov/nt a drop of prepared specimen in glycerine ; paint a
mug of cement rovmd the cover-glass ; amd examine toith the high
power.
L The cells, which often remain side by side in little
groups, are columnar in shape, with nuclei near
their inner or deeper ends.
b. Cells in situ.
Take a prepared section of dog^s stomach which has been stained^
EPITHELIUM 163
avd then cleared in oil of doves. Moumt in balsam, a/nd exemiine
with the high power,
i. The superficial layer consists of long narrow
columnar cells, packed together side by side, with
nuclei at their inner or deeper ends.
m. Ciliated Epithelium. In this the cells, which are usually
columnar, bear at their free ends tufts of exceedingly fine hair-
like processes — cilia — ^which, when living, exhibit active lashing
movements.
a. Isolated cells. From trachea of rabbit : isolated by
maceration for 24 hours in Ranvier's alcohol ; stained
with picro-carmine, and scraped into glycerine.
Motmt a small drop of the prepared specimen in glycerine ;
paint a ring of cement rou/nd Die cover-glass ; eocamine toith the
high power, and note :
L The shape of the cells : their nuclei ; and the tuft
of cilia at one end of each cell.
b. Oells in situ: ciliary moTement.
Snip off a smaU piece of epUheliwm from the roof of the
mouth of afrestdy hiUedfrog, nea/r the eyebaU; m^yu/nt in normal
saU sokbtion, a/nd add a sm»aU d/rop of gamboge water to rcTider
the m4}vements m>ore clearly visible ; exa/mine wiUh the high power :
note:
i. The currents due to the ciliary motion.
ii. The movements of the individual cilia : best seen
when the specimen is beginning to die, and the
movements to slacken in speed.
IV. Stratified Epithelium. This is characterised by the
epithelium being several cells in thickness.
Take a prepared section of oesophagus of rabbit, or of conjvmc-
tiva of rabbit or pig, which has been hard&iMd in chromic cund,
stained, and cleared in oU of cloves. Mount in balsam, examine
with the high power, and note :
i. The stratification of the epithelium.
ii. The transition from the deeper spherical or columnar
cells to the superficial squamous cells.
164 ELEMENTARY HISTOLOGY
B. Glands.
A gland consists essentially of a layer of epithelial cells
secreting some special fluid. The epithelial surface may be flat,
but is more usually folded or pitted, often in a very complicated
manner, so as to increase the extent of the secreting surface.
L Simple Glands. In simple glands the epithelial surface
is increased by simple pit-like depressions, whose mouths serve
to discharge the secretion on the free surface.
Take a prepared section of large integtine of rabbit which has
been hardened in chromic acid, stained, and cleared in oil of
cloves, Motmt in balsam, and examine first with the low power ^
then with the high. Note the following points :
i. The glands are simple tubular depressions of the
surface,
ii The glandular epithelium lining the pits is a single
layer of short columnar granular cells, many of
which are swollen to form goblet cells.
n. GomiK)iind Glands. In compound glands each depression
instead of being a simple pit is itself subdivided or branched,
often in a very complicated manner. There are two chief
varieties: (1) tubular glands, in which the several sub-
divisions are tubular, and of tolerably uniform diameter
throughout : and (2) racemose glands, in which the blind ends
of the pits are dilated into globular chambers or alveoli, to
which the special glandular epithelium is usually confined.
a. Compound tabular glands. Take a prepared section of
kidney of frog. Examine under a hand lens, and draw on
a large scale. Notice thai the dorsal surface may be
distinguished from the ventral.
i. The tubular gland cavities are cut at various angles.
If cut transversely a tube appears as a circular
ring : if cut obliquely, as a more or less elongated
elliptical ring: if cut longitudinally, as two
parallel rows of epithelial cells.
ii. The gland cells form a single layer of cubical granular
cells, lining the tubes.
iil The Malpighian bodies are spherical dilatations on
the tubes, into which project little knots of capillary
Fig. 38, — Section Ibrough mucous membrane of (be cardiac end of
a dog's stomach, x 140.
B, bloodvessel; CC. cubic or peptic cells; CM, cohimnii
□void cells ; OB, fundus or bottom of gland cavity ; CK, eland
Cit across ; QM, mouth of gland ; MC, circular muscle Snces;
glludiaol muscle fibres ; P, connective tissue layer between
158 ELEMKNTARY HISTOLOGY
delicate elastic sheath — the sarcolemma — which
will be visible in but few cases; it is most
readily seen at places where the fibre has been
torn across.
li. The alternate light and dark bands with which
the muscle fibres are marked transversely,
and from which the name, striated muscle, is
derived.
ill. The readiness with which the fibres split up
longitudinally into fibrils.
b. Frog's mnscle. Shred gently apiece of fresh frog^s muscle
in normal salt soltdion : cover, and examine with the
high power: note:
i. The transYerse striations.
ii. The sarcolemma : best seen by slightly crushing
the specimen.
iii. The nuclei in the fibres: seen on addition of
acetic acid.
n. Non-striated, or InYolnntary Mnscle.
Take a prepared specimen of fro^s bladder which has been
macerated in Ranvier^s alcohol for 24 howrs ; pencilled with a
fine brush to remove the epithelium of the inner surface ; stained^
and cleared with oil of cloves. Mount in balsam^ and examine
with low and high powers : note :
i. The bands of muscular fibre.
ii. The formation of each band by a number of
elongated, fusiform, nucleated muscle-cells.
iii. The absence of transverse striation in the muscle.
D. ConnectiYe Tissues.
Under the name " connective tissue " are included various
tissues whose functions are mainly psissive, and which serve
CONNECTIVE TISSUES 159
to support, strengthen and bind together the various organs
and parts of the body. Histologically the connective tissues
consist of elements of four kinds, united together in very
varjdng proportions in different situations : (1) white fibrous
tissue; (2) yellow elastic tissue; (3) connective tissue cor-
puscles, which are comparatively slightly altered cells, usually
branched; and (4) ground substance, or intercellular sub-
stance.
I. White Fibrons Tissue. This consists of a number of
fine transparent fibres of a more or less cylindrical
shape, and with a very characteristic wavy outline;
between the fibres are connective tissue cells, usually
in small numbers. The fibres are arranged side by
side in bundles, and each fibre presents a number of
longitudinal fibrillar striations. The cellular origin
of white fibrous tissue is difficult to determine. The
fibres are believed to be formed by modification of the
intercellular matrix rather than from the bodies of
the cells themselves.
a. Tendon of rat's tail. Pull out a small piece of ten-
don from the tail of a rat; place it on a slide
in a drop of normal salt solution; spread it ovl
with needleSy cover and examine vnth low and high
powers : note :
i The fibres, with wavy outlines.
ii. The fibrillsB, indicated by longitudinal wavy stria-
tions within the fibres.
Add a drop of acetic acid to the preparation : note thai
iii. The fibres swell up and become transparent.
iv. Longitudinal rows of tendon cells, with nuclei,
become visible between the fibres.
n. Yellow Elastic Tissue. This consists of fine branching
homogeneous fibres, with great power of resisting
160 ELEMENTARY HISTOLOGY
chemical reagents ; the fibres are formed from an inter-
cellular matrix, and not from cells directly.
a. Ligamentnm nach» of ox. Tease finely a smali shred
in water; examine vnth low and high powers : note :
i. The branching fibres, with very sharp outlines.
ii. The tendency of the branches to anastomose with
one another and so form networks.
iii. The tendency of the fibres and branches to curl up
at their broken ends.
Add a drop qfaeetio add : note thai
iv. No alteration whatever is produced in the fibres.
V. No nuclei appear.
m. Areolar tissue. This is a meshwork composed of both
white fibrous and elastic tissues.
a. Subcutaneons tissue of mammal. Ttike ajreshly killed
ratf and snip off a smaU piece of the loosefihrous tissue
which connects the skin tvith the suhjacent parts ; stretch
it tUl quite flat with a pair of needles^ breathing con-
stantly upon it to make it adhere to the sUde ; cover,
and examine toith low and high powers : note :
i. The meshwork, composed of white fibrous tissue
with wavy outlines, mingled with which are
branched elastic fibres.
Add acetic a^sid : note that
ii. The white fibrous tissue swells up and becomes
transparent.
iii. The elastic tissue is unaltered.
iv. Connective tissue corpuscles, with nuclei, become
visible.
IV. Adipose tissue. This consists of a fine network of
vascular connective tissue, in the meshes of which are
fat cells, i.e., connective tissue corpuscles in which large
quantities of fatty or oily matter have accumulated.
a. Omentum of rabbit or kitten. Mount a small piece of
fresh omentum in normal salt solution ; protect it from
CARTILAOiS 161
the preaaiire of the cover glaae ; extwnine vfith low <md
high powers : note :
i. The vaficular oohnectiye tissue meshwork, in which
lie the fat cells.
ii. The &t cells: large, spherical, or fi-om mutual
pressure polyhedral, cells ; distended with fatty
matter, and with their nuclei near the surface.
b. Osmic Acid specimen.
Note the reduction of the osmic add by the fat, which
becomes stained a dark brown or black colour.
R Cartilage*
In cartilage or gristle the interoeUular substance, which in
most other tissues is only present in small quantity, is greatly
increased so as to far exceed in bulk the cells which it connects
together. The intercellular substance forms a dense translucent
matrix resembling an extremely stiff jelly, in which are im-
bedded the cartilage cells, either singly or in groups. In young
cartilage the intercellular substance is much less abundant, and
the cells consequently closer together than in older or more
mature specimens.
Cartilage when free from other tissue is called hyaline carti-
lage, from the clear or glassy appearance of the matrix, in
contradistinction to fibro-cartilage, in which the matrix is
fibrous from admixture with white fibrous or elastic tissues.
I. Hyaline Cartilage,
a. Cartilage of newt. TcJee a emaU piece of cartilage from
the shoulder girdle of a newt ; scrape away genUy any
musde or other tisstie that may adhere to it ; moimt in
normal soli eolutiony and examine with low aiid high
powers,
i. The intercellular matrix is either hyaline or faintly
granular.
ii. The cartilage cells are imbedded in the matrix ;
each cell is nucleated, and occupies a cavity or
lacuna in the matrix. In places the ceUs are in
groups of twos or fours owing to recent division.
L
162 ELEMRNTARY HI8T0L0OT
Wash the specimen thorougldy in uxUer ; sUiin vnth canning ^
and, mount as a permanent preparation in glycerine ; examine
urOh the high power, and note that
iii. The ceU nuclei are stained deeply, and the matrix
very slightly; the layer of matrix immediatdj
surrounding each cell — ^the capsule— stains more
deeply than the other parts.
b. Articular cartilage. This forms caps covering the
ends of those bones which fit together to form mov-
able joints ; the caps act as elastic cushions to break
the force of shocks.
Afornit in balsam a prepared section of articular carUhgefrom
the head ofthefemurj the section being made perpendicular to tfie
(Miicvlar swrfa^se ; exa/mine with low and high powers.
i. The matrix is hyaline or faintly granular.
ii. The cartilage cells. Towards the free surface the
cells and cell groups become gradually flattened,
and arranged parallel to the surface.
F. Bone.
Bone consists of a dense fibrillar intercellular matiix, in
which are imbedded cells which lie in cavities connected with
one another by fine branching canals. The matrix is richly
impregnated with inorganic salts, chiefly phosphate and car-
bonate of lime, which form about two-thirds by weight of the
substance of the bone, and give it its great hardness and
strength. The matrix, with its contained bone-cells, is arranged
in concentric layers or lamellse, around tubular passages, the
Haversiaii canals, in which lie the bloodvessels, which pene-
trate the bone in great numbers. A Haversian canal with its
contained bloodvessels, and its surrounding layers of matrix
and cells, are together spoken of as a Kaversiaii system.
1. Examine wOh both low and high powers prepa/red trans-
verse sections of a long bone.
i. The Havetsian ssrstems form the greater part of
the bone, and arc readily recognised by the con-
centric arrangement of the lamellie, and the
'^'^ntral canals.
I
I
BONE 163
ii. The interstitial lamella fill up the spaces between
the Haversian systems. They form parts of circles
which are in many cases of much larger radius
than the circles of the Haversian systems.
ill. The lacuna are the spaces in the matrix in which
the bone-cells lie. In sections of dried bone the
lacuna appear black, through being filled either
with air or with dirt.
iv. The canalicxQi are very fine branching canals con-
necting the lacunse together ; they are occupied
while tiie bone is living by branching processes
of the bone-cells. At the outer part of each
Haversian system, some of the canaliculi are
looped, opening at both ends of the loop into
the same lacuna.
v. The large central mednllary cavity of the bone is
occupied during life by the marrow, which
consists of adipose tissue, with very numerous
bloodvessels and large nucleated reddish marrow
cells.
vi. The peripheral or circumferential lamella are a
series of concentric lamella parallel to the surface
of the bone, and forming its most superficial
layer.
rii. The perimednllary lamella are a series of concen-
tric lamella lining the central medullary cavity
of the bono.
INDEX.
[Refereneet to defMlopmstU or* printed in thick nutnsrdU.]
Abdominal viscbra, 18-23
Acetabulum, 62
Adipose tissue, 160
Adrenal bodies, 23
Afferent nerves, 68
Alimentary canal, 21, 126-180
Ampulla, 95
Ankle, 63
Annulns tympanicus, 48
Aorta, 30, 131
Aortic arch, 30-32, 181 tea.
Aperture, doacal, 16, 96, 129
external, 16, 17
Aponeurosis, 66
Apparatus, 1
Appendicular skeleton, 49-64
Aqaeductus Sylvii, 78, 122
Aqueous humour, 88, 89
Arches, aortic, tee alto Carotid,
Systemic, Pnlmo-outaneous
Arches
byoid, tee alto Hyoid Arch
of jaws, tee alto Jaw, Visceral
Arches
yisceral, tu alto Visceral
Arches, Branchial Arches
Archinepbric duct, 146
Areolar tissue, 160
Arteries, 24, 29-32, 39
Artery, anterior mesenteric, 81
carotid, 30
CGBliac, 31
ooeliaco-mesenteric, 81
cutaneous, 32
dorsal aorta, 30
epigastric, 32
external carotid, 80
Artery, gastric, 31
hsemorrhoidal, 32
hepatic, 31
hypogastric, 32
iliac, 80, 82
internal carotid, 80
laryngeal, 30
lingual, tee external carotid
lumbar, 81
mesenteric, 31
occipital, 31
oocipito-vertebral, 81
oesophageal, 30
peroneal, 82
posterior mesenteric, 81
pulmonary, 32
sciatic, 32
splenic, 31
subclavian, 31
tibial, 32
urino-genital, 31
vertebral, 31
Articular cartilage, 168
process, 42
Arytenoid cartilage» 18
Atlas, 43
Auditory capsule, 43, 46, 141
organ, 94, 96 ; fig. 80a, p. 129
Auricle, 24, 34
Axial skeleton, 42-49
Axis cylinder, 86
B
Baokbonb, 42
Basi-bianchial, 148
Basi-hyal, 148
Basilar plate, 141
Bile-duct, 22, 127
166
INDEX
Bladder, gaU, 22, 127
urinary, 19, 97, 98, 127
Blastocoele, 110
Blastopore, 116
Blind spot, 91
Blood, 87-39
Body-oavity, 18, 116, 136
Bone, stractnre of, 162
Bones and cartilages of the skele-
ton: —
angnlo-splenial, 47
arytenoid, 18
astragalus, 63
calcanenm, 53
carpal, 61
clavicle, 50
colamella, 49, 95, 143
coracoid, 50
dentary, 48
epicoracoid, 50
exoccipital, 44
femur, 53
fronto-parietal, 46
girdle, 44
humerus, 51
hyoid, tee aUo H joid bone and
arch
ilium, 19, 52
ischium, 53
maxilla, 47
Meckel's cartilage, 47, 142
mento Meckelian, 48
metacarpal, 52
metatarsal, 64
nasal, 45
omosternum, 50
OS cruris, 53
palatine, 46
palato-pterygoid, 142
parasphenoid, 45, 141
phalanges, 52, 54
precoracoid, 50
premaxilla, 47
pro-otic, 46
pterygoid, 46
pubes, 53
quadrate, 47, 142
•* quadratojugal," 47
radio-ulna, 51
scapula, 50
Bones, sphen-ethmoid, 44, 45, 141
squamosal, 47
stapes, 143
stemam, 50
suprascapala, 60
tarsal, 63, 54
tibio-abula, 68
vomer, 45
Brachial plexus, 76
Brain, 68, 70-74, 120-123
Branchial arch, 129 teq,, 143
chamber, 129
cleft, 129
Baccal cavity, 17, 18
Calcar, 54
Calcareous or periganglionic
glands, 77
Canal, semicircular, 96
CanaUculi, 163
Capillaries, 24, 39
Capsule, auditory, 43, 45, 95
olfactory, 43, 45
Cardiac plexus, 77
Carotid arch, 30, 137
Carotid gland, 30, 137
Cartilage, 161-2
Cartilage-bone, 40, 43
Cauda equina, 75, 76, 77
Cavities of brain, 72, 73
Cell, 151
Central canal of cord, 86
Centrum, 42
Cerebellum, 72, 122
Cerebral hemisphere, 70, 123
vesicle, 123
Chiasma, optic, 73
Choroid, 88, 90, 91, 93, 126
plexus. 71, 122, 123
Choroidal fissure, 126
Cilia, 153
Ciliary movement, 163
muscle, 90
nerves, 90
processes, 90
vessels, 90
Circulation of blood, 24, 38, 89
in tadpole, 130-137
Cistema lymphatica magna, 19
INDEX
167
Cloaca, 21, 96-98
Gloacal apertnre, 16, 129, ue alto
Proctodseuin
Cochlea, 94
Coelom, 116, 188, 189
Columella, 49, 95, 126, 148
Condyle, occipital, 44
Cones and rods of retina, 93, 126
Conjunctiva, 88
Connective tissue, 158-161
Contraction of muscle, 55
Coracoid foramen, 50
Cornea, 87, 88
Corpora adiposa, 23, 104
Cranial flexure, 122
nerves, 77-84, 123
Cranium, 43-49, 140
Crura cerebri, 74, 122
D
Dbhtdbation, 11
Development, 99-160
general account, 99-104
detailed account, 106-160
of nervous system, 117-128
of sense organs, 128-126
of alimentary canal, 128-129
of gill arches and clefts, 129,
180
of circulatory system, 180-
138
of ccelom, 188, 189
of skeleton, 189-148
of urinary system, 148-160
Digestive organs, 20-22
Digit, Mte also Hand, Foot
Dissection, 2
Dorsal aorta, 30
Drawing, 2, 3
Duct, bile, 22, 127
Ductus endolymphaticus, 94
Duodenum, 21
B
Eab, 94, 95, 126
Efferent nerves, 68
Egg, 100
fertilisation of, 108-109
formation of, 104-106
maturation cf , 106
Bgg> segmentation of, 109 113
Elastic tissue, 159, 160
Endolymph, 94
Epiblast, 113 teg,
derivatives of, 117
epidermic layer of, 117
nervous layer of, 118
Epicoracoid, 50
Epiphysis, 51
Epistemum, 50
Epithelium, 151-153
ciliated, 153
columnar, 152
glandular, 154-167
squamous, 162
stratified, 153
Eustachian tube, 17, 95, 126
External characters, 15-17
openings, 16, 17
Eye, 87-93, 124, 126
frog, 87, 88, 92, 93
ox, 88-91
F
Fat-body, 23, 104
Fat-cells, 160, 161
Female organs, 97, 98
pronucleus, 108
Fenestra ovalis, 49, 95
Fertilisation, 108, 109
Fibrous tissue, 159
Filum terminale, 74
Fissure of cord, 86
Follicle of ovum, 104
Fontanelle, 44
Food-yolk, 106, 112
Foot, 16, 54
Foramen, intervertebral, 42, 74
magnum, 44'
of Monro, 73, 128
Fore-brain, 122, 123
Fore-limb, 16, 51, 52
Fourth ventricle, 72, 73, 122
G
Gall bladder, 22, 127
Ganglion, of spinal nerve, 77
Gasserian, 79
cells, 84-86
Grasserian ganglion, 79
General anatomy, 15-23
168
INDEX
G^enital plexus, 77
ridge, 104
Qerminal layers, 118-117
spot, 106
vesicle, 106
GiU arohes, 129, 160
defto, 102, 103, 129
Gills, external, 102, 129
opercular, 103, 180
Girdle, tee alto Pectoral, Pelvic
Gland, 16i-157
carotid, 30, 187
compound, 164
gastric, 166
cement, 100, 101
racemose, 164
simple, 154
thymus, 24
thyroid, 24
tubular, 166
Glenoid cavity, 60
Glottis, 18
Grey matter, 86
H
HiBMOBRHOIDAL PLEXUS, 77
Hand, 16, 62
Hallux, 64
Hardexung, 10, 11
Haversian system, 162
Head, 16
kidney, 148, 146-148
Heart, 19, 24, 26, 32-36, 180
Hepatic plexus, 77
portal system, 27-29
Hind-brain, 121, 122
Hind-limb, 16, 63. 54
Histology, 84-86, 91-98, 161-163
Hyaline cartilage, 161
Hyoid cartilage, 18, 43, 48, 49
arch, 49, 120, 142, 148
Hyomandibular cleft, 126, 129
Hypoblast, 116-117, 127
derivatives of, 117, 188
Imbedding, 11, 12
Impregnation, 108, 109
Infun£bulum, 78, 122, 128
Insertion of muscle, 66
Intercellular substance, 151
Intestine, 19, 21, 162
Iris, 87, 88
Iter, 73, 122
Jaw, 43, 46-48, 101, 141, 142
lower, 17, 47, 48
upper, 17, 46, 47
larval, 100
KiDNBT, 22, 148-160
Labial OABTiLAaB, 142
Lacuna in bone, 163
Laiyngeal chamber, 127
Lateral plate, 188
ventricle, 73, 123
Lens, 87, 89, 91, 124, 126
capsule and ligament, 91
Ligamentum nuchae, 160
Limbs, 16, 101, 102
skeleton of, 51-64
Linea alba, 66
Lips, 101
Liquor sanguinis, 87
Liver, 19, 21, 127
Lower layer cells, 118
Lang, 19, 102, 127
Lymphatic sjstem, 19, 36, 87
Lymph hearty 36, 37
sacs, 36
M
Maoebation, 7, 8
Male organs, 96, 97
pronucleus, 108
Malpighian body, 140, 164
Mandibular bar, 47, 48, 129, 141,
142
Marrow, 163
Maxillary bar, 46, 47
teeth, 17
Meckel*s cartilage, 47, 142
Medulla oblongata, 72
Medullary cavity, 163
sheath, 84, 86
INDEX
169
Medallated nerves, 84
Membrane-bone, 40, 43
Mesenteron, 116-117, 126
Mesentery, 20
Mesoblast, 116, 117
derivatives of, 117, 138, 139
Mesostemum, 51
Metamorphosis, 101, 102, 137
Metasternum, 51
Methods, hardening, 10-11
imbedding, 11, 12
macerating, 7, 8
mounting, 6, 7
section-cutting, 12, 13
staining, 8-10
table of histological, 14
teasing, 7
washing, 11
Microscope, 3-6
Mid-brain, 121, 122
Migration of blood corpascles, 39
Motor nerve, 68
Mounting media, 6, 7
Mouth, 16-18, see also Stomodaeum
Miillerian duct, 149
Muller's fibres, 93
Muscles, of head, 58-61
of hind-limb, 61-67
of trunk, 56-58
adductor brevis, 65
adductor longus, 62
adductor magnus, 62
biceps, 64
ciliary, 90
cucullaris, 57
depressor palpebrse inferioris,
60
depressor mandibalae, 57, 59
extensor cruris, 67
extensor dorsi communis, 58
gastrocnemiuSj 66
geniohyoid, 58
glutseus, 58, 64
hyoglossus, 59
ilio-psoas, 65
infraspinatus, 67
intertransversales, 58
involuntary, 55, 158
latissimus dorsi, 57
levator anguli scapul», 57
Muscles, levator bulbi, 60
masseter, 60
mylohyoid, 58
non-striated, 158
obliquus extemus, 56
inferior, 61
intemus, 56
superior, 61
obturator, 66
pectineus, 65
pectoralis, 56
peroneus, 67
petrohyoid, 59
pterygoideus, 59
pyriformis, 64
quadratus femoris, 66
rectus abdominis, 56
rectus anticus femoris, 64
rectus externus, 60
rectus inferior, 61
rectus intemus, 60
rectus intemus major, 62
rectus inter nus minor, 62
rectus superior, 60
retractor bulbi, 61
retrahens scapulae, 57
sartorius, 62
semimembranosus, 64
semitendinosus, 65
sternohyoid, 68
striated, 157
structure of, 157
submandibular, 58
temporalis, 59
tibialis anticus, 67
tibialis posticus, 66
triceps extensor femoris, 62
vastus externus, 64
vastus intemus, 64
voluntary, 55, 157
Muscular system, 55-67
Myotome, 138
N
Nabbs, antebiob, 17, 124
posterior, 17, 124
Nephrostome, 146, 149
Nerve
abducens, 80
auditory, 82, 95
M
170
IKDKX
Nenre, biachial, 7ft
cells, 84-86
ciliary, 90
coccygeal, 76, 77
ooraoo-claTiciilar, 7ft
cranial, 77-88, 188
cmral, 76
facial, 81, 82
fibres, 84, 86
ffloflflo-pharyngeal,
hypoglotsal, 76
ileo-hypogasfcric, 76
medullat^ 84
motor ocali, 79
non-medullated, 86
olfactory, 78
optic, li, 87-88
pathetic, 79
peroneal, 76
pnenmogttrtrici 88
radial, 76
roots, 86
•ciatio, 76
spinal, 74-77, 183
sympathetic, 77, 84
tibial, 76
trigeminal, 79, 80
ulnar, 76
vagus, 83
Nervous system, 68>86, 117-183,
84-86
Neural arch, 42
canal, 42
fold, 118
groove, 118
plate, 118
spine, 42
tube, 119
Neurenterio caaal, 119
Neuroglia, 86
Nodes of Banvier, 86
Non-medullated nerves, 8ft
Nose, development, 183
Nostril, 17
Notochord, 118, 189
OOOIPITAL CX)NDTLE, 44
Occipital-atlantal mambrane, 43
(Esophagus, 21
Olecranon proceas, 51
Olfactory capsule, 43, 45. 141
lobe, 70, 198
organ, 198
Operculum, 100, 101, 199
Optic capsule, 46, 141
chiasma, 73
cup, 195
lobe, 71, 199
thalami, 71, 199
vesicle, 199, 194, 195
Ora senata, 90
Origin of muscle, 66
Ovaiy, 19, 97, 105
Oviduct, 20, 98
Oviposition, 99
Ovisac, 98
Ovum, 9ee Egg
Palato-pteryooii>, 149
Pancreas, 22, 197
Parachordal, 140
Pectoral girdle, 18, 49, 60
Pelvic girdle, 62, 63
Pericardial cavity, 19, 188
Periganglionic glands, 77
Perimedullary lamella), 163
Perineurium, 84
Periotic capsule, 94
Peripheral lamellaB, 163
nervous system, 74-84, 193
Peritoneum, 20, 189
Pia mater, 70, 86
Pigment layer of retina, 93, 195
Pineal body, 71, 199
Pituitary body, 74, 198
Plexus, brachial, 76
sciatic, 76
cardiac^ 77
solar, 77
Polar bodies, 107
PoUez, 52
Portal system, 27, 29
Post-anal gut, 197
Post-axial surface, 61
Preaxial surfaoe, 61
Preserving, 10, 11
Presternum, 60
INDEX
171
Primitive sheath, 84, 86
Proctodeeum, 126, 128
Pronephros, 148-148
Pronnolens, 107, 108
Palmo-cutaneous arch, 32, 137
Pupil, 87, 88
Pylangium, 35
Pylorus, 21
Q
Quadrats, 47, 142
R
Reagents, habdbnino, 10, 11
macerating, 7, 8
mounting, 6, 7
staining, 8-10
Renal plexus, 77
portal system, 28
Reproductive organs, 96-98
Retina, 88, 91, 92, 93, 126
Rhinal processes, 46
Rods and cones of retina, 93, 128
Rules for drawing, 2, 8
for dissection, 3
for use of microscope, 3-6
8
Sacgulus, 94
Sacrum, 43
Sarcolemma, 168
Sciatic plexus, 76
Sclerotic, 46, 87, 88, 93, 128
Section cutting, 11-13
Segmental duct, 146
Segmentation cavity, 110, 116
nucleus, 108
of the egg, 109, 112
Semicircular canal, 96, 126
Sense capsules, 46, 141
organs, 122-126
Sensory nerve, 68
Sheath of Schwann, 84, 86
Shoulder girdle, 18
;Sinus venosos, 26-27, 32, 33
Skeleton, 40-64
appendicular, 49-64
axial, 42-49
Skin, 16
SkuU» 43-49, 140-148
Somatopleure, 188
Solar plexus, 77
SpawA, 99
Spermatozoa, 96, 99, 108
Splanchnopleure, 138
Spleen, 23, 37
Splitting of mesoblast, 116, 188.
Spinal chord, 68, 74, 86, 86, 117-
120
ganglia, 77
nerves, 74-77, 128
Spinous process, 42
Staining reagents, 8-10
Stapes, 143
Sternum, 49, 60
Stomach, 21
Stomata, 36
Stomodseum, 126, 128
Subcutaneous tissue, 160
Subocular bar, 142
Suspensorium, 47
Suspensory ligament, 91
Swammerdam, glands of, 77
Sympathetic nervoni system, 70,
77,83
Symphysis, 62
Synangium, 36
Systemic arch, 30-32, 187
Systole, 26
TASTB PAPIlXjE, 126
Teasing, 7
Teeth, 17, 46, 47
Tendo Achillis, 66
Tendon, 169
Testis, 20, 96, 108
Thalamencephalon, 71, 122
Third ventricle, 71, 73, 122
Thyroid gland, 26
Tongue, 18
TrabBculse cranii, 140
Transverse process, 42
Truncus arteriosus, 24, 36
Tuber cinereum, 73
Tympanic cavity, 17, 95
membrane, 16, 17, 49, 96, 126
172
INDEX
Ubbtsb, 23, 96-98, 150
Urostyle, 19, 42, 43, 140
Utrioalas, 94
Uvea, 90
Vas deferens, 96, 97
Vasa efferentia, 96, 150
Vasonlar system, 24-39, 130-188
Vein, 24-29, 39
anterior abdominal, 18, 28, 66
anterior vena oava, 26, 27
brachial, 26
cardiac, 29
dorso-lambar, 28
external jugular, 26
femoral, 28
gastric, 29
hepatic, 27
hepatic portal, 28, 29
innominate, 26
internal jagular, 26
intestinaJ, 29
lingual, 26
mandibular, 26
musoulo-cutaneous, 26
ovarian, 27
parietal, 28
pelvic, 28
portal, 27-29
posterior vena cava, 27
pulmonary, 27
renal, 27
renal portal, 28
sciatic, 28
splenic, 29
spermatic, 27
subclavian, 26
subscapular 26
vesical, 28
Ventral fissure, 74
Ventricle, of brain, 73, 120-128
of heart, 24, 34
Vertebrae, 42, 43
Vertebral column, 42, 43, 189,
140
plate, 138
Vesical plexus, 77
Vesicula seminalis, 23, 96, 150
Vestibule of ear, 94
Viscera, abdominal, 18-23
Visceral arches, 120, 141-148
clefts, 120
skeleton, 141-143
Vitelline membrane, 106
Vitreous humour, 88
Vomerine teeth, 17
W
Washing, 11
White fibrous tissue, 169, 160
matter, 86
Wolffian body, 145, 148, 149
duct, 140, 150
Wrist, 61
XiPHISTBBNUM, 61
TbLLOW ELASTIC TISSUE, 169
Tolk, 101, 104, 112, 113
cells, 113
hypoblast, 116
plug, 115
Z
Zonule of Zinn, 91
Zygapophysis, 42
A
Printed by Ballanttne, Hanson &* Co.
Edinburgh ^ London
m^
t
\ -
«"
Live Music Archive
Librivox Free Audio
Metropolitan Museum
Cleveland Museum of Art
Internet Arcade
Console Living Room
Books to Borrow
Open Library
TV News
Understanding 9/11