GUIDES FOR VERTEBRATE DISSECTION

THE FROG

AN ANUROUS AMPHIBIAN

BY

J. S. KINGSLEY

Professor of Biology in 'J'ujts College

NEW YORK

fENTCY TTOLT AND COMPANY

GUIDES FOR VERTEBRATE DISSECTION

THE FROG

AN ANUROUS AMPHIBIAN

BY

J. S. KINGSLEY

Professor of Biology in Tufts College

NEW YORK

HENEY HOLT AND COMPANY

Copyright, 1907,

BY

HENRY HOLT AND COMPANY

INTRODUCTION

THESE directions for dissection are intended eventually to
include representatives of all the major groups of vertebrates.
Each is complete in itself and is issued separately so that
laboratories may select those forms best adapted for their courses.
The directions have been tested by several years' use and are
thought to have a distinct pedagogic value in that they do not
so much tell the student what he will find, but instead ask him
what he does find. He thus obtains his information from the
specimen, not from the printed page. For similar reasons illus-
trations have been omitted; students sometimes find it easier
to copy the published figure than to work out the points for
themselves.

No attempt has been made to follow out every system of
organs completely, but each has been traced far enough to give
a good knowledge of the more important structures to use as a
basis for comparisons. The student by following the directions
may obtain a knowledge of the general anatomy of the animal
studied, but this knowledge of itself has little value. More
important is the benefit to be gained by comparing the different
forms dissected, tracing as far as possible their resemblances and
differences. Hence in his dissection the student should con-
tinually recall the conditions existing in all other animals as he
is tracing out each part.

More than this: he should read the general statements given
in manuals of vertebrate structure as he takes up each organ
or system of organs, thus correlating his discoveries and making
them a part of one general whole. It would be well to go farther
and read the accounts of the development of the organs in
question in some of the text-books of vertebrate embryology. It
is only in this way that an explanation of many peculiarities of
structure may be obtained.

Unless explicitly used otherwise the terms right and left in
the following directions apply to the right and left of the animal
being dissected, not of the student. Anterior and posterior

111

iv INTRODUCTION

indicate relative position with regard to head and tail, while
dorsal and ventral are used for the anterior and posterior of
human anatomy. Medial is used to imply proximity to the
middle line, lateral being the contrasting term. Proximal refers
to that part of an organ or structure nearest to its centre or to
its attachment to the body, distal being the opposite adjective.
In speaking of muscles the fixed point of attachment is the
origin, the attachment on the part to be moved is its insertion.
INJECTING. — In many cases it is almost impossible for the
beginner to trace the blood-vessels unless they are filled with some
colored substance which renders them more easily seen. This
is especially true of the smaller vessels. Injection is also frequently
convenient in tracing other vessels like those of the urogenital
system.

Various substances ('injection masses') have been devised for
filling the vessels. The essential features of a mass are that it
have color, that it flow freely when injected, and that it soon harden
so that it will not escape from a vessel accidentally cut. Within
recent years a starch mass has been largely used, and as this answers
all the purposes of these guides it is described here :

Corn-starch 400 pts. by volume

2% chloral hydrate in water 400 "

95% alcohol 100 "

Color and glycerin (equal parts) . . 100 ' '
The mixture should be thoroughly mixed by stirring and
strained through cheese-cloth or paper cambric, stirring during
the operation. The starch and color quickly settle, hence the
mixture has to be stirred while using. It will keep indefinitely,
but of course must be thoroughly mixed each time before using.

The colors commonly used are vermillion,* insoluble Prussian
blue, chrome green, and chrome yellow. The vermillion is usually
used for the arterial, the blue for the venous system, but it is
often advantageous to use chrome yellow instead of blue, as it
contrasts better with dark organs like the liver and kidneys, while,
when a blood-vessel occasionally bursts in injection, the viscera
are not so badly stained.

* Care should be taken to get true vermillion (mercuric sulphide), as much
that masquerades under that name is red lead colored with cosine. This
works disadvantageously as the cosine dissolves in the liquids in which the
specimens are preserved and stains everything indiscriminately.

INTRODUCTION V

An extremely fine chrome yellow may be made by dissolving
200 parts of acetate of lead and 105 parts by weight of chromate
of potash in separate dishes of water. After complete solution
mix and allow the precipitate to settle. Pour off the supernatant
fluid and wash the precipitate with several waters so as to remove
the potassium acetate which would injure the specimen.

Many instruments — syringes, water-pressure apparatus, etc.,
have been proposed for injecting, but a considerable experience
has led to the conclusion that for small animals there is nothing
better than a large rubber bulb for the pressure. This is con-
nected by rubber tubing with the canula which is inserted in the
vessel to be filled. Use the largest canula possible and keep it
free from precipitated mass.

SKELETONS. — The skeletons made by the average student are
likely to be imperfect, but the knowledge which he obtains in
preparing them is of value. The laboratory should have skeletons
well prepared, but the student should clean those which he studies.
In the case of fishes it is sufficient to remove the skin from the
body, next to place the animal for a few minutes in water
near the boiling-point, and then to remove the flesh by hand.
With other animals the tissues are more resistant, and in these
cases the animal, after removal of the skin, should be boiled in a
soap solution made as follows:

Thoroughly mix with heat 75 grams of hard soap, 12 grams
of potassic nitrate (saltpetre), 150 cc. of strong ammonia, and
2000 cc. of soft water. For use, one part of this ' stock ' is diluted
with three of water and the body is boiled in this, the length of
time varying with the size and consistency of the animal, care
being taken not to boil it long enough to soften the ligaments
unless it be desired to separate the bones from each other.

For decalcification of skulls in order easily to get at the
brains, nitric alcohol, made by mixing equal parts of ten per cent
nitric acid and ninety-five per cent alcohol, is useful.
Material for dissection can be obtained from

Supply Department, Marine Biological Laboratory, Woods
Hole, Mass.

Dr. F. D. Lambert, Tufts College, Mass.

H. H. and C. S. Brimley, Raleigh, N. C.

H. A. Ward, Rochester, N. Y.

Kny-Scheerer Co., 225 Fourth Ave., New York.

THE FROG

THERE are many species of true frogs (Rana) in the United States, but
the following directions may be used with any; the larger the speci-
mens the better.

For study each student should be provided with two specimens, a
male and a female, besides a prepared skeleton. One, preferably the
male, should have both arterial and venous systems injected. This should
be used for the general study of the viscera and the circulatory organs,
while the other should be used for the muscles, the internal structure
of the heart, the female urogenital organs, and the nervous system.

The arterial system may be filled by inserting the canula in the bul-
bus cordis and ligating it in the truncus arteriosus impar. The venous
system is injected by forcing the injecting fluid in both directions through
the ascending portion of the abdominal vein, and forward and back
through the postcava.

Skeletons may be prepared by skinning the frog, separating the fore
limbs and the supporting girdle, and heating for a short time in soap
solution. Then pull and brush away as much of the flesh as possible,
repeating the use of the soap solution if necessary. Care must be used
not to boil too long in the soap solution, as this will soften and eventually
destroy the ligaments uniting the elements.

It is also well to have in the laboratory some disarticulated skeletons
in which the bones have been separated from each other and completely
cleaned by longer boiling in soap solution or in weak caustic soda.

THE SKELETON

In the frog, as in most Amphibia, the exoskeleton is practically
absent, and all parts may be regarded as forming the endo-
skeleton.

This is composed of axial and appendicular portions. The
axial skeleton consists of the vertebrae, ribs, and skull; the appen-

2 GUIDES FOR VERTEBRATE DISSECTION

dicular skeleton of the supports of the limbs and the girdles which
connect these to the body. The sternum, usually regarded as a
part of the axial skeleton, is, in the case of the frog, best treated
in connection with the shoulder-girdle.

The vertebral column is composed of separate bones, the
vertebrae, plus an elongated bone, the urostyle or coccyx, at the
posterior end. How many vertebrae do you find?

Study a single vertebra from about the middle of the column.
In it make out a centrum or body from which arises dorsally on
either side a neural arch terminating above in a neural spine.
Is the centrum procoelous (hollow in front), opisthoccelous (hollow
behind), or amphiccelous (concave both in front and behind)?
Notice that each vertebra bears a pair of lateral projections, the
transverse processes (diapophyses). Do they arise from the cen-
trum or from the neural arch? Examine the distal end of each;
see the hollow in which, in life, the end of a short rib was inserted.
On the anterior and posterior ends of the neural arch notice the
articular facets (zygapophyses) by which the vertebrae are con-
nected together. Does the postzygapophysis face upwards or
downwards? What is the condition in the prezygapophysis?

Draw dorsal, lateral, and end views of this vertebra, X4,
naming the parts.

Compare other vertebrae with the one studied. How does the
first (the atlas) differ from those behind? Does it have both
pre- and postzygapophyses? Notice the anterior prolongation
of its centrum. Draw this vertebra. Examine the ninth verte-
bra (sacrum). Is it procoelous? What is the character of the
centrum behind? With what parts does the pelvis articulate?

Draw three or four vertebrae together, viewed from the side,
showing the way in which the zygapophyses overlap and the
openings (intervertebral foramina), between the neural arches of
the successive vertebrse, for the passage of the spinal nerves.
Is there an intervertebral foramen between the first vertebra and
the skull?

Behind the sacrum the vertebral column is continued by
the coccyx or urostyle, which must be regarded as composed of
coalesced caudal vertebrae. Notice the articular facets by which
it is connected with the sacrum, and above them the neural
arch. Does this arch continue the length of the bone? Is there
an intervertebral foramen between the sacrum and the urostyle?

THE FROG 3

On either side, five or six millimeters from the anterior end, are
small foramina for the exit of the eleventh nerve.*

The Skull. — In the skull notice that you can distinguish a
median portion, the cranium, and visceral or facial portions on
either side, the two being connected behind by lateral projections
(otic capsules) from the cranium and in front by means of nasal
capsules. (These last do not show well in the dried skull, as
the cartilage of which they are composed shrivels in drying.)

Examine the hinder surface (base) of the skull, making out the
following points. In the centre the large opening (foramen
magnum) leading into the cranial cavity, bounded on either side
by rounded prominences (occipital condyles) for articulation with
the atlas. These condyles are borne on the exoccipital bones.
Obliquely in front of and lateral to each exoccipital is a pro-
otic bone forming the wall and roof of the otic capsule, and
extending from the outer end of each prootic down to the angle
of the jaws is a hammer-shaped squamosal bone. In front of,
below, and parallel to the shank of the squamosal is a pterygoid
bone to be studied later. (The columella auris, lying beneath and
outside of the prootic, is often missing from the prepared skull.)

Draw the base of the skull, X4, naming the parts.

The dorsal surface of the skull shows the exoccipitals, prootics,
and squamosals, already made out. On either side of the middle
line, in front of the foramen magnum, the roof of the cranium is
formed by a fronto-parietal bone, which extends forward to the
anterior margin of the orbits. Joining the fronto-parietals in
front is the sphenethmoid bone, the extent of which is better seen
from the side or from below. It is largely covered above by
the fronto-parietals and in front by the paired, triangular nasal
bones.

In front of the nasals and forming the tip of the jaw are a
pair of premaxillary bones, each consisting of an alveolar process,
bearing teeth, and an ascending process which unites it to the
nasal. Most of the rest of the upper jaw is formed by the maxil-
lary bones, each of which joins the premaxillary in front and
is connected, a little farther back, with the nasal bone by a frontal
process. Behind, each maxillary is connected, by an overlapping
joint, to a slender, splint-like quadratojugal bone which extends

* For this numbering see the section on the nervous system, infra.

4 GUIDES FOR VERTEBRATE DISSECTION

back to the angle of the jaw, where it comes in close connection
with the squamosal already mentioned. This is a T-shaped bone,
one arm of the T articulating with the prootic, the other extend-
ing forward, above the pterygoid (better seen from the ventral
surface), until it nearly joins the maxillary. In addition to
these, portions of the vomers and pterygoids can be seen from
above, but their study is best undertaken from the ventral surface.

Draw a dorsal view of the skull, X4, being careful to trace
exactly the limits of each bone; label all parts.

Also draw the ventral surface of the skull on the same scale,
showing the exoccipitals, sphenethmoid, premaxillaries, maxillaries,
and quadrate jugals already seen as well as the following elements.
Beginning in the middle line behind, just in front of the exoccipi-
tals is an unpaired, sword-shaped parasphenoid bone, the guard of
which overlaps the prootics, the blade covering the sphenethmoid.

From just in front of the sphenethmoid a palatine bone extends
laterally on either side to meet a palatine process of the maxillary,
while just in front of the palatines are a pair of irregularly shaped,
tooth-bearing vomers. With what bones do these last articulate?
The pterygoid bones are shaped like the Greek letter A- In front
they articulate with the lateral end of the palatines, the posterior
inner limb joins the prootic, the outer the squamosal at the angle
of the jaw. At the junction of squamosal and pterygoid is a
quadrate cartilage, to which the lower jaw is hinged.

In a side view of the skull notice the gap in the bony wall of
the cranium between front o-parietals, sphenethmoid, and para-
sphenoid. See also the mode of articulation of maxillary and
quadratojugal.

In the lower jaw make out an angulo-splenial bone extending
forward from the angle of the jaw on the inner and lower surface.
It bears on its posterior third a small coronoid process above, and
on its outer surface a groove for Meckel's cartilage. The upper
and outer surface of the jaw is composed of a dentary bone which
overlaps the angulo-splenial and extends forwards to meet a
pair of mento-Meckelian bones which touch at the symphysis of
the jaw.

Draw a side view of the skull, X4.

The Hyoid Apparatus, placed between and behind the halves
of the lower jaw, also belongs to the skull. It consists of a broad,
thin cartilaginous body with processes, anterior and posterior, for

THE FROG 5

the attachment of muscles. From its anterior margin the anterior
cornua pass outwards, then backwards, and lastly upwards to
connect with the prootic, while the posterior cornua arise from
the posterior margin of the body and pass outwards and back-
wards on either side of the larynx.

Draw the Hyoid, X4.

Shoulder-girdle and Sternum. — These form an incomplete ring
or girdle around the body, but are without direct connection with
the axial skeleton. The girdle may be divided into dorsal and
ventral halves by the socket, glenoid fossa, for the articulation
of the fore limb. Dorsal to the fossa is the scapula, which is
surmounted by the suprascapula, composed partly of bone,
partly of cartilage. Does the scapula contribute to the forma-
tion of the glenoid fossa?

Below the fossa are two bones on either side, extending
towards the median line, an anterior clavicle and a posterior
coracoid bone, including between them an opening, the coracoid
fontanelle. Connected with the clavicle is a cartilaginous bar,
the procoracoid, not usually easily seen in the prepared skeleton.

Clavicles and coracoids of the two sides are connected in
the middle line by the sternal structures. Of these, in the prepared
skeleton, the following parts may be made out. Joining the
clavicles in front, an omosternum, and in front of this an epister-
num. Behind the coracoids is the sternum proper, terminated by
abilobed xiphisternum.

Draw this apparatus, turning the scapula and suprascapula
outwards in the sketch, so that all parts may be seen in a ventral
view.

In the fore limb make out and draw the following parts. In
the region of the upper arm a long bone, the humerus, with en-
larged articular processes on either end. On its inner or under
surface is a strong ridge, the crista deltoidea, for the attachment
of the deltoid muscle. On the posterior distal angle of the
humerus, beside the articular surface, is a prominent trochlear
process.

The skeleton of the forearm is composed of the coalesced
radius and ulna (radio-ulna or os antebrachii), the radius being on
the anterior side. Notice the shape of the articular face of the^
proximal end of this bone. Its size is increased by a projecting;
olecranon process.

6 GUIDES FOR VERTEBRATE DISSECTION

The carpus, or wrist, contains six bones arranged in two
rows, with the sixth element, the centrale, between them. The
proximal row consists of a radiale and an ulnare, the distal of three
carpales.

The digits or fingers consist of a basal row of metacarpal
bones, corresponding to the palm, and succeeded by several
phalanges (finger bones). The digits are numbered beginning on
the radial side. Do you find any trace of a first digit (pollex)?

The Pelvic Girdle is articulated with the axial skeleton by
means of the diapophyses of the sacral vertebra. It is V-shaped
and consists of paired bones meeting hi the midventral line.
On the outer side of the posterior or ventral end is a hemispherical
cup, the acetabulum, for the articulation of the hind limb. Study
the pelvis from the side. Three bones will be seen to meet in
the acetabulum ; in front a long ilium which also articulates with
the sacrum; behind a small ischium, and below a much smaller
pubis.

Draw the pelvis from the side, showing these points as well
as the strong process, the crista ilii, on the dorsal surface of the
ilium, and the place where the ilium articulates with the sacrum.

The Hind Limb corresponds closely to the fore limb in its
structure. In the thigh and articulating with the acetabulum
is the femur, which is expanded distally to articulate with the
united tibia and fibula (os cruris), the bone of the shank. Notice
the grooves indicating the compound nature of the bone. The
inner or anterior element is the tibia.

The ankle or tarsus is composed of two rows of bones, the two
of the proximal row being greatly elongate and united at either
end. These bones are the tibiale or astragalus, the fibulare or cal-
caneum. The bones of the distal row are not readily made
.out in the prepared skeleton.

The tarsal bones are succeeded by the elongate metatarsals
•and those in turn by the phalanges, composing the digits. The
first digit is on the tibial side.

Draw the skeleton of the hind limb.

THE FROG

EXTERNAL ANATOMY

In the entire animal notice the shape of the body. Is there
a distinct neck? From your study of the skeleton explain the
'hump' on the back. Where is the ankle? How many toes on
each foot? Are all of the feet webbed and how far does the
web extend along the digits? Is the anus dorsal or ventral in posi-
tion? Examine the surface of the body with a hand lens; do
any parts appear to be markedly glandular? Do you find any
signs of scales?

Examine the head for eyelids and external nostrils (nares).
Are there fleshy folds about the nares? See the circular area,
the tympanic membrane on either side of the head behind the
eye. Note the gape of the mouth; is it large or small? Are
there teeth in both jaws? Look in the median line between or
a little in front of the eyes for a dark spot, the brow spot or frontal
organ.

Draw a dorsal view of the frog, natural size.

Cut through the tympanic membrane at its border and free
it from the ring (tympanic annulus) on which it is stretched.
This lays open the tympanic cavity, shaped like a flat funnel and
ending below in a large short Eustachian tube. Attached to the
inner surface of the tympanic membrane is a slender bone, the
columella auris. Where is the other end of this element? Note
that the columella is held in position by a thin membrane, the
plica plectri. Would the existence of this tend to show that the
columella was morphologically outside the tympanic cavity?
From which side has it extended into the cavity? Probe the
Eustachian tube ; open the mouth and see where the probe appears.

Draw the tympanic cavity hi diagrammatic section, showing
tympanic membrane, columella, Eustachian tube, etc.

INTERNAL STRUCTURE

Cut through the ventral skin (the skin only) from the hind
limbs forward to the symphysis of the lower jaw and carefully
lift it up. Is it free from the underlying muscles at all points?
The free portions correspond to subcutaneous lymph spaces.

MUSCLES. — On either side of the median line in the abdominal
region is the large rectus abdominis muscle, divided transversely

8 GUIDES FOR VERTEBRATE DISSECTION

by inscriptiones tendineae. Each rectus arises from the pubic bone
and divides, as it passes forward, into medial and lateral portions.
The medial part continues forward to the sternum, the lateral
becomes confluent with the portio abdominis of the pectoralis
muscle, to be studied later.

Lateral to the rectus is the external oblique muscle. What
is the direction of its fibres? On lifting the external the internal
oblique is seen. What is the direction of its fibres? (Beneath
this is a third layer, the transversus. These layers are not to be
separated until later.)

The pectoralis muscle, already mentioned, extends from the
midventral line to the shoulder. It presents three divisions.
Beginning behind, these are (1) the portio abdominis, continuous
with the rectus abdominis; (2) portio sternalis, arising from the
sternum and xiphisternum, and (3) portio epicoracoidea, arising
from the sternum proper and the medial end of the coracoid.

In front of the pectoralis is the coraco-radialis muscle, arising
from the omo- and episternum and from the coracoid. Its
fibres run outward and backward and are continued as a tendon
passing through the deltoid (infra) to reach the radial side of the
radio-ulna.

The deltoideus muscle arises by two heads, one (clavicularis)
from the clavicle and omosternum, the other (scapularis) more
external and arising from the clavicle and especially from the
scapula. Its fibres are largely inserted on the crista deltoidea of
the humerus.

Sketch the muscles so far made out.

In front of these muscles in the floor of the lower jaw is a
transverse submaxillaris muscle. Can you distinguish in it an
anterior and a posterior (subhyoideus) portion? Are the fibres
continuous across the median line?

Add these muscles to your sketch. Then cut the submaxil-
laris in the median line and reflect the halves.

In front, at the symphysis of the jaw, is a small transverse
submentalis muscle. Behind this the floor of the mouth is formed
by a median hyoglossal and a pair of geniohyoid muscles. The
latter are partly double, having two heads which are attached
to the lower jaw, while behind two similar heads are attached to
the hyoid. The hyoglossal is also divided behind, while in front
it extends into the tongue. Open the mouth and follow its course.

THE FROG 9

Sketch these muscles on a separate sheet, leaving room
behind for the following muscles.

Cut through the deltoid and portio sternalis of the pectoral
near the median line, exposing the coraco-brachialis brevis and
longus muscles, which run from the coracoid, near the sternum, to
near the middle of the humerus. Add these to sketch, then
split the sternum longitudinally, exposing the deeper muscles, as
follows: On either side of the median line a strong sternohyoid
muscle, arising in part from the coracoid and sternum, in part
as an anterior prolongation of the rectus abdominis, and passing
forward to be inserted on the hyoid between the geniohyoids.
The omohyoid muscle is inserted on the hyoid just lateral to the
sternohyoid. It runs obliquely outwards and backwards to its
origin on the inner surface of the scapula. Other muscles are
exposed by deeper dissection. Lift the sternohyoid and see the
thyreoid gland lying in the hinder angle of the body of the hyoid.
Farther back, in the angle between sterno- and omohyoid muscles,
is the pseudothyreoid body. Sketch these in their proper rela-
tions.

Remove the skin from the dorsal surface of head and trunk,
noting the expanded tympanum surrounded by a ring of carti-
lage, the annulus tympanicus, supporting the tympanic mem-
brane. Note that in removing the skin the membrane is often
split, indicating that it is composed of more than one layer.

A little behind the tympanum is the depressor mandibulse
(digastric) muscle, which extends downward from the dorsal
fascia to its insertion on the posterior angle of the lower jaw.
Separate this muscle and turn it outward, exposing its other head,
attached to the posterior arm of the squamosal, as well as the
cucularis muscle, arising from the prootic and squamosal and
inserted in the anterior border of the scapula, external to the
origin of the deltoid. The thymus gland may now be seen hi the
angle between the cucularis and the head of the deltoid. It
should be inserted in the drawing which you will make later.

Just in front of the tympanum is the temporalis muscle, which
arises from the upper part of the prootic and passes under the
anterior arm of the squamosal and over the pterygoid bone to its
insertion on the coronoid process of the lower jaw. In front of
and partly covered by the temporalis is the pterygoideus muscle,
arising from the fronto-parietals and passing downwards and

10 GUIDES FOR VERTEBRATE DISSECTION

backwards to its insertion on the inside of the lower jaw behind
the temporalis.

The masseter major is easiest traced from its insertion on the
outside of the lower jaw opposite the temporalis to its origin from
the tympanic annulus and the squamosal. The masseter minor
is attached to the hind end of the jaw and to the quadratojugal.

Sketch these muscles, tympanum, etc., in side view.

The rhomboideus anterior muscle extends, on the dorsal
surface of the body, from the dorsal fascia and the fronto-parietals
back to its insertion on the ventral surface of the suprascapula.
The anterior part of the dorsal surface of the scapula and supra-
scapula is covered by the dorsalis scapulas muscle, the fibres of
which converge to their insertion on the crista deltoidea. Behind,
the scapular elements are covered by the latissimus dorsi muscle,
which also extends back on to the back itself. It arises from the
dorsal fascia and its fibres unite above the shoulder with those
of the dorsalis scapula. Lateral to and deeper than the rhom-
boideus anterior is the levator scapulae anterior, the anterior end
of which is covered by the cucularis, while behind it is inserted
on the ventral surface of the suprascapula.

Sketch these muscles.

Next cut the rhomboideus anterior and the latissimus dorsi
and attempt to lift the suprascapula. It will be found to be
held by several muscles, most prominent of which are the
rhomboideus posterior, visible in part on the medial side of the
suprascapula, and the serratus superior, which overlies the postero-
lateral portion of the rhomboideus posterior.

Draw these, then remove them and the suprascapula, exposing
the large longissimus dorsi muscle, which extends from the
occipital region of the skull back to the pelvis and to the anterior
third of the urostyle. Is it attached to any of the vertebrae?
Does it present inscriptiones tendinise? Is it attached behind to
other parts than the urostyle?

Behind and lateral to the longissimus dorsi is the coccygeo-
iliacus muscle, extending from the posterior part of the urostyle
to the iliac bone. The other and deeper muscles of the back may
be omitted.

Aside from the muscles from the shoulder-girdle region already
mentioned, there is but a single muscle, the triceps brachii or
anconseus, in the upper arm. This has four heads (scapularis,

THE FROG 11

arising from the scapula, the mediate, laterale, and profundum
from the humerus). All unite to give rise to a tendon which
passes over the elbow-joint to be inserted on the ulno-
radius.

The muscles of the forearm and hand are more numerous and
may be grouped as flexors and extensors accordingly as they bend
or straighten the parts.

In the dorsal view of the hind limb make out the following
muscles. The iliacus externus, extending from the outer side of
the ileum to the trochanter of the femur. The pyriformis, extend-
ing from the tip of the urostyle to the crista femoris. The
triceps femoris, on the outside of the limb, which arises from the
girdle by three heads: a, the glutasus maximus (vastus externus),
most posterior on the dorsal surface, from the posterior end of
the ileum between the iliacus externus and the pyriformis; b, the
caput medium (rectus femoris anticus), passes below the external
iliac to its origin from the ventral surface of the ileum; c, the
cruralis (vastus externus), not visible from above. All divisions
unite in a broad tendon inserted on the distal end of the femur
and the proximal end of the tibio-fibula.

Medial to the distal end of the triceps the posterior part of
the ileo fibularis muscle can be seen. Its origin from the hinder
end of the ileum is concealed beneath the glutaeus maximus. Its
tendon passes over the knee and is inserted on both femur and
the upper fibular portion of the os cruris.

Next nearer the median line is the large semimembranosus
muscle, crossed by an oblique inscriptio tendinea. It arises
from the ischiopubic region of the pelvis and is inserted on the
proximal end of the os cruris.

The gracilis minor lies on the medial side of the limb. It
arises, with its fellow of the opposite side, from a tendon in the
middle line, just ventral to the anus, and unites distally with the
gracilis major (infra).

Sketch these superficial dorsal muscles of the thigh.

On the ventral surface of the thigh recognize the median head
of the triceps femoris and also the gracilis minor. Also see the
cruralis head of the triceps which arises from the wall of the ace-
tabulum (its origin will be seen later).

A flat muscle, the sartorius, crosses the proximal end of the
cruralis and extends along the nrdventral surface of the thigh

12 GUIDES FOR VERTEBRATE DISSECTION

to the'knee, where it becomes tendinous and passes to its insertion
on the proximal end of the tibia.

Just lateral to the gracilis minor is the gracilis major, crossed
by an oblique inscriptio tendinea. It arises from the pelvis and
is inserted, together with the minor, by two tendons, one going
to the proximal end of the tibia, the other to the hinder surface
of the tibio-fibula. Between the sartorius and the gracilis major
is the proximal end of the adductor magnus muscle.

Sketch these ventral muscles; then cut through the sartorius
and both gracilis muscles and make out the following parts:

Nearly the whole of the adductor magnus is now seen; note
its two heads, dorsal and ventral, arising from the ischium, and
trace its insertion on the distal end of the femur.

Lateral to the proximal end of the adductor major a bit of the
pectineus muscle is seen and still farther to the outside the adduc-
tor longus, which crosses the proximal end of the crural. Separate
these, making out the origin of the pectineus from the anterior
part of the pelvic symphysis, and that of the adductor longus
from the lateral surface of the ileum. Their insertions, respectively
on the middle of the femur and on the tendon of the adductor
magnus, should be found.

On the medial side of the adductor magnus is the slender
semimembranosus muscle arising by two heads.

Sketch the three deeper muscles of the ventral side. There
are several smaller and deeper muscles which may be omitted.

The largest muscle in the shank of the leg is the plantaris
longus, which arises by two heads, a larger one in the angle of
the knee, a smaller from the tendon of the triceps on the outside
of the knee. Below, the muscle passes into a long tendo Achillei
which passes over the 'heel' to the plantar surface of the foot.

Lateral to this plantaris, as seen from above, is the peroneus
muscle, which arises from the convex side of the knee and is
inserted by two tendons on the distal end of the tibio-fibula.

Add these muscles to the sketch of the dorsal superficial
muscles of the thigh.

Turn the frog ventral side uppermost. The os cruris appears
in the middle line of the shank. The extensor cruris brevis is
attached to the proximal half of its lateral side, its origin being
on the distal end of the femur. On the medial side the tibialis
posticus is similarly attached to the os cruris. The tibialis anticus

THE FROG 13

brevis (partly covered by the extensor cruris brevis) is attached
to the lateral distal side of the os cruris, while the tibialis anticus
longus lies between the external cruris brevis and the tibialis
anticus brevis on the one side and the peroneus on the other.

Work out the origin and insertion of these muscles and add
them to the drawing showing the ventral superficial muscles of
the thigh.

The muscles of the ankle and foot are numerous. They may be
grouped as flexors on the plantar side, extensors on the dorsal
side.

THE ORAL CAVITY. — Open the mouth and find the in-
ternal nares (choanae) by inserting a probe in the external naris.
Look in the roof of the mouth, between the choanse, for the
vomerine teeth. What is the shape of the tongue? How is it
attached? How does this differ from the condition in yourself?
Note where the Eustachian tube enters the mouth or, better,
pharynx. Look in the floor of the back part of the pharynx for
a longitudinal slit, the glottis, bounded by a pair of semicircular
elevations (caused by the arytenoid cartilages beneath the
lining membrane). Behind the glottis the oesophagus begins.

Draw the floor and roof of the mouth, illustrating all of these

points.

VISCERAL ANATOMY.— With the injected specimen lay
back the skin of the ventral surface as before and note, on its
inner surface, two branches of the great cutaneous artery and a
single trunk of the great cutaneous vein. Note where they enter
the deeper structures.

Lay open the body cavity (peritoneal cavity or coelom) by
a cut a little to one side of the median line, extending from the
origin of the hind legs to the shoulder-girdle, which can be felt
through the flesh, cutting nothing but the body wall.

Lift the wall very carefully and see the abdominal vein extend-
ing dorsally from the wall to the liver and running backwards in
the ventral body wall. Follow it posteriorly to its origin by the
union of two femoral veins, one coming from each hind leg. Sketch
these in blue on a sheet intended to show the entire venous
system.

Make transverse cuts, in front and behind, through the body
wall and pin out the flaps, exposing the viscera. Most conspicu-
ous are the dark-colored liver in front and the coils of the intes-

14 GUIDES FOR VERTEBRATE DISSECTION

tine behind. Insert a blowpipe in the anus and inflate the
urinary bladder in the posterior part of the peritoneal cavity.

Sketch these viscera without disturbing them.

Study the parts more carefully. See that the whole cavity is
lined with a smooth membrane, the peritoneum, and that this
covers the enclosed viscera as well. Insert a probe in the oesoph-
agus, then turn the liver to the right and note the posterior end
of this tube. Just inside the ccelom the alimentary canal enlarges
to form the stomach, which is somewhat U-shaped. Its two
ends have received special names. That joining the oesophagus
is the cardiac, the other the pyloric end. Note the constriction
(pylorus) at the pyloric end, formed by a sphincter muscle.

Follow the course of the intestine backwards from the stomach,
taking pains not to tear the membranes binding the various
parts of the alimentary tract together and to the body wall.
The first portion of the intestine, paralleling the pyloric part
of the stomach is the duodenum. The rest of the tube, of about
the same size throughout, is the small intestine. Near the
posterior end there comes a sudden enlargement, the rectum,
which proceeds backwards until the urinary bladder is reached,
where this enlarged portion is known as the cloaca, a region which
extends to the anus.

See the membranes which bind the alimentary canal together
and to the body wall. Those connecting the different parts
of the tract are called omenta, those connecting the canal to
the body wall are mesenteries. The mesentery supporting the
stomach is the mesogaster, that of the small intestine the mesentery
proper, that of the rectum the mesorectum. The hepato-duodenal
omentum connects liver and duodenum, the gastro-duodenal
omentum lies between stomach and duodenum, and the gastro-
hepatic omentum between stomach and liver.

Examine the liver more carefully. How many lobes are
there? See on the postero-dorsal surface of one of its lobes the
spherical gall-bladder (greenish in life). Trace the bile-ducts
from it into the liver and into the intestine.

Look in the gastro-duodenal omentum for the elongate
pancreas. Does it extend into the mesentery or into any other
omentum? Look in the mesentery between the rectum and the
bend of the stomach for the round, dark-colored spleen.

Make a more detailed drawing of the alimentary tract.

THE FROG 15

Tip the intestine to the right and note, in the mesentery
just in front of spleen, the common intestinal artery coming from
the mid-dorsal line. It soon divides into anterior and posterior
branches. The anterior is the cceliac axis, the other the anterior
mesenteric artery. Follow the coeliac axis to its division into a
ventral gastric artery going to the stomach and a gastro-hepatic
trunk which soon divides into an hepatic artery going to the liver
and a dorsal gastric artery going to the stomach. What regions
of the stomach do the two gastric arteries supply? Does the
hepatic artery enter the liver near the bile-duct? Where does
the pancreatic artery connect with the other vessels?

In the same way follow the branches of the anterior mesen-
teric: first a short splenic artery to the spleen, next branches to
the intestine and a haemorrhoid artery to the anterior part of the
rectum.

Sketch these arteries in red on a sheet which shall later con-
tain the whole arterial system.

Note also small veins in the mesentery paralleling the arterial
twigs. These unite to form the intestinal vein. Trace this forward,
finding the entrance of splenic and gastric veins. How many
trunks of the latter come from the stomach? All these veins
unite to form the hepatic-portal vein which enters the liver.
Does the abdominal vein connect with this? Does the portal
enter the liver by a single trunk?

Sketch this portal system in the drawing of the veins.

At either side of the liver, projecting into the body cavity
from in front, is a lung. Inflate with blowpipe from the glottis.
Sketch the blood-vessels (pulmonary arteries and veins) in the
walls. How many branches to each? Cut across a lung. Are
there small air-sacs throughout or are they confined to the walls?
Which are more internal, arteries or veins? Sketch a bit of the
lung showing these points.

Cut across the stomach at the cardiac end and across the
intestine just in front of the rectum. Trim the mesenteries close
to the alimentary tract and remove the latter from the body.
Split it lengthwise with the scissors and examine the internal
surface with the hand-lens, noting the differences in the character
of the lining in the different parts.

The urogenital organs are now exposed. In the posterior part
of the body cavity will be found a pair of mesonephroi (' kidneys ')i

16 GUIDES FOR VERTEBRATE DISSECTION

oval in outline and dark-colored. Resting on each is a rounded
yellow body, the testis, attached only by its medial border. Each
testis bears a number of yellow finger-like bodies, the fat bodies
or corpora adiposa. Tip one of the testes toward the median line
and see, on the ventral surface of the mesonephros, a long band-
like structure, bright yellow in the fresh specimen, the suprarenal
gland. The deferent duct (Leydig's duct) leads back from the
lateral margin and hinder angle of the mesonephros. Trace
those of the two sides back to their entrance into the cloaca.
Do they unite before entrance? Do you find an enlargement
(seminal vesicle) on them?

Sketch the organs.

Examine more closely with hand-lens and note the renal and
genital (spermatic) arteries and veins entering and leaving the
kidneys and testes on the medial side. How are the fat bodies
supplied? Then turn one of the testes to the median side and
see the fine vessels (vasa efferentia) passing from the testis into
the mesonephros. Are they connected longitudinally? Sketch
these relations.

In the female the lobulated ovaries, enormous in size and
crowded with eggs, are prominent on either side, especially near
the breeding season. Without tearing see that each ovary is
attached to the dorsal body wall by a thin membrane, the meso-
varium, in which .the ovarian arteries and veins run. In front
of the ovary look for the fat body. Is this attached to the
ovary?

Cut into the ovary ; is it solid or hollow? Sketch the sec-
tion showing the relative position of the eggs.

Remove the ovary from one side, exposing the oviduct (Mtil-
lerian duct), a greatly contorted tube extending the whole length
of the peritoneal cavity. Trace it backwards. Does it com-
municate with the cloaca or does it open directly to the exterior?
Is it free during most of its course, or is it supported from the
body wall? Is any part expanded to form a thin-walled uterus?
Trace it forward, removing the stomach and liver if necessary
to see its anterior end. Do tue tubes of the two sides unite?
Do you find one or two anterior openings (ostium tubae) ?

Remove the oviduct. Do you find any tube connected with
the kidney comparable to Leydig's duct in the male (Wolffian
duct)? What differences can you recognize? Slit open the

THE FROG 17

cloaca on one side and find the openings of the bladder and the
urogenital ducts.

Sketch the urogenital organs of the female.

Returning to the injected male, tip the liver forward so that
its dorsal surface may be seen. See proceeding from each lobe
of the liver an hepatic vein which enters a large venous trunk,
the postcava, the anterior portion of which will be traced later.
At present follow it backwards. To which side of the oesophagus
does it pass? Note the vessels it receives from the fat body,
genitalia, and mesonephroi. How far back of the latter organs
does it extend?

On the lateral dorsal surface of each mesonephros is a vessel
(Jacobson's vein) which, behind the organ, is called the common
iliac vein. Trace it backwards to its origin by the union of two
vessels, an external iliac vein, coming from the more ventral
part of the leg, and an ischiadic vein, entering the pelvis dorsal to
the hip-joint. Which of these veins is connected with the femoral
vein (p. 13)? Follow both into the leg.

Add these vessels to the sketch of the venous system.

Part of the blood coming from the hind limb finds its way to the
heart, by passing, via the common iliac and the postcava, through the
kidney. This forms the renal portal system. Another portion passes,
by way of the femoral and abdominal vein, through the liver, and hence
this forms a part of the hepatic portal system, the rest of which has
already been seen.

The dorsal aorta lies just dorsal to the postcava. In front it
can be seen to arise from the union of two arterial trunks, the
radices aortae, the rest of which will be followed later. Trace the
aorta backwards from this point, noting the origin of the various
arteries already seen to arise from it. From what does the com-
mon intestinal artery arise? Are the urogenital arteries single
or paired in origin?

Behind the last urogenital artery the aorta gives off a median
posterior mesenteric artery which supplies the posterior part of
the rectum. Just behind this point it divides into a pair of
common iliac arteries, each of which soon gives off an epigastric
artery supplying the urinary bladder and ventral body wall.
Each common iliac now divides into a femoral and an ischiadic

18 GUIDES FOR VERTEBRATE DISSECTION

artery, which should be followed into the leg, the latter being
much the larger and forming the chief supply of the limb.

Add these vessels and their branches to the sketch of the
arterial system.

The liver abuts in front against a thin membranous partition,
the false diaphragm or septum transversum. Cut through this
and lay open a second part of the coslom, the pericardial cavity,
lined with a pigmented epithelium, the pericardium. In this
cavity lies the heart, free behind but attached dorsally and in
front.

Make out in the heart a posterior conical portion, the ventricle,
and in front a pair of thinner walled auricles. Which of these
latter is the larger, the right or the left? Crossing the right auricle
and extending forward from the ventricle is an enlargement, the
bulbus cordis, from the anterior end of which arises a cylindrical
tube, the truncus arteriosus impar, which extends forward to the
anterior wall of the pericardial cavity, where it divides into two
trunci arteriosi.

Tip the heart forward, tearing the membrane (dorsal meso-
cardium) holding it, and see, on its dorsal surface, a large triangular
sac, the sinus venosus, formed by the union of three vessels, the
postcava behind and a precava in front on either side. Follow
the postcava backwards to the liver region (p. 17) and then
follow out the right precava beyond the pericardium to its divi-
sion into three trunks, the anterior (external jugular vein) going
straight forward, the middle (vena anonyma), and the posterior
(subclavian vein). The anonymous vein soon divides into the
internal jugular and the scapularis veins, the subclavian into
the great cutaneous and the brachial veins. Trace the former
into the skin, the latter into the arm.

Tip the heart forward again and see the two pulmonary veins,
one coming from each lung. Into which auricle do they empty?

Sketch these blood-vessels in their respective sheets, inserting
the heart, in outline in both, showing respectively its arterial and
venous connections.

Follow the truncus. Each one soon divides into three curved
vessels, the anterior the common carotid artery, the second the
radix aortae, and the third the pulmo-cutaneous artery. Note in
the carotid, soon after its separation from the rest, an enlargement,
the carotid gland, at which the vessel divides into external and

THE FROG 19

internal carotid arteries. Follow the external carotid into the
lower jaw, the external dorsally to the posterior end of the head.
In the same way follow out the pulmo-cutaneous arch to its
division into the pulmonary artery going to the lungs and the
great cutaneous artery which ascends alongside the cucularis
muscle. How many branches to the pulmonary artery before
it enters the lungs? After these vessels have been dissected
out the ramifications of the internal carotid may be followed.
Then follow the radix aortae, note and follow the subclavian
artery into the fore limb and the main trunk of the radix to the
union with its fellow to form the dorsal aorta.

Sketch these vessels in the arterial drawing. Your two
sketches should now present the main arterial and venous trunks
of the body.

The heart should now be studied in the uninjected specimen.
Cut the connecting vessels (cavae, pulmonary veins, and the
great arterial trunks beyond their division into carotid, radix,
etc.) and remove heart from body.

Note the position of the entrance of the cavffi and the
pulmonary veins and of the giving off of the truncus arteri-
osus impar. Cut across the two paired arterial trunks close to
their origin from the unpaired trunk and note the cut ends.
Are they simple tubes? What relations do you recognize to vessels
already seen?

Pin the heart out under water, the ventral side uppermost,
and carefully dissect away the ventral side of the auricles, ventricle,
and the bulbus. Wash out the coagulated blood.

Are the two auricles completely separated by a partition
(interauricular septum)? Which one connects with the sinus
venosus? What relative difference is there in the positions of
the sinu-auricular opening and that of the pulmonary veins?

Is there a separate opening (auriculo-ventricular canal) from
each auricle into the ventricle? To settle this examine first the
auricles and then the ventricle. How are the differences to be
explained? Which has relatively the thicker wall, the right or
left auricle or the ventricle?

In the bulbus cordis notice a longitudinal, somewhat twisted
fold, the spiral valve or septum bulbi, practically dividing the
bulbus into right and left halves. Probe through the cut ends of
the arterial trunk with three bristles and note where each appears.

20 GUIDES FOR VERTEBRATE DISSECTION

On which side of the septum bulbi is the pulmo-cutaneous and
on which the carotid-systemic cavity? Would this imply a
separation of two kinds of blood as it leaves the ventricle?

Draw the dissected heart, X4, showing these points. Then
read some good account of the parts and their function (e.g.,
Gaupp, 2, pp. 247-265; Huxley and Martin, pp. 17-19).

THE NERVOUS SYSTEM. The spinal nerves.— In the dorsal
wall of the body cavity after removal of the viscera see several
pairs of white nerve trunks, the spinal nerves. Each nerve
makes its appearance in the midst of a grayish white mass, the
so-called periganglionic gland or calciferous body (in reality an
extension of the endolymph space of the spinal cord filled with fine
granules of calcium carbonate). The spinal nerves in the posterior
region pursue a course nearly parallel with the main axis of the
body; those in front are nearly at right angles to that axis. How
many * of these nerves do you find? Which of the anterior
pairs is the largest? Follow this one out, noting the region to
which it is distributed. Can you now explain its greater size?
Examine more closely and see if any of the other spinal nerves
send twigs to this large one, thus forming a brachial plexus.
Compare your results with those found by others in the laboratory
and see if there be any variation.

Note that several of the posterior nerves, by means of cross-
branches, form a lumbo-sacral plexus. What nerves, by number,
enter into its constitution? Are variations found in number
or in arrangement of these nerves and branches? Besides
several smaller nerves two larger ones arising from this plexus
enter the hind limb, a smaller and more lateral crural nerve,
and a larger ischiatic nerve nearer the median line. What is
the distribution of each?

Examine the spinal nerves more closely and see that each,
soon after leaving the periganglionic gland, gives off a small nerve,
ramus communicans, from its ventral surface. These rami
communicantes run toward the median line to join, either side
of the vertebral column, a longitudinal nervous structure, the
truncus sympatheticus. This sympathetic trunk runs on either

* In the young there is a spinal nerve in front of the first vertebra. This
disappears long before the adult condition, but still the first of the per-
sistent nerves, issuing between vertebra? 1 and 2, must be called the second
of the spinal nerves.

THE FROG 21

side just dorsal to the aorta and the radices aortse. It is com-
posed of a number of enlargements, the sympathetic ganglia and
smaller nerves — rami intergangliares — connecting them. How
many sympathetic ganglia do you find? Do the rami communi-
cantes join the ganglia or the interganglionic nerves? Note
several (how many?) small nerves arising from the median
sympathetic ganglia to form a trunk (splanchnic nerve) which
follows the common intestinal artery to be distributed to the
viscera.

Remove carefully the corresponding vertebral centra and
then follow the proximal portion of the eighth, ninth, and tenth
spinal nerves through the periganglionic glands to the spinal
cord, which lies just dorsal to the centra. At the gland each
nerve is divided into two portions or roots, one dorsal, the other
ventral in position. Which of these, the dorsal or ventral, bears
an enlargement (ganglion)?

Draw the spinal nerves, their roots and ganglia, together
with the sympathetic system as far as made out, X2.

Spinal Cord. — Place the frog for several days in nitric alcohol
(equal parts of 95% alcohol and 10% nitric acid) so as to decalcify
the skeleton. When the bone is softened remove the neural
arches from the vertebrae, exposing the spinal cord. What parts
of it are larger than the rest? Does the distribution of the
spinal nerves suggest any reason for the enlargements? Notice
its small hinder portion, the filum terminale. Is there a groove
(dorsal fissure) running along the dorsal surface of the cord? Can
you see any of the dorsal roots of the spinal nerves and note
where they enter the cord?

Make a drawing of the spinal cord, X4, leaving space in
front to insert the brain later.

The Brain. — Lay open the brain cavity by removing the skin
and fronto-parietal bones and underlying cartilage, beginning
a little in front of the eyes and proceeding backwards in the
most careful manner, taking great pains not to injure the underly-
ing structures. In this operation you will doubtless tear the
slender stalk (paraphysis) passing from the brain to the roof of
the cranium beneath the brow spot. Continue to develop until
the whole dorsal surface of the brain is exposed back to its con-
nection with the spinal cord. In this brain make out the follow-
ing regions.

22 GUIDES FOR VERTEBRATE DISSECTION

In front an elongate telencephalon composed of right and left
halves (cerebral hemispheres, separated by the sagittal fissure) .
Anteriorly the two hemispheres are separated by a slight con-
struction from an olfactory lobe (rhinencephalon). Are the two
olfactory lobes distinct from each other?

Behind and between the diverging posterior ends of the
telencephalon and lying at a somewhat lower level is the dien-
cephalon or 'twixt-brain, which in turn is bounded behind by
a pair of prominent rounded optic lobes, the mesencephalon.

Immediately behind the mesencephalon and lying at a lower
level is a transverse band, the greatly reduced metencephalon
or cerebellum. The lateral ends of this rest upon the myelenceph-
alon or medulla oblongata, a large region which extends back to
the posterior end of the cranium.

Study these parts more in detail. Make out in the roof of the
diencephalon the position of the paraphysis already noted, and
immediately behind it a short longitudinal stalk (pedicle of the
epiphysis) which reaches back to its origin from about the middle
of the 'twixt-brain. Pull the optic lobes slightly toward the
posterior end and see, at the hinder end of the diencephalon, a
narrow optic tract extending outwards and forwards between the
optic lobe and 'twixt-brain. Note the direction of the major
axis of the mesencephalic hemispheres.

If carefully opened the dorsal surface of the myelencephalon
will be found to be covered with a loose tissue, the taenia or roof
of the fourth ventricle. Removal of this exposes a triangular
opening (fossa rhomboidalis) into the cavity (fourth ventricle).
The lateral walls of this fossa form the eminentia acustica. Is
the fossa rhomboidalis continued backwards as a groove upon the
dorsal surface of the spinal cord?

Draw the brain, X4, from the dorsal surface on the drawing
of the spinal cord, leaving space on one side of the sheet to insert
the cranial nerves, to be studied next.

The Cranial Nerves.* — The frog, like other Ichthyopsida,
has but ten pairs of nerves given off from the brain inside the

* The study of the cranial nerves is facilitated by allowing the head to
remain from two to four weeks in the nitric alcohol, after which the muscles,
etc., are readily removed with the forceps. This, however, renders the brain
very tender and interferes with its subsequent removal from the head and
its dissection. The complete dissection of the nerves takes more time than

THE FROG 23

cranium. These are known both by name and by number. In
some cases the roots of two or more of these nerves may be
so closely associated that only microscopical analysis can dis-
tinguish them as they emerge from the brain or as they pass
through the skull.

Carefully pick away the muscles lying between the brain and
the tympanum until at about the level of the side of the brain
the nerve-trunks are reached. These will be recognized by their
white color and, in this region, the transverse direction of the
larger trunks.

Most posterior of the series is the complex of the ninth and
tenth nerves. This emerges from the brain at about the level
of the middle of the fossa rhomboidalis, passes outwards behind
the otic capsule and soon presents a small enlargement, the
jugular ganglion, beyond which the trunk separates into several
branches.

The two anterior of these form the glossopharyngeal (IX)
nerve. At their common base these have a small glossopharyn-
geal ganglion, lateral to which the two branches separate. The
posterior is the glossopharyngeal proper, which passes outwards
and then downwards. The other is Jacobson's commissure,
which should be traced laterally to the medio-posterior side of
the tympanum, where it unites with a branch (hyomandibular)
of the seventh nerve, the common trunk now passing straight
outwards behind the tympanum.

The other branches, which extend laterally, form the vagus
nerve (X), while from the jugular ganglion a commissure of the
sympathetic system extends backward to the second spinal
nerve (so-called hypoglossus, XI), which leaves the spinal cord
between the first and second vertebrae.

The roots of the fifth and seventh nerves are also closely
associated with each other. They arise from the antero-lateral
side of the medulla and unite almost immediately to form a
large common prootic ganglion* inside the cranium. Outside
the skull four main trunks are immediately visible. Of these the
hindermost is the hyomandibularis branch of the facial nerve (VII)

can be given in the ordinary course, and hence only the roots and main
trunks are noticed here. Those wishing to follow it farther should use
Gaupp's description.

* A fusion of Gasserian (5th) and geniculate (7th) ganglia.

24 GUIDES FOR VERTEBRATE DISSECTION

already mentioned. Trace it outwards and backwards to its
union with Jacob son's commissure.

The next branch is the common maxillo-mandibularis trunk
of the trigeminal nerve (V). Trace it laterally to its division into
maxillaris and mandibular nerves. The next anterior is the
superficial temporal branch of the trigeminal nerve, the distal
portion of which has probably been removed with the muscles.
It supplies the adjacent region of the top of the head.

Now carefully remove the tissues between the eye and the
cranium, keeping watch for the ophthalmic branch of the fifth
nerve, which runs forward from the common prootic ganglion,
parallel to the side of the cranium. Trace it forward to the tip
of the snout.

The ophthalmic nerve crosses the optic nerve (II), which runs
obliquely forwards and outwards from beneath the diencephalon
to the eyeball.

Running parallel to the ophthalmic nerve in the orbital region, but
passing beneath the optic nerve, is a part of the oculomotor nerve (III).
The branches of this, as well as the whole of the trochlearis (IV) and
abducens (VI) nerves, are difficult to trace.

The olfactory nerve (I) should be traced forward from the
rhinencephalon to its distribution to the olfactory organ.

Add these nerves to the sketch of the brain.

Turn the head on the side and follow the branches of the
vagus, hyomandibular, glossopharyngeal, maxillaris, and man-
dibular nerves outwards and downwards, and trace all but the
maxillaris on to the lower jaw.

Now remove the lower jaw; remove the skin from the roof
of the mouth, taking great care not to take the underlying nerves
with it. Several nerves will be exposed. Of these note especially
the palatine nerve, a branch of the seventh, which passes nearly
straight forward near the middle line across the inner side of the
floor of the orbit. Trace its branches as far as convenient and
also follow it dorsally to its origin from the common prootic
ganglion.

Add all these nerves to your drawing.

Cut through the olfactory and optic nerves, and with the
forceps grasping the olfactory, carefully lift up the brain, cutting
the successive nerves as close to the cranial wall as is possible.

THE FROG 25

Remove the brain from the cranium and study its lateral and
ventral aspects.

In the side view recognize all the regions seen from above,
noting the prominent way in which the metencephalon projects
above the myelencephalon. Recognize also the broad fibre
tract — the optic tract, part of which has been seen in the dorsal
view — extending downward over the side of the diencephalon.
Examine carefully the side of the olfactory lobe and see if you
can recognize two roots to the olfactory nerve. Find the roots
of nerves V, VI, IX-X already seen and look for the root of III
near the ventral surface, immediately below the middle of the
optic lobe. Examine carefully the contour of the ventral surface.

Draw a side view of the brain, X4.

Also make a similar drawing of the ventral surface. Note
the distance to which the inferior roots of the olfactory nerves
extend backwards. Follow the optic tracts to the union (in
reality a crossing — chiasma) on the ventral surface and note the
origin of the optic nerves from this chiasma. Immediately
behind the chiasma is a median swelling, indistinctly two-lobed, the
infundibulum, with the roots of the III nerve projecting between
its sides and the main part of the brain. Behind the infundib-
ulum is a three-lobed projection, the hypophysis, lying on a bed
of cross-fibres, the lemnisci. In the ventral surface behind the
hypophysis see the median groove, the ventral fissure, marking
off the two sides of the brain and extending back on the cord.
The two regions thus differentiated have the special name of
pedunculi or crura cerebri from the mesencephalic region forward,
beneath the infundibulum and chiasma, to the telencephalon.

With a sharp scalpel carefully slice off the roof of the brain
until the cavities are exposed in its whole extent, from the front
of the cerebral hemispheres back to the limits of the fossa rhom-
boidalis. Of these make out a pair of cavities (ventricles) lying
in the telencephalon (ventricles 1 and 2). Do they extend into
the olfactory lobes? Also an unpaired cavity (third ventricle) in
the diencephalon, connecting in front with the first and second
by transverse passages (foramina of Monro; interventricular
foramina) . The wall between the anterior end of the third ventri-
cle and the sagittal fissure (p. 22) is the lamina terminalis. In
the optic lobes are a pair of mesencephalic ventricles, which, since
they do not occur in mammals, are not numbered. The median

26 GUIDES FOR VERTEBRATE DISSECTION

part of the cavity in this region is the aqueduct or iter; it extends
back to the fourth ventricle in the myelencephalon, the extent
of which is slightly larger than the fossa rhomboidalis.

Draw the brain thus dissected, showing the various cavities
and the relative thickness of the walls in different regions.

THE FROG 27

BIBLIOGRAPHY

Gaupp's extensive work on the frog (quoted below) should be in
every laboratory where this animal is dissected in other than a super-
ficial manner. These volumes contain such a complete bibliography
of the subject that scarcely any mention of works is needed here. There
are therefore given only a few manuals of dissection, a few general works
dealing with the morphology of the Anura, and some papers, mostly of
American origin, which have appeared since Gaupp's volumes were in
print.

MANUALS OF DISSECTION

BOURNE, G. C. An introduction to the study of the comparative anatomy

of Animals. Vol. 1. London, 1900. (Frog, pp. 16-107.)
DODGE, C. W. Introduction to elementary practical Biology. N. Y.,

Harper's, 1894. (Frog, pp. 202-263.)
HUXLEY, T. H., and MARTIN, H. N. A course in elementary instruction

in Practical Biology. Revised edition by G. B. Howes and D. H.

Scott. London, 1889. (Frog, pp. 1-172.)
MARSHALL, A. M. The Frog: An introduction to anatomy, histology,

and embryology. London, 1896.
PRATT, H. S. A course in Vertebrate Zoology. Boston, Ginn, 1905.

(Frog, pp. 94-134.)

GENERAL WORKS

Handbuch der vergleichenden und experimentellen Entwicklungslehre.

Jena. 6 vols.
COPE, E. D. The Batrachia of North America. Bulletin U. S. National

Museum, No. 34. Washington, 1889.
ECKER, A. The anatomy of the Frog. Translated by George Haslam.

Oxford, 1889.
GADOW, H. Amphibia and Reptiles. Cambridge Natural History,

vol. 8. London, 1901.
GAUPP, E. A. Ecker's und R. Wiedersheim's Anatomie des Frosches.

Braunschweig. 1. Abth., 1896; 2. Abth., 1899; 3. Abth., 1904; pp.

1738.
GEGENBAUR, C. Vergleichende Anatomie der Wirbelthiere. Leipzig,

1898-1901. 2 vols.
HERTWIG, R. A manual of zoology. Translated and edited by J. S.

Kingsley. N. Y., Holt, 1902.

HOLMES, S. J. The Biology of the Frog. N. Y., Macmillan, 1906.
HUXLEY, T. H. A manual of the anatomy of Vertebrated Animals.

London, 1882.

GUIDES FOR VERTEBRATE DISSECTION

JOHNSTON, J. B. The nervous system of Vertebrates. Philadelphia,
Blakiston, 1906.

KINGSLEY, J. S. Text-book of Vertebrate Zoology. N. Y., Holt, 1899.

OWEN, R. On the' Anatomy of Vertebrates. London, 1866-68. 3 vols.

PARKER, W. K., and BETTANY, G. T. Morphology of the Skull. London.

PARKER, T. J., and HASWELL, W. A. A text-book of Zoology. London,
1897.

STANNIUS, H. LehrbuchderZootomie. Berlin, 1856.

WIEDERSHEIM, R. Elements of the comparative anatomy of Verte-
brates. Adapted by W. N. Parker. London, 1897.

Vergleichende Anatomie der Wirbeltiere. 6. Auflage. Jena, 1906.

MORGAN, T. H. The development of the frog's egg. N. Y., Macmillan,
1897.

SHORTER ARTICLES

BERNARD, H. M. Studies on the Retina. Quar. Jour. Micr. Sci., 43,

1900; 44, 1901; 46, 1906.
CAMERON, J. Origin of the epiphysis in Amphibia as a bilateral structure.

Rept. Brit. Assoc. Adv. Sci., 73, 1904.
COLE, F. J. Some variations in the spinal nerves of the Frog. Trans.

Liverpool Biol. Socy., 15, 1901.
Fox, H. Development of tympano-Eustachian passage and associated

structures in the toad. Proc. Acad. Nat. Sci. Philadelphia, 1901.
HEMPSTEAD, M. Development of the lungs in frogs. Proc. Am. Assoc.

Adv. Sci., 49, 1900. ,

JONES, W. C. Development of sympathetic nervous system in the toad.

Jour. Comp. Neurol., 15, 1905.
KING, H. D. Maturation and fertilization of the egg of Bufo lenti-

ginosus. Jour. Morphology, 17, 1901.
REED, M. Formation of the interior cells in the segmentation of the

frog's egg. Biolog. Bulletin, 8, 1905.
SAMPSON, L. V. Contribution to the embryology of Hylodes martini-

censis. Amer. Jour, of Anat., 3, 1904.
STRONG, 0. S. Cranial nerves of Amphibia. Journal of Morphology, 10,

1895.
WILSON, H. V. Formation of the blastopore in the frog egg. Anatom.

Anzeiger, 18, 1900.
YERKES, R. M. Sense of hearing in frogs. Jour. Compar. Neurology,

15, 1905.

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