ALBERT R. MANN LIBRARY AT CORNELL UNIVERSITY DATE DUE CCCTa hn | eee = bict 7B 107% dca GAYLORD sit QL 668.E2M35 1896 “Wan Cornell University Library The original of this book is in the Cornell University Library. There are no known copyright restrictions in the United States on the use of the text. http://www.archive.org/details/cu31924002877078 THE FROG: AN INTRODUCTION TO ANATOMY. HISTOLOGY, AND EMBRYOLOGY. THE FROG: AN INTRODUCTION TO ANATOMY, HISTOLOGY, AND EMBRYOLOGY. 4 6 ty 1 LIBRA m. BY THE seal A. MILNES MARSHALL, M.D..D.Sc., M.A, F.R.S., FORMERLY FELLOW OF ST. JOHN’S COLLEGE, CAMBRIDGE ; PROFESSOR IN THE VICTORIA UNIVERSITY ; BEYER PROFESSOR OF ZOOIOGY IN OWENS COLLEGE EDITED BY G. HERBERT FOWLER, B.A., Pu.D., LATE BERKELEY FELLOW OF THE OWENS COLLEGE, MANCHESTER; ASSISTANT PROFESSOR OF ZOOLOGY, UNIVERSITY COLLEGE, LONDON SIXTH EDITION REVISED AND ILLUSTRATED NEW YORK MACMILLAN AND CO. 1896 Med. >1\ PREFACE TO THE FIRST EDITION. Tue Owens College Course of Elementary Biology, which forms part of the scheme of study prescribed by the Victoria University, is of a rather more extended and comprehensive nature than the courses held elsewhere under the same name; and experience has shown me that there is want of a book that will guide and direct the student through the practical part of his work, the whole ground of which is covered by no one of the existing manuals. It is to meet this want that the present little work has been prepared. This first instalment of the work consists of an Introduction, containing practical instruction in the methods employed in biological investigation ; followed by the application of these methods to the examination, both anatomical and histological, of an actual animal. For this purpose the frog has been selected 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 reason, 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 dis- section, etc., have throughout been printed in italics, It is not expected that the student should do the whole of iv PREFACE the work here given the first time he goes over it. The dis- section of the muscles and of the cranial nerves should only be attempted if time remain after the other work is completed. In preparing this first part I have received very valuable assistance from Dr. Hartog, Demonstrator of Biology in the College, and from my friend and pupil Mr.C. H. Hurst. Iam also much indebted to Prof. Gamgee and to Mr. Waters for the important help they have given me in the histological portions. OWENS COLLEGE, August, 1882. PREFACE TO THE SIXTH EDITION. A Few additions and alterations, based upon a continuous working knowledge of the book since its first issue, have been made in this edition. Professor Marshall had so complete a grasp of the difficulties which confront a student of Elementary Zoology, and so great a sense of proportion and arrangement, that no substantial alteration could increase the educational value of his work. Two new figures have been added, Figs. 30 and 37. For the second of these, taken from Professor Marshall’s “ Vertebrate Embryology,” I am indebted to Messrs. Smith, Elder & Co. UNIVERSITY COLLEGE, Lonpon, 1896. CONTENTS. INTRODUCTION. Apparatus required. Dissection. Drawing. Use of the Microscope. Preparation of Microscopical Objects. Section cutting. Table of Histological Processes CHAPTER I. GENERAL ANATOMY OF TIE FROG. External Characters. Buccal Cavity. Abdominal Viscera. Peritoneum. Digestive Organs CHAPTER II. THE VASCULAR SYSTEM OF THE FROG. The Heart. The Veins. The Arteries. The Structure of the Heart. Microscopical Examination of Blood CHAPTER III. THE SKELETON OF THE FRoG. The Axial Skeleton. The Appendicular Skeleton CHAPTER IV. Tue Muscucar SYSTEM OF THE FRoG. Muscles of Trunk. Muscles of Head. Muscles of Hind- limb : ; : CHAPTER V. Tue NERvous SYSTEM OF THE FROG. The Central Nervous System. The Peripheral Nervous System. Histology of Nerves PAGE I-14 15-23 24-39 40-54 55-67 68-86 vili CONTENTS CHAPTER VI. THE Eye anpD Ear. The Eye of the Frog. The Eye of the Sheep or Ox. OEE of the Eye. The Ear ofthe Frog. ; . 87-95 PAGE CHAPTER VII. THE REPRODUCTIVE ORGANS AND THE CLOACA. The Male Frog. The Female Frog . ‘ i ‘ ¥ 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 Cceelom. Development of the Skeleton. Development of the Urinary System . : 7 : ‘ ; 99-148 CHAPTER IX, ELEMENTARY HISTOLOGY, Epithelium. Glands, Muscle. Connective Tissue. Carti- lage. Bone ‘ ‘ 3 . 149-160 - INDEX : . F ‘ 161-167 INTRODUCTION. I.—LIST OF APPARATUS REQUIRED. Tue following apparatus is recommended 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, 7.¢., 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. a. A pocket lens, containing two or three lenses mounted in a handle, and giving when combined a magnifying power of at least six diameters. 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 HB pencil, and a piece of indian-rubber. 11. A cheap pair of compasses, for measuring the dissections. A Ne DISSECTION AND DRAWING II—ON DISSECTION. The object of dissection is to separate the several parts and organs from one another, so as to define their boundaries 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; é.g., when dissecting the bloodvessels or nerves of the throat, distend it by passing a small roll of paper or the handle of a seeker down the cesophagus; 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 under water, which sup- ports the parts and greatly facilitates the operation. A stream of water allowed 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. TII—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 MICROSCOPE 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, i.¢., 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 finishing 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 x2; if half the size, x 4, 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; ¢.g., 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 MICROSCOPE. The microscope consists essentially of a stand and a body, the latter of which bears at its ends the lenses 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 diaphragms. Through this hole light is directed on the object to be examined by means of a mirror 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 low power ; one magnifying many times (200 diameters or more), a high power. Similarly 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 varying with different objectives, and toa slight extent with different observers. The higher the power employed the closer must the objective be brought to the object. As the position of the object on the stage of the microscope is fixed, this distance is regulated by moving 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 adjustment. The sliding of the body should be 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. This fine adjustment is effected by a screw witha 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. Tn using the microscope attend to the following rules: 1. Always examine an object first with the low power. Having adjusted the eyepiece and objective, direct the light up the tube THE USE OF THE MICROSCOPE 5 of the microscope 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 close 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 clean 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 will 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 clearness 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 OF 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 PREPARATION OF MICROSCOPICAL 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 gradually 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 clean one. The most important mounting media are the following : 1. Normal Salt Solution : a 0°75 per cent. solution of com- mon salt in water. This is very 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. Glycerine 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 fix it to the slide. For permanent THE PREPARATION OF MICROSCOPICAL OBJECTS 7 preparations it is better to use glycerine jelly ; a drop of this should be melted on the slide, and the object transferred to it from glycerine: ring with cement as before. 3. Canada Balsam 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 cloves or turpentine in order to clear them, z.e., render them permeable by the balsam. Canada balsam, if too thick, may be diluted with chloroform, turpentine, 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 shreds 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 very 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 separate the component parts from one another. C. Maceration. The process of teasing is in many cases facilitated by pre- viously macerating the specimen, #.e., soaking it in some fluid, which, while preserving the individual cells, tends to loosen them from one another. The most important macerating fluids are as follows : 1, Ranvier’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. Miiller’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 (selective stains). The most important are the following : 1. Hematoxylin. There are various preparations of hema- toxylin, or logwood, used in microscopical work: the best is that proposed by Delafield. It is prepared thus : dissolve 4 grammes of crystallised hematoxylin 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 3to 4 days. Filter, add 100 ¢.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 hema- 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. Hematoxylin stains the nuclei of cells much more strongly than the other parts. 2. Borax-Carmine, 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-aleohol—.e., 70 per cent. alcohol to which a few drops of hydrochloric acid 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 acid-alcohol will vary, according to the nature and size of the specimen, from a quarter of an hour up to a day or more. 3. Picro-Carmine is a very useful, and to a certain extent a differential stain, as it colours the several tissues different tints. It may be prepared thus. Dissolve 1 gramme of carmine in 4 cc. of liquid ammonia and 200 c.c. of distilled water. Add 5 grammes of picric acid; shake the mixture well for some minutes, and then decant from the excess of acid. Leave the decanted liquid for some days, stirring it occasionally; then evaporate it to dryness, and to every 2 grammes of the dried residue add 100 c.c. of distilled water. Picro-carmine answers best with specimens preserved in 70 per cent. alcohol. They should be left in the stain for a day, and ‘then placed in 70, and afterwards in 90 per cent. alcohol. Some specimens give better results when washed freely with water on removal from 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 4 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 specimens 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 specimens, acetic acid may 10 PRESERVING AND HARDENING conveniently be mentioned here, as it is used for the same pur- pose as the true stains, i.¢., 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. VION PRESERVING AND HARDENING. The reagents in common use for killing and preserving small animals are valuable also from their power of “ fixing” the tissues, z.¢., 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 staining 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. 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. After removal it is thoroughly washed with water or weak alcohol, and then transferred to 70 per cent. alcohol before staining. 4. Chromic Acid. 70% B. Staining. 50% + 30% Water Borax Hematoxylin Carmine or Picro-Carmine Silver Nitrate > Water <— ) 30 % 50% Acid spirit —___-+ 70% C. Section cutting 4 % 7 and Mounting. Absol. Alec. Turpentine or Benzole + + YF Melted Canada Stron i g Glycerine Paraffin, Balsam. Glycerine. Jelly. CHAPTER I. GENERAL ANATOMY OF THE FROG. Fic. 1.—The Common Frog (Rana temporaria) (from Ecker). A. External Characters. Lay the frog on a board before you ; note, and make drawings showing the following points : 1. The division into head, trunk, and limbs ; and the absence of neck and tail. 2. The two great surfaces. a. The dorsal surface, or back, is directed upwards when the frog is in the natural position. b. The ventral surface, or belly, is directed downwards towards the ground. 16 GENERAL ANATOMY 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. Each eye has two eyelids, of which the upper is thick, pigmented, and almost immovable, while the lower is semi-transparent and freely movable. W101 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, Arm. il, Forearm. ii. Hand, with four digits, corresponding to the four fingers of man; the thumb being very small and inconspicuous. In the male frog there is a thicken- ing 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. li. 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 apertures: or openings on the surface of the body. a. Median apertures. j. The Mouth is a wide horizontal slit. ii. The Cloacal 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 projec- tion formed by the posterior end of the urostyle. THE BUCCAL CAVITY 17 b. Paired apertures. i. The Nostrils or anterior nares are two small open- ings on the dorsal surface of the head, close to its anterior end. B. The Buccal Cavity. Open the mouth to its full extent : note the wide buccal or mouth cavity, of which the hinder part or pharyna is continued back into the esophagus. Note also the following structures : 1. On the Roof of the Mouth. a. The Teeth. i. The maxillary teeth are a row of fine teeth, attached round the edge of the upper jaw. 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. 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. Pass bristles through the nostrils, and see that they come out through the posterior nares into the buccal cavity. c. 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. Perforate the tympanic membrane on one side with a needle, and pass a bristle or seeker through 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 with your finger, and note that it can be made to project very considerably into the 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 B 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. Turn the tongue forwards 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. Pass bristles through the glottis into the lungs. If any difficulty is experienced in finding the glottis snip through the angles of the mouth with scissors, so as to allow the mouth to be opened more widely. C. The Abdominal Viscera. Lay the frog on its back under water, and fasten it down to the dissecting board by pins through the limbs. Cut through the skin, along the middle line, the whole length of the ventral sur- face. Separate the skin from the underlying parts, noticing its very loose attachment to these parts, and the large space—a lymph cavity—beneath it. Turn the flaps of skin outwards, and pin them back. 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 with forceps the muscular body-wall, and cut through it into the body-cavity or celom with scissors a little to one side of the median line, being careful not to injure the anterior abdo- minal vein which runs along the inner surface of the body wall in the middle line. ; Continue the cut backwards to the hinder end of the body, and forwards to the jaw, cutting through the pectoral girdle with strong scissors, and taking care not to injure the parts beneath. Note on the inner surface of the larger flap the anterior abdo- minal vein, and carefully dissect this from the flap. Pull the two flaps apart, cutting through them transversely at their posterior ends to facilitate the process, and turn them back so as to display the viscera. ABDOMINAL VISCERA 19 Inflate the lungs with a blow-pipe through the glottis, and inflate the bladder through the cloacal aperture. Note and draw the general arrangement of the viscera,showing the following structures : 1. The heart, enclosed in the pericardium, is situated in the middle ventral line, and in the natural condition of the parts is covered by the pectoral girdle and the sternum. 2. The liver is a large reddish-brown bilobed organ, behind and at the sides of the heart. 3. The lungs are two thin-walled elastic sacs at the sides of the heart : they lie dorsal to the liver, and are often hidden by it. Note the bristles already passed into the lungs through the glottis. 4. 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 posterior end of the body cavity. Fic. 2.—A diagrammatic transverse section across the posterior part of the body of a female frog. a, urostyle; 4, muscles of body wall; d, large intestine; d.a, dorsal aorta; i, ilium; 2, lymph space between the skin and the muscular body wall; 7, spinal nerves ; 0, kidney; ov, oviduct ; 4, peritoneum ; s, skin; ¢, fold of skin at groin; z, ureter ; v, posterior vena cava. 6. In the female frog note, in addition to the above parts, a. The ovaries: two large bodies of irregular shape, each consisting of a mass of spherical black and white eggs, like small shot. 20 GENERAL ANATOMY OF THE FROG b. The oviducts: two long, very much convoluted tubes with thick white walls, lying at the sides 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. D. The Peritoneum. Notice the thin pigmented membrane—the peritoneum— which lines the body cavity. Trace this to the mid-dorsal line, where it is reflected downwards as a double layer—the mesen- tery—which embraces at its edge the alimentary canal, and binds its several coils together. (See Fig. 2.) Notice 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. E. The Digestive Organs. = (- _ FIG. 3.—General view of the viscera of the male frog, from the right side. a, stomach 4, bladder; c, small intestine; cZ, cloacal aperture ; d, large intestine ; ¢, liver; /, bile duct ; & gall bladder; 4, spleen; z, lung; &, larynx ; 7, fat body; m, testis; 7, ureter; o, kidney; Pp. pancreas ; 7, pelvic symphysis ; s. cerebral hemisphere ; sf, spinal cord ; Z, tongue; 7, auricle; «7, urostyle; v, ventricle; v.s, vesicula seminalis; ww, optic lobe; «+, cerebellum; y, Eustachian recess; z, nasal sac. ; DIGESTIVE ORGANS 21 Turn the liver forwards, and note the stomach lying beneath its left lobe. Pass the handle of a seeker through the mouth and down the esophagus into the stomach. [ff the specimen be a female, remove the ovaries and oviducts completely, taking cure not to damage the alimentary canal. | 1. The Alimentary Canal. a. The esophagus isa 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 stomach longitudinally along its left side, and wash out its contents: note the handle of the seeker already in- serted through the mouth; also the longitudinal folds of the mucous membrane lining the stomach, which increase the extent of its surface. i 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 GENERAL ANATOMY OF THE FROG a. The gall-bladder is a small spherical sac 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 trace the bile duct turn the liver forwards so that the point of attachment of the gall-bladder is clearly seen ; and slightly stretch the duodenum by a pin passed through the pylorus. De- termine the position of the two ends of the bile duct from the description given above, 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 tts contents : squeeze the gall-bladder so as to drive the bile along the duct into the duodenum: note the point at which it enters, and insert a bristle through the opening into the duct. Notice also the strong wavy transverse folds of the mucous mem- brane of the duodenum. 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. Cut through the mesentery along its attachment to the intestine : uncotl the intestine, leaving it attached at both ends, and spread it out on your dissecting board: measure 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 VISCERA 23 the other viscera, are outside the abdominal ccelomic 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 vesicula 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 fat 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. CHAPTER IT. THE VASCULAR SYSTEM OF THE FROG. Tue 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 veins, 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. : Pin down the frog on its back wnder water and open the body cavity as before, taking special care to preserve the anterior ab- dominal vein. Dissect this vein carefully from the body wall. In freeing the pectoral girdle from the underlying muscles take care not to injure the neighbouring bloodvessels. Open the pericardial cavity and dissect the pericardium from the heart and the roots of the great vessels, examine and draw the heart in situ, showing 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 truncus arteriosus is a cylindrical body arising from the right anterior border of the ventricle, and running obliquely forwards across the auricles. THE VEINS 25 Lift up the ventricle and turn its apex forwards so as to expose the sinus venosus. iv. The sinus venosus is a thin-walled sac, lying dorsal to the ventricle and behind the auricles; it receives the three large vene cave. 2. The pulsation of the heart. a. Note 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 series of contractions and dilatations. c. Note further that in each contraction or systole of the heart all four divisions of the heart contract, but not simultaneously. The sinus venosus contracts first, then the two auricles, then the ventricle, and finally the truncus arteriosus. B. The Veins. from the right side. a, stomach ; a.v, Matinee abdominal vein ; 4, bladder ; 4.v, brachial yein; c.d, cloacal aperture ;~c.v, cardiac vein; d, large intestine ; é, liver ; e.v, external jugular yein ; fv, femo al vein ; g, gall- -bladder ; a spleen j 46, en peli ae 2.0, inno’ n; 7-v, interna) ein ; left_pelvic_vein; 7.v, wilted le rere Ganey: ro ia atic Sy poral vet vein ; 7, right pelvic vein; ~v, xenal_portal vein ; Sy Sue yenosus ; s.c, sciatic vein ; s.v, subclavian vein; Z, tongue; 4a, truncus arteriosus; #, right auricle ; vy, Ta tricle ; v.v, vesical veins, Fic. 4.—Diagrammatic e of the venous system of the frog, 26 THE VASCULAR SYSTEM 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. Dissect from the ventral surface. In cleaning a vein take hold with the forceps, not of the vein itself but of the tissue sur- rounding it ; and take especial care not to prick the vein, as by doing so you allow the blood to escape and obsewre the dissection, and also render the vein itself difficult to see owing to the loss of colour. Always dissect along and not across « bloodvessel, and pin out the parts so as tu 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 returni 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, A. The external jugular vein is formed by i. The lingual vein, from the floor of the mouth and the tongue. ii, 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 A. 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. 73. The subclavian vein, the largest of the three, is formed by «i. The brachial vein, from the fore limb. vii. The musculo-cutaneous vein: a very large vein returning blood from the skin and THE VEINS bo b i muscles of the side and hack of the body, and of the head as far forwards as the nose. 4b. The left anterior vena cava corresponds in its course and branches to the right one. J* The posterior vena cava is a median vein which, com- mencing between the kidneys, runs forward, dorsally to the liver, 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 : Vi. The right and left hepatic veins, from the liver: these open into the posterior vena cava just before it joins the sinus venosus. vw 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. II. 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. III. 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 VASCULAR SYSTEM OF THE FROG Ze a. The renal portal system. Trace back the anterior abdominal vein to the hinder end of the body, where it will be seen to be formed by the union of the two pelvic veins. Follow back the pelvic vein of one side to the base of the hind limb ; here it will 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 the femoral vein is the renal portal vein, which is to be followed: to the outer side of the kidney. 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-lumbar 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 from the hind limbs; and partly by veins returning blood from the alimentary canal. . 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 ARTERIES 29 iii. A cardiac vein, from a network of vessels on the truncus arteriosus. YY 2. The hepatic portal vein is a wide vein 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 from the walls of the alimentary canal, and is formed by the union of the follow- ing veins: j. The gastric vein, from the stomach. ii. Intestinal veins, from the whole length of the intestine, both small and large. iii. The splenic vein, from the spleen: this usually joins one of the intestinal veins. C. The Arteries. Fic. 5.—Diagrammatic figure of the arterial system of the male frog, from the right side. a, stomach; 4, nostril; c, small intestine; ¢.a, carotid artery; cg, Carotid gland; c.#, coeliaco mesenteric artery ; ¢.7, cutaneous artery; d, large mtestine ja, Cecsal aorta J Fem %, spleen ; f.a, hepatic artery; 7, right lung; Za, Timgual artery; m, testis; o, kidney ;' 0.2, gccipito-vertebral_artery; #.@, pulmonary artery ; r, pelvic girdle; s, sternum; s.@, subclavian artery; 5.c, sciatic artery; Z, tongue; 4a, i ; #.a, urogenital arteries; v, ven- tricle ; 1, carotid arch ; 2, systemic arch >~3putmre-curateons arch. ~—SE ee —_—<—<—<——— ——— 30 THE VASCULAR SYSTEM OF THE FROG Dissect as for the veins. Pass a roll of paper or plugs of cotton-wool down the esophagus, so as to distend it and stretch the aortic arches. Clean carefully the aortic arches, commencing at the truncus arteriosus ; and follow the several arteries to their distribution, removing the veins and other structures which over- lie them. Note the division of the truncus arteriosus in front into right and left branches, each of which again divides into three aortic arches—the carotid arch, the systemjc arch, and the ma a arch. oh Fake wan O. I. The Carotid Arch is the most anterior of the three arches: it runs round the side of the csophagus, and is connected dorsally with the second or systemic arch ; its chief branches are as follows : 1. The lingual artery is a small artery supplying the tongue. Immediately beyond the origin of the lingual artery the carotid arch presents a small spongy swelling, the carotid gland. 2. The carotid artery runs round the side of the cesopha- gus 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 external carotid artery, supplying the roof and sides of the buccal cavity, and the orbit. ii. The internal carotid 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 to form 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 esophageal arteries are one or two branches arising THE ARTERIES 31 from the upper part of the arch and entering the dorsal wall of the esophagus. 3. The occipito-vertebral artery is a short branch arising from the dorsal part of the arch: it runs 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 celiaco-mesenteric artery is a large median artery arising 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 intestines. Its branches are as follows: i. The celiac artery: which divides into a. The gastric artery, supplying the stomach. B. 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. B. The posterior mesenteric artery, supplying the distal part of the intestine. y. The splenic artery, supplying the spleen. 2. The urinogenital 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. 3. The lumbar arteries are small paired lateral branches supplying the body-walls. 32 THE VASCULAR SYSTEM OF THE FROG 4, The hemorrhoidal 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 supplying 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. III. The Pulmo-cutaneous Arch is the hindmost of the three aortic arches: it divides about the level of the carotid gland into the following branches: J. 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 runs with somewhat sinuous course along the outer side of the whole length of the lung, giving off branches into its walls. D. The Structure of the Heart. Having completed the dissection of the bloodvessels, cut them across, about half an inch from the heart ; remove the heart com- pletely, and dissect it carefully under water. It is well to cut the vessels of unequal lengths on the two sides, as this will facilitate the recognition of the sides of the heart during the dissection, Place the heart at first with the dorsal surface upwards. 1. The Sinus Venosus (Fig. 4, p. 25) is a thin-walled sac on the dorsal surface of the heart; it is triangular in shape, with the apex directed backwards. Into its anterior angles the right and left anterior vene cave open, and into its posterior angle or apex the posterior vena cava. Cut away with scissors the dorsal wall of the sinus venosus so as to expose its cavity : wash out any contained blood. THE HEART 33 The sinu-auricular aperture (Fig. 6, SV) leading from the sinous venosus to the right auricle, is a transversely oval opening, arded by imperfect anterior and posterior valves, in the ventral wall of the sinus venosus, close to its anterior end, and very nearly in the median plane. Fic. 6.—The frog's heart seen from the ventral surface, and dis- sected so as to show its structure. The ventral walls of the truncus arteriosus, and of the auricles and ventricle have been removed. (From a‘drawing by Dr. Hurst.) ‘A, auriculo-ventricular aperture and one of its valves ; B, aperture leading from ventricle to truncus arteriosus, with one of its valves ; C, left carotid arch; C’, style passed down right carotid arch into the truncus arteriosus; LA, left auricle; P, left pulmo-cutaneous arch ; P’P’, style, passed down right pulmo-cutaneous arch into the truncus arteriosus; PV, opening of pulmonary vein into left auricle: RA, right auricle; S, left systemic arch; S/, style passed down right systemic arch into the truncus arteriosus; SV, opening from sinus venosus into right auricle; V, ventricle. ? Cc 34 THE VASCULAR SYSTEM OF THE FROG 2. The Auricles. Zurn the heart over, with its ventral sur- face upwards. Cut away the ventral wall of both auricles with jine 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, RA) 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 dorsally 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, taking care not to damage the truncus arteriosus. 3. The Ventricle (Fig. 6, V) is conical in shape with the apex backwards, and has a small central cavity, with thick spongy walls. The 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 Cut away carefully, with fine scissors, the ventral wall of the truncus arteriosus so as to expose its cavity and the contained valves. 4. The Truncus 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—8,S’. The cavity is partially divided by a vertical tongue-like projection from its dorsal wall: on the ventral surface of this tongue are two small openings, very close together, which lead into the right and left carotid arches, C,C’. Cut across the aortic arches, just beyond the division of the 36 THE VASCULAR SYSTEM OF THE FROG truncus into right and left branches, and note that though each branch is apparently a single vessel its cavity is really divided into three vessels corresponding to the three aortic arches. Pass bristles down these aortic arches, and note the points at which they severally open into the truncus arteriosus. E. The Lymphatic 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 subcutaneous 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.17.) The body-cavity itself also communicates with the lymphatic system through small openings or stomata in the peritoneum. 3. 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 37 vertebra, and beneath the shoulder girdle : they 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. 23). 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 ; dilute it with a drop of normal salt solution (0°75 per cent.) ; put on a thin cover-glass, and run a ring of oil round the edge to pre- vent evaporation : examine with 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. i. Red 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°0235 mm. in length by 0:0145 mm. in width ; or about 755 X rey Of an inch. ii, White corpuscles. These are much fewer in number and of smaller size: they are colourless, granular, subspherical in shape, and exhibit “ ameceboid” movements. Sketch one half a dozen times at intervals of half a minute. 2. Action of acetic acid on blood. Place a fresh drop of blood on a clean slide: add a drop of acetic acid : cover, and examine with the high power: note the changes produced. i. Red corpuscles: the nuclei become much more apparent than before, and the red colour dis- appears. 38 THE VASCULAR SYSTEM OF THE FROG ii, White corpuscles: become clearer, and show nuclei, sometimes more than one in a single corpuscle. II, Human Blood. 1. Normal. Prick the tip of your finger, and place a small drop of the blood on a slide: add a drop of normal salt solution, cover, and examine as before. Note the following points : i. Red 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- leaux, like piles of coins. Their average diameter is 0-008 mm., or about z555 of an 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 zeoo Of an inch in diameter: their ameboid 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. Circulation of the Blood in the Web of a Frog’s Foot. The web uniting the toes of the frog’s foot is so thin and transparent, that with the microscope the blood in it can readily be seen coursing along the capillaries. Examine a frog prepared to show the circulation in the web of the foot. Note the following points : 1. With a low power. a. The irregularly branched pigment cells to which the colour of the frog’s skin is due. 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- CIRCULATION OF BLOOD 39 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 : note the following points : 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. c. The variations in calibre of the small arteries and capillaries: whilst under observation an artery or capillary may be seen to change its size to a consider- able 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 corners of the capillary net- work. 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 IIT. THE SKELETON OF THE FROG. THE 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 membrane-bone, in contradistinction to the former kind, or cartilage-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, t.¢., have calcareous salts deposited in its 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 limb-girdles which attach them to the body. Examine the prepared skeletons, and make careful drawings to scale of the several parts. In your drawings colour the cartilage blue, the cartilage-bones yellow, and the membrane-bones white or red, Prepare skeletons for yourself by dipping the parts in hot water, and carefully brushing away the soft tissues until the skeleton is clean. THE abies OF THE FROG Fic. 7.—The skeleton of the frog, seen from the dorsal surface ; the left suprascapular and scapular have been removed. a, astragalus; c, caleaneum; d, suprascapular; e¢, exoccipital ; ¥, femur; /, frontoparietal ; g, metacarpals; 2, humerus ; 2, ilium; 2, metatarsals ; Z, carpus; , maxilla; , nasal; 0, pro-otic; 4, pterygoid ; pm, premaxilla; g, ‘‘quadratojugal”’; 7, radio-ulna; s, squamosal; se, sphenethmoid ; s.v, sacral vertebra ; ¢, tibio-fibula; ~, urostyle. 41 42 HE SKELETON OF THE 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 vertebra, and a posterior unsegmented portion of about equal length—the urostyle. At the sides of the tube, between the successive vertebra, are the intervertebral foramina through which the nerves pass out from the spinal cord to the various parts of the body. a, Structure of a vertebra. Haxamine one of the vertebra, say the third, more closely: draw it, showing the Sollowing 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 vertebre 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. . The transverse processes are a pair of large processes projecting horizontally outwards from the 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 vertebre in front and behind, and so serve to link the vertebre together. a. The anterior articular processes, or prezyg- apophyses, face upwards and slightly in- wards. 8. The posterior articular processes, or post- zygapophyses, face downwards and slightly outwards. < ; {HE VERTEBRAL COLUMN 43