: oS | eteaugtraee econ: _ et _ ae eet a - e L : : Ei a: RTH eae ae a ts 5 nana BS i Pots sy EMR ah Rens uae iat me ERE Se ne tr = 3 felts 3 iia Bee erences oe erie, ett ie oe ote ie ae Sites sen : f ite ae oe e tea Pree it tet 8 ie See sires ae He ee ee een re ra a te sh Ps aes She ime 2 ee ne cata ry apart! See eeasagrete isa ee gonna os Cornell Mniwersity Library BOUGHT WITH THE INCOME FROM THE SAGE ENDOWMENT FUND THE GIFT OF Henry W. Sane 1891 CP a | eK | iD | 5931 “TA. 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/cu31924024584231 A GUIDE TO THE DISSECTION OF THE DOG A GUIDE TO THE DISSECTION OF THE DOG BY O. CHARNOCK BRADLEY M.D., D.Sc., F.B.S.E., M.R.C.V.S. PRINCIPAL OF THE ROYAL (DICK) VETERINARY COLLEGE, EDINBURGH ; LECTURER ON COMPARATIVE ANATOMY, UNIVERSITY OF EDINBURGH WITH 69 ILBUSTRATIONS LONGMANS, GREEN, AND CO. 39 PATERNOSTER ROW, LONDON NEW YORK, BOMBAY AND CALCUTTA 1912 All rights reserved N267080 PREFACE Durinea recent years the feeling that there was room for a book in English dealing with the anatomy of the dog has been steadily gaining ground. Doubt- less, in the opinion of many, the gap can be adequately filled only by a systematic treatise similar to the classic ‘ Anatomie des Hundes’ of Ellenberger and Baum. Until some such work makes its appearance, perhaps the present publication may serve as a temporary means of supplying the deficiency. For several reasons it was decided to introduce the subject of canine anatomy to the English reader in the form of short notes planned to aid him in his desire to obtain first-hand knowledge by dissection. Herein are given directions which experience has shown will help the student to display the various organs and structures in an orderly and profitable manner. The descriptions which follow are purposely given in the briefest possible form, in order that the dissection of the whole animal may be accomplished within a reasonable length of time. While primarily intended for the guidance of the student who proposes to join the ranks of a profession which already contains a fair number of special- ists whose work necessitates a close knowledge of the anatomy of the dog, it is hoped that these notes may be of some service to the student of comparative anatomy who wishes to gain an acquaintance with the structure of an easily obtained mammal. The nomenclature herein employed is that which may be called the Baden and Stuttgart modification of the B.N.A. of the human anatomist; and, if not perfect, is infinitely better than the nondescript nomenclature, crowded with synonyms, formerly found in works on comparative anatomy. My thanks are due to my colleague, Mr. W. M. Mitchell, M.R.C.V.S., for assistance in many directions; and to the publishers for the cordial manner in which they have met my suggestions. 0. CHARNOCK BRADLEY. EpinsuraH: June 1912. COLOURED ILLUSTRATIONS IN TEXT Dissection of the Pectoral Region ‘ Diagram ‘of the Cranial Mesenteric Artery Coeliac Artery Portal Vein Medial Surface of the Right Tang Medial Surface of the Left Lung Diagram of the Thoracic Duct ‘ : Diagram of Part of the Sympathetic oe ee : Diagram of the Urinary Organs of the Male Superficial Dissection of the Inside of the Thigh . Dissection of the Inside of the Thigh . Superficial Dissection of the Gluteal Region and Thigh Dissection of Gluteal Region and Lateral Aspect of the Thigh Deep Dissection of the Gluteal Region ‘ Diagram of the Arteries on the Dorsum of the Pes Diagram of the Nerves on the Dorsum of the Pes Dissection of the Lateral Aspect of the Leg : ‘ Dissection of Origin of Tibial and Common Peroneal Nerves Dissection of the Medial Aspect of the Leg Superficial Dissection of the Sole . Diagram of the Plantar Nerves of the Pes . Diagram of the Plantar Arteries of the Pes Diagram of the External Diac and Hypogastric Aeterles Superficial Dissection of the Shoulder and Neck . Dissection of the Shoulder Dissection of the Medial Aspect of tite Sidon ‘Aa wuld edanni Dissection of the Lateral Aspect of the Shoulder, Arm, and Forearm Dissection of the Arm Dissection of the Medial Aspect of ihe Horan Diagram of Nerves on the Dorsum of the Manus ‘ Diagram of Nerves on the Volar Aspect of the Manus . Diagram of the Arteries on the Dorsum of the Manus . Diagram of the Volar Arteries of the Manus Dissection of the Side of the Neck and Thorax 100 101 lil 117 119 121 125 127 131 133 138 139 140 147 ILLUSTRATIONS Superficial Dissection of the Face Dissection of the Tongue, Pharynx, etc. Dissection of the Larynx . a Dissection of the Larynx Dissection of the Orbit Dissection of the Orbit Arteries at the Base of the Brain BLACK AND WHITE ILLUSTRATIONS IN Diagram of the Brachial Plexus Position of the Abdominal Viscera Extent of the Abdominal Muscles Transverse Section of the Abdomen Transverse Section through the Abdomen Transverse Section of the Abdomen Transverse Section of the Thorax Transverse Section of the Thorax Outline of the Spleen Outline of the Stomach Lateral Surface of the Right Liang Lateral Surface of the Left Lung Diaphragmatic Surface of the Lungs . Outline of the Heart as seen from the Left . Outline of the Heart as seen from the Right Diagram of the Branches of the Bronchi Outline of the Diaphragmatic Surface of the Tiger Outline of the Visceral Surface of the Liver i Diagram of the Gall-bladder and its Associated Ducts Plexus of Lumbar and Sacral Nerves Tendons on the Dorsum of the Manus Transverse Section of the Spinal Cord . . Transverse Sections of the Spinal Cord at Different Levile : Lateral Surface of the Cerebral Hemisphere Dorsal View of the Cerebral Hemisphere Medial Surface of the Cerebral Hemisphere . ‘ Dissection to show the Floor of the Lateral Ventricle . Lateral Aspect of Hind-brain and Mid-brain . 113 132 204 204 212 . 213 » 214 216 220 DISSECTION OF THE DOG Tue dissection of the dog is best begun with the animal lying on its back. If the subject is a female, the mammary glands should be examined as far as possible before any dissection is carried out. MammMa.—The mammary glands of the dog are eight to ten in number, arranged in a double row—often asymmetrical—along the ventral aspect of the thorax and abdomen. When ten glands are present, they can generally be designated as four thoracic, four abdominal, and two pubic. Though nipples are present in both sexes, mammary glands, as such, are seldom demonstrable in the male. Even in the female, except during the period of lactation, the mamme do not generally form very distinct projections. Each nipple (papilla mammez) is conical in form, covered with hairless skin, and pierced at its apex by numerous (eight to twelve) openings. Each small orifice leads to a milk canal (ductus lactiferus), which traverses the length of the teat from a milk sinus (sinus lactiferus) at its base. Dissection.—Make a longitudinal incision along the mid-ventral line from the middle of the neck to near the external genital parts, and a transverse incision from the medial aspect of one elbow to a corresponding point on the opposite limb. Two short and two long flaps of skin should now be reflected. In doing this, be careful to avoid removal of the mammary glands. On arriving at the base of the nipple, make a circular cut through the skin so as to leave the teat uninjured and attached to its gland. In turning outwards the flaps in the neck, avoid removal of the cutaneous muscle which here forms a thin layer of transverse fibres. If the mammary glands are not active the gland-tissue will be scanty and form merely a thin layer under the skin. Generally the thoracic glands are separate from each other, while those over the abdomen are continuous. The character of the glands is much more obvious during lactation, when they form a continuous, lobulated sheet stretching from the oral border of the pectoral muscles to the neighbourhood of the external genital parts. Dissection Remove the thoracic mammary glands and define the pectoral muscles. Then proceed with the dissection as in the male. 2 DISSECTION OF THE DOG Dissection of the Male.—If the subject be a male, make an incision along the mid-ventral line from the middle of the neck to the prepuce. Then make a transverse incision from the medial aspect of one elbow to a similar point on the opposite limb. Turn back four flaps of skin. In doing so, an extensive but thin subcutaneous muscular sheet —m. cutaneus— will be exposed in the neck, where its fibres are transverse, and over the abdomen, where they are oblique. Define the pectoral muscles. In clearing the fascia from the pectoral muscles note small vessels and nerves (about six), which appear close to the mid-ventral line and proceed laterally over the surface of the muscles. These vessels are the perforating branches of the internal mammary artery. In the xiphoid region an artery and vein (branches of the cranial epigastric vessels) appear from under the border of the deep pectoral muscle, and run obliquely in caudo-lateral direction to supply the mammary region. These are larger in the female. M. PECTORALIS SUPERFICIALIS.—In the main the fibres of the superficial pectoral muscle run transversely to the long axis of the body. The origin of the muscle is from the first two segments of the sternum and from the septum between it and its fellow muscle, and its insertion—hidden as yet by the brachio-cephalic muscle—is to the line on the humerus running distalwards from the tuberculum majus. Crossing the superficial pectoral muscle close to its insertion, and lying in a narrow space bounded by this and the brachio-cephalic muscle, are two vessels : namely, a communicating branch from the cephalic vein to the external jugular vein, and the deltoid ramus of the thoraco-acromial artery. Dissection.—Reflect the superficial pectoral muscle by cutting across it close to its origin, and notice in doing so that the muscle increases in thickness towards its cranial border. Clean the surface of the deep pectoral muscle and dissect out the various structures lying in a triangular space at the root of the neck. In reflecting the superficial muscle observe its nerve of supply (from the brachial plexus) bending round the cranial border of the deep pectoral muscle, accompanied by the pectoral ramus of the thoraco-acromial artery and its satellite vein. M. PECTORALIS PROFUNDUS.—The deep pectoral muscle is much more extensive than the superficial member of the same group. Consequently, a considerable extent of it is visible before the superficial muscle is reflected. Its fibres are disposed with a varying degree of obliquity. The most caudal fibres are the most oblique. The caudal border of the muscle is rendered some- what indefinite from the presence of a narrow band sometimes isolated, and always separable, from the rest. DISSECTION OF THE DOG 3 The origin of the muscle is from the sternum from the level of the second costal cartilage to the xiphoid process. Its insertion is into the medial tubercle (tuberculum minus) of the humerus, and, by means of a thin tendon which crosses the origin of the biceps, to the lateral tubercle (tuberculum majus) ~—~y, jugularis externa _= m. brachio-cephalicus --7 m. sterno-cephalicus A m. pectoralis superficialis ~\\ eee 7n, medianus - 3 vom. biceps brachtt - eo F j : (aa = a ' i a4 1 1 oa it oi 1 = ney x { H t ay, seg { I ™ \ | 4 n. ulnaris : J. tte os . Ng | ™. triceps brachii © 4 i \ | ~ Ng im, tensor fascte antibrachit N NX ne \ = yy m. pectoralis profundus [ i a m. rectus abdominis Fig. 1.—Dissection of the pectoral region. also. The most caudal fibres of the muscle are connected with the cutaneous muscle of the abdomen. At the root of the neck is a triangular space of some moment on account of the structures contained therein. The lateral limit of the space is formed by the brachio-cephalic muscle. Medially the sterno-cephalic muscle forms its boundary ; while its base—caudal in position—lies at the edge of the pectoral muscles. B2 4 DISSECTION OF THE DOG Within the space will be found the external jugular vein, the communicating branch from the cephalic vein, the omo-cervical arterial and venous trunks, and the nerves to the superficial pectoral and brachio-cephalic muscles. In the depths of the triangle is part of the brachial plexus of nerves. All these structures will be more fully exposed during the dissection of the axillary space. Dissection Reflect the deep pectoral muscle by cutting through it a short distance from its origin. This exposes the axillary space, the structures in which should be cleaned with great care. In reflecting the muscle, a lymph-gland (the axillary gland) will be found lying over the distal part of the teres major and between this and the pectoral muscle. Branches from the brachial plexus of nerves should also be noted bending round the axillary vessels to sink into and supply the deep pectoral muscle. Axitta.—The axillary space is bounded laterally by the scapula and the subscapular muscle, and medially by the chest-wall and the muscles clothing it. In the natural condition the space can scarcely be said to exist except as a narrow chink filled by areolar and fatty tissue ; but on dissection its medial and lateral walls are separated from each other and a definite space is produced. Dorsally the two walls meet at a very acute angle; ventrally, however, they diverge slightly, and the floor of the space is defined by the pectoral muscles. Cranially the axillary space communicates with the neck ; whereas caudally it is closed in by the latissimus dorsi and cutaneous muscles. The principal contents of the space are the axillary vessels and the brachial plexus of nerves. V. ET A. AXILLARIS.—The axillary vessels are among the structures first exposed on reflecting the deep pectoral muscle. The vein drains the blood from the limb, and joins the external jugular vein at the entrance to the chest, thus constituting one of the roots of the innominate vein. Its collateral tributaries are the lateral thoracic, subscapular and omo-cervical veins, and small vessels from the pectoral muscles. Occasionally the omo-cervical venous trunk joins the jugular direct. Of the tributaries the subscapular vein is by far the largest, its volume depending mainly upon the circumstance that it continues the cephalic vein. The axillary artery is a continuation of the subclavian artery. Com- mencing on a level with the first rib, it rans down the limb as far as the tendon of insertion of the teres major muscle. Most of its branches will be examined in connection with the scapular and arm regions, but two should now be noted as supplying the pectoral muscles and their neighbourhood. These are the thoraco-acromial and lateral thoracic arteries. A. THORACO-ACROMIALIS.—The thoraco-acromial artery soon divides into two branches, a ramus deltoideus and a ramus pectoralis. Both of these have DISSECTION OF THE DOG 5 been already noted in connection with the dissection of the pectoral muscles. Very commonly the deltoid ramus arises from the omo-cervical trunk. ‘A. THORACALIS LATERALIS.—The lateral thoracic artery arises some little distance from the border of the first rib, supplies the pectoral muscles, runs along the deep face of the deep pectoral muscle in company with the nerve to the latissimus dorsi, and ends in the cutaneous muscle over the abdomen. TRUNCUS OMO-CERVICALIS.—The omo-cervical trunk is a branch of the subclavian artery. Arising within the first rib, it passes medial to the axillary artery and the brachial plexus to divide before long into the ascending cervical and transverse scapular arteries. The ascending cervical artery (a. cervicalis ascendens) runs for a distance on the medial surface of the brachio-cephalic muscle, and finally enters the substance of this muscle. The transverse scapular artery (a. transversa scapule) passes towards the cranial border of the subscapular muscle and divides into several branches, the main one of which accompanies the suprascapular nerve. N. PHRENICUS.—The phrenic nerve will be found between the axillary vein and artery dorsal to the omo-cervical arterial trunk. It arises by three roots from the ventral divisions of the fifth, sixth, and seventh cervical nerves, and leaves the present dissection by entering the thorax. Later, it will be traced to the diaphragm to which it carries motor impulses. Dissection.—Cut across the axillary vessels, close to the first rib. This allows of-a more complete inspection of the brachial plexus. PLEXUS BRACHIALIS.—The brachial plexus is a somewhat complex arrange- ment of nerves formed by branches from the last three cervical and the first two thoracic nerves. It is placed between the axillary vessels and the scalenus muscles. Its branches are: Nerve to the brachio-cephalic muscle, n. suprascapularis, n. subscapularis, n. musculo-cutaneus, n. axillaris, n. radialis, n. medianus, n. ulnaris, n. thoraco-dorsalis, nn. thoracales ventrales, and n. thoracalis longus. _ Most of these will be examined in the dissection of the limb; but the following points should be noted at the present moment. The ventral thoracic nerves (nn. thoracales ventrales) are mainly distributed to the pectoral muscles. One of them supplies the skin and cutaneous muscle on the side of the chest and abdomen. The thoraco-dorsal nerve (n. thoraco-dorsalis) supplies the latissimus dorsi muscle. If the subject is a male the external genital organs should next be dissected. PARTES GENITALES EXTERN#Z.—The male external genital parts consist of the scrotum, the penis, and the prepuce. The scrotum is a membranous bag with a double cavity in which the testes B3 6 DISSECTION OF THE DOG are lodged. It lies between the thighs, and forms a rounded prominence crossed in a cranio-caudal direction (generally obliquely) by a shallow groove in which a faint line, the raphe scroti, may be detected. The skin of the scrotum is thin and provided with comparatively few hairs. Dissection.—Make an incision through the skin along the raphe of the scrotum, and expose the underlying tissues. nn, cervicales Ss nn thoractt? fo-------n, phrenicus . thoracalis longus ~~ _To m. brachio-cephalicus Sediteiciase un. musculo-cutaneus n. thoraco-d orsalis ae -----in. suprascapularis S, ‘an. subscapulares N -\To m. pectoralis profundus . Ae n, axillaris nn. medianus et ulnaris “sn. radialis Fic. 2.—Diagram of the brachial plexus. The wall of the scrotum can be resolved into three layers. Of these the most superficial consists of skin. Under this is a fibrous tissue known as the tunica dartos, which, with its fellow of the other side, forms the median septum between the two scrotal cavities—the septum scroti.- Removal of the dartos exposes the third layer composed of a fascial tunic lined within by a serous membrane. The fascia is continuous with that covering the deep face of the transverse abdominal muscle, and is in the form of a pear-shaped sac the narrow end of which is connected with the superficial end of the inguinal canal. Along the dorsal side of the narrower part of the sac there is a muscular slip, the external cremaster muscle (m. cremaster externus), associated with the internal oblique muscle ‘of the abdominal wall, on the one hand, and terminating, on the other hand, in an aponeurosis within the substance of the wall of the scrotum. _- To m. pectoralis superficialis DISSECTION OF THE DOG 7 TUNICA VAGINALIS.—The tunica vaginalis is a serous membrane continuous, through the inguinal canal, with the peritoneum. Like other serous membranes it is composed of a parietal and a visceral part. The parietal portion lines the interior of the scrotum and is. continuous with the visceral part in the caudo-dorsal region of the scrotal cavity. The visceral part of the tunic will be displayed after an examination of the testis and its surroundings has been made. Dissection.—Cut through the parietal tunica vaginalis with a pair of scissors, and examine the contents of the cavity of the scrotum. TESTIS ET EPIDIDYMIS.—The testes are two oval organs, slightly flattened laterally (especially on the medial surface), each lodged in its own compartment ofthe scrotum. The long axis of each testis is oblique, and runs caudo-ventral. The two surfaces, medial and lateral, are smooth and convex, as is also the ventral border. The dorsal border and the two extremities are connected with the epididymis. The epididymis of the dog is relatively large. It consists of an elongated, laterally compressed mass formed by the tortuous windings of a long tube held together by dense connective tissue and covered by the tunica vaginalis. The body (corpus epididymidis), or main part of the structure, lies dorsal to the testis. The cranial and caudal ends, known respectively as the head (caput epididymidis) and the tail (cauda epididymidis), are adherent to the extremities of the testis. The cauda is also firmly bound to the wall of the scrotum. From the cauda the ductus deferens takes origin. The duct runs cranialwards dorso-medial to the testis, at first somewhat convoluted but afterwards straighter, and passes into the inguinal canal. FUNICULUS SPERMATICUS.—The spermatic cord consists of the ductus deferens and certain blood vessels, nerves, and lymphatics. The vein within the cord is richly coiled and forms a prominent object, extending from the head of the epididymis into the inguinal canal. Partly buried within the venous mass is the internal spermatic artery (a. spermatica interna). The visceral part of the tunica vaginalis may now be followed. It is applied closely to the testis and epididymis, and dips in between these two organs, to form a cavity, the sinus epididymidis, the entrance to which is lateral in position. Continued towards the inguinal canal, the tunic encloses the spermatic cord, from which and from the epididymis it is reflected as a mesentery containing fine nerve filaments. It will be observed that the deferent duct and the external spermatic artery are enclosed in a special sheath of the tunic which is an offset from the covering of the other constituents of the spermatic cord. Praputium.—The prepuce is a tubular sheath of integument enclosing the glans of the penis, and, in the retracted state of this organ, circumscribing a cavity which communicates with the surface by a slit-like orifice. The prepuce Bp4 8 DISSECTION OF THE DOG consists of two layers: a double parietal layer, and a visceral layer intimately adherent to the glans penis. The superficial sheet of the parietal layer is composed of skin differing little from that of the neighbourhood. This should be reflected by making a longitudinal incision from the preputial orifice to the scrotum. Care must be taken to preserve the preputial muscle (m. pre- putialis) which, as a flattened band, springs from the abdominal aponeurosis in the xiphoid region, and runs immediately under the skin to form a loop round the orifice of the prepuce. If the deep sheet of the parietal layer be now slit open with a pair of scissors, its resemblance to mucous membrane will be manifest. The surface looking towards the penis is longitudinally folded and studded with irregular rows of flattened, rounded elevations caused by masses of lymphoid tissue. The membrane is directly continuous with the visceral layer of the prepuce, which extends over the glans penis to the urethral orifice where it meets the urethral mucous membrane. It should now be noted that the prepuce is supplied with blood by the external pudendal artery. Dissection.—The penis should now be freed from its surroundings as far as its root. In cleaning the organ, note the presence of a narrow, pale muscle, the retractor of the penis, running along its urethral surface from the anus to the vicinity of the glans. Preserve also the dorsal vessels and nerves which will be found on the opposite surface. Prnis.—The penis consists of a middle part (its body or corpus penis), a root (radix penis) attached to the arch formed by the two ischial bones, and a free extremity (glans penis) of considerable length. The term dorsum penis is applied to that aspect of the organ which is in contact with the abdominal wall; whereas the opposite aspect is called the urethral surface (facies urethralis). The body of the penis lies in the middle line, dorsal to the testes and crossed laterally by the ductus deferens. It is composed of three bodies running parallel to each other. The corpora cavernosa penis are two rods of erectile tissue surrounded and bound together by a dense fibrous envelope, the tunica albuginea [corporum cavernosorum], and separated from each other by a fibrous septum (septum penis). In the ischial region the two corpora diverge, each forming a crus penis which is attached to the border of the ischium. From about the middle of the penis, the corpora are continued onwards as a bone, the os penis. This may be considered as formed by two narrow plates joined dorsally and diverging ventrally to enclose a groove in which the urethra lies. The caudal end of the bone is truncated. The cranial end is more pointed, and to it is appended a curved process composed of fibrous tissue of cartilage-like density. The third erectile body of the penis is the corpus cavernosum urethre, DISSECTION OF THE DOG 9 associated, as its name indicates, with the urethra, and lodged in a groove on the urethral surface of the penis. The two crura penis, forming the radix penis, are covered by the ischio- cavernous muscles. In the middle line, between the crura, the corpus caver- nosum urethre forms a double expansion, the bulbus urethre, also covered by a muscle, the M. bulbo-cavernosus. The glans penis of the dog is of considerable length. Thinnest about the middle, it expands into a swelling at each end. The caudal expansion known as the bulbus glandis is the larger, and is developed on the dorsal side of the os penis. From it the dorsal veins of the penis take origin. The smaller expansion of the glans forms the free extremity of the penis and is more cylindrical in form than the bulbus glandis. Its terminal part is pointed and has the urethral orifice at its apex. Running along the urethral surface of the glans is a feeble fold of prepuce known as the frenulum preputii. Vv. DORSALES PENIS.—The two dorsal veins of the penis arise in the erectile tissue of the bulbus glandis. Running along the dorsum of the penis to its root, they bend round the border of the ischia and form the main roots of the internal pudendal veins. AA. DORSALES PENIS.—The dorsal arteries of the penis lie lateral to the veins of the same name. Each forms the largest terminal branch of the internal pudendal artery of its own side, and, leaving the pelvis by curving round the border of the ischium, can be readily followed to the glans penis. NNW. DORSALES PENIS.—The dorsal nerves of the penis, branches of the internal pudendal, accompany the dorsal vessels, each nerve lying lateral to the artery which it follows. Dissection.—Cut through the roots of the brachial plexus and clean up the ventral and lateral aspects of the thorax and abdomen. In doing this, first define the extent and attachments of the cutaneous muscle as far as the present dissection permits. Preserve the branches of the intercostal and lumbar vessels and nerves which appear about midway between the mid-dorsal and mid-ventral lines of the trunk. Observe that the branch from the third or fourth intercostal nerve is of good size and crosses the border of the latissimus dorsi muscle to reach the lateral aspect of the arm. It is connected with a ventral thoracic nerve from the brachial plexus. In removing the remains of the prepuce of the male, or the mamme of the female, take care not to destroy the external pudendal vessels and the external spermatic nerve which may be found embedded in an accumu- lation of fat in the inguinal region. The artery and vein can generally be traced to an anastomosis with branches from the cranial epigastric vessels. 10 DISSECTION OF THE DOG A group of lymph-glands will also be found in the inguinal region. Several vessels and nerves pierce the abdominal wall. Of these the circum- flex iliac vessels and the lateral cutaneous femoral nerve are the most conspicuous. They appear close to the angle of the ilium and are distri- buted in the cutaneous muscle of the abdomen and in the skin over the lateral proximal part of the thigh. A little cranial to the point of appearance of the circumflex iliac vessels, the phrenico-abdominal artery pierces the oblique muscle of the abdomen. Branches of the ilio-inguinal and ilio-hypogastric nerves should also be noted and preserved. A. PUDENDA EXTERNA.—The external pudendal artery is generally a branch of the deep femoral, but it may arise from a common trunk from which springs the caudal epigastric artery as well. The origin of the vessel cannot be examined at present. Crossing the spermatic cord medially, the artery runs towards the umbilicus and supplies the inguinal lymph-glands, the prepuce, and the surrounding skin. In the female it is larger and furnishes blood to some of the mammary glands. N. SPERMATICUS EXTERNUS.—The external spermatic nerve is a part of the genito-femoral nerve. Appearing through the abdominal wall along with the external pudendal vessels, it crosses the spermatic cord and is expended in the external genital parts, or the mammary glands, and the skin of the inner part of the thigh. It is sometimes connected with the ilio-inguinal nerve. M. OBLIQUUS EXTERNUS ABDOMINIS.—The external oblique is the most superficial muscle of the abdominal wall. Its origin is by digitations from the ribs from the fifth or sixth to the last, and from the lumbo-dorsal fascia. The muscular fibres run in a caudo-ventral direction as far as the border of the straight abdominal muscle. Here they give place to an aponeurosis which is attached to the linea alba. An important part of the aponeurosis stretches from the lateral angle of the ilium to the pubis near the symphysis. This constitutes the inguinal ligament of Poupart (ligamentum inguinale [Pouparti]), in association with which is the superficial opening of the inguinal canal. Opposite this opening the pectineus muscle takes part of its origin from the inguinal ligament. At the same point a thin reflection of the aponeurosis to take part in the formation of the scrotum should be noticed. Medial to the inguinal canal, i.e.in the pubic region, the aponeurosis is thin and mixed with fibres from the other abdominal muscles. From the aponeurosis of the oblique muscle a thin, ill-defined sheet is reflected onto the medial aspect of the thigh. M. LATISSIMUS DoRSI.—As its name indicates, this is a very broad muscle covering the dorso-lateral part of the thorax. Its origin is from the lumbo- dorsal fascia, by means of which it is attached to the spinous processes of the last seven thoracic vertebra, and by fleshy strips from the last two or three ribs. Its insertion is by a flattened tendon into the crista tuberculi minoris of the DISSECTION OF THE DOG ll humerus. The whole of the muscle cannot be conveniently examined at the present stage of the dissection. M. TRANSVERSUS CosTARUM.—This is a small, thin, irregularly triangular muscle placed over the union of the bony and cartilaginous segments of the first two or three ribs. Its origin is from the first rib, and its insertion, by means of a thin and not very definitely bounded aponeurosis, is into the sternum from the third to the fifth or sixth costo-sternal joint. Dissection.—Until such time as the inguinal canal has been examined the abdominal wall should be dissected on one side only. Reflect the external oblique abdominal muscle after making two incisions. The first incision should be longitudinal and through the aponeurotic tendon of the muscle a short distance from the fleshy margin. The second incision must be transverse and through the fleshy part of the muscle about midway between the thigh and the last rib. Now turn the aponeu- rosis as far towards the middle line as possible. Note that, forming the medial boundary of the superficial opening of the inguinal canal, there is a stout band connected with the origin of the pectineus muscle. This is formed by a blending of the aponeuroses of the two oblique and the transverse abdominal muscles. Immediately cranial to this the external oblique aponeurosis can be reflected to the middle line (linea alba), but as the xiphoid region is approached the reflection becomes less and less complete owing to a fusion with the internal oblique aponeurosis. M. OBLIQUUS INTERNUS ABDOMINIS.-Many of the fibres of the internal oblique muscle of the abdomen run in a cranio-ventral direction, but towards the region of the groin they become more and more transverse. The margin of the fleshy part of the muscle forms a sinuous curve, the most caudal part of which is superficial to the rectus muscle. The muscle has a dorsal attachment to the lumbo-dorsal fascia, the inguinal ligament, and the last two ribs. Ven- trally it is continued as an aponeurosis in which two layers can be demonstrated. The superficial layer assists the aponeurosis of the external oblique muscle in the formation of the superficial sheath of the rectus muscle, and thus reaches the linea alba. The fibres of the two aponeuroses become interwoven medial to an oblique line stretching from the middle line of the body near the pubis to about the middle of the breadth of the rectus muscle at the costal margin. The deep layer of the aponeurosis is incomplete and does not exist caudal to the umbilicus. As will be seen later, it blends with the aponeurosis of the transverse muscle of the abdomen to form the deep sheath of the rectus muscle. Dissection.—Make a longitudinal incision through the superficial sheath of the rectus muscle along a line a few millimetres within the lateral edge of the muscle. The incision should be made with care as the sheath is not very thick. Turn the sheath towards the middle line, and in 12 DISSECTION OF THE DOG doing so observe its firm attachment to the tendinous intersections of the underlying straight muscle. Cut across the fleshy part of the internal oblique muscle in the same manner as was done with the external oblique. Reflect the transverse muscle of the ribs. M. RECTUS ABDOMINIS.—The fibres of the straight muscle of the abdomen run parallel to the middle line of the body. Generally six tendinous inter- sections cross the muscle in an irregularly oblique manner. Of these one occurs on a level with the umbilicus; two are caudal, and three are cranial to this point. As has already been seen, the superficial sheath of the muscle is firmly adherent to the intersections. The cranial attachments of the rectus muscle are by means of a thin aponeu- rotic tendon (partly covered by the expanded insertion of the m. transversus costarum) from the cartilage of the first rib, and from the sternal end of the cartilages of the ribs from the second to the seventh. In addition fleshy slips leave the sternal ends of the eighth and ninth ribs. Caudally the muscle is attached to the border of the pubis. Dissection.—Reflect the straight muscle by a transverse incision about the middle of its length, and thus expose the cranial and caudal epigastric vessels as well as certain intercostal and lumbar nerves and vessels. A. EPIGASTRICA CAUDALIS.—The caudal epigastric artery either arises in- dependently from the deep femoral artery, or it has an origin in common with the external pudendal. The artery lies on the deep face of the rectus muscle of the abdomen and runs in a cranial direction. It is accompanied by the caudal epigastric vein (v. epigastrica caudalis). A. ET V. EPIGASTRICA CRANIALIS.—The cranial epigastric vessels appear between the xiphoid process of the sternum and the ninth or tenth costal cartilages ; that is, not far from the middle line. They follow a slightly oblique direction on the deep surface of the rectus muscle. Superficial branches, piercing the muscle or crossing its medial border in the xiphoid region, have already been noticed. NERVES OF THE ABDOMINAL WALL.—Lying deep to the internal oblique and rectus muscles on the one hand, and superficial to the transverse muscle on the other, are branches of the last five thoracic and the first two lumbar nerves. The last four intercostal nerves are derived from the ventral primary divisions of the ninth, tenth, eleventh, and twelfth thoracic nerves. They appear from beneath the costal arch, cross the surface of the transverse muscle, and disappear under therectus. The last thoracic nerve is similarly disposed. The thio-hypogastric and ilio-inguinal nerves arise from the ventral primary divisions of the first two lumbar nerves. N. ilio-hypogastricus——The ilio-hypogastric nerve divides into lateral and DISSECTION OF THE DOG 13 medial branches ; the former piercing the two oblique muscles to pass ventral- wards on the surface of the external oblique about midway between the last rib and the ilium. The medial branch is disposed between the internal oblique ‘and the transverse muscles in the same manner as are the thoracic nerves. N. ilio-inguinalis.—The ilio-inguinal nerve has a distribution very similar to that of the ilio-hypogastric. M. TRANSVERSUS ABDOMINIS.—As the name suggests, the fibres of the transverse muscle of the abdomen run, for the most part, in a direction at right angles to the long axis of the body. Towards the inguinal region, however, the fibres assume a caudo-ventral obliquity. The margin of the fleshy part of the muscle is mainly continued towards the middle line dorsal to the rectus muscle, but about midway between the umbilicus and the pubis the margin of the transverse overlaps the ventral surface of the straight muscle. The fibres of the transverse muscle arise from the extremities of the transverse processes of the lumbar vertebre, and from the medial surface of the last four or five ribs and their cartilages. The aponeurotic tendon which succeeds the fleshy part of the muscle, and by which it finds attachment to the linea alba, is blended with the deep layer of the internal oblique aponeurosis cranial to the umbilicus. Thus it shares in the formation of the deep sheath of the rectus muscle. Caudal to the umbilicus the aponeurosis becomes thin, and, about midway between the umbilicus and ‘the pubis, suddenly changes from the dorsal to the ventral side of the rectus. VAGINA M. RECTI ABDOMINIS.—The constitution of the sheath of the straight muscle can now be fully determined. The superficial or ventral layer of the sheath is mainly formed by the aponeuroses of the external and internal oblique muscles; but, in the inguinal region, to these is added the aponeurosis of the transverse muscle. The superficial layer, therefore, is complete and of considerable stoutness. The deep or dorsal layer of the sheath, on the contrary, is incomplete. Where the straight muscle lies over the costal cartilages no sheath interposes. From the region of the xiphoid process to a variable distance from the umbilicus the sheath is formed by the aponeurosis of the transverse muscle and the deeper layer of the aponeurosis of the internal oblique muscle. Caudal to this the transverse aponeurosis alone forms the sheath to a point about midway between the umbilicus and the pubis where, the aponeurosis passing ventral to the rectus, the deep sheath becomes deficient. Where the sheaths of the two recti muscles meet in the middle line a strong cord is formed. This is known as the linea alba, and stretches from the xiphoid process of the sternum to the symphysis pubis. Composed of interlacing and decussating fibres derived from the aponeuroses of the oblique and transverse muscles, it is broadest in the xiphoid region, gradually narrowing towards the pubis. At a point on the linea alba, generally about two-thirds of the distance from the pubis to the xiphoid, is a scar, the umbilicus, marking the point of attachment of the umbilical cord of embryonic life. 14 DISSECTION OF THE DOG CANALIS INGUINALIS.—The inguinal canal is a passage through the ab- dominal wall in which lies the spermatic cord of the male or the round ligament of the female. In the male, moreover, the testis descends into the scrotum by way of the canal. The canal is lined by a tubular connection between the peritoneum of the abdomen and the tunica vaginalis of the scrotum. In direction it is oblique ; its superficial extremity (annulus inguinalis subcutaneus) being caudal and medial to its deep extremity (annulus inguinalis abdominalis). The canal should be examined on that side of the abdomen where the muscles are yet intact. Introduce a probe or glass rod into the canal, and clean, in the first place, the subcutaneous ring. This is, speaking generally, an oval opening in the aponeurotic tendon of the external oblique muscle of the abdomen. The lateral boundary of the ring is formed by the stout part of the aponeurosis connected with the origin of the pectineus muscle. The medial boundary is formed by the combined aponeuroses of the external and internal oblique and the transverse muscles. The canal itself has a length of some 30 mm. and crosses the lateral border of the rectus muscle obliquely. Near the subcutaneous ring, it runs along the grooved surface of the inguinal ligament (Poupart’s ligament). The canal may be considered as possessing a ventral and a dorsal wall. The ventral wall throughout the whole of its extent is formed by the aponeurosis of the external oblique, and, throughout the greater part of its extent, by the fleshy part of the internal oblique muscle also. Close to the abdominal ring, some fibres of the transverse muscle may assist in the formation of this wall, but more com- monly the abdominal ring occurs solely in the fascia of the transverse muscle. The dorsal wall, close to the subcutaneous ring, is formed for a very short distance by the combined oblique and transverse aponeuroses. For the greater part the wall is constituted by the fascia of the transverse muscle covered internally by the peritoneum. In the female, the inguinal canal contains a funnel-shaped cavity continuous with that of the peritoneum. Outside its serous lining there is a considerable accumulation of fat:. In intimate association with the inguinal canal are the external pudendal and caudal epigastric vessels. The external pudendal artery and vein cross the medial aspect of the spermatic cord at the subcutaneous ring. The caudal epigastric vessels also cross the medial side of the canal, but they do so obliquely and at a variable point in its length. Running for a distance parallel to the inguinal canal, and in its dorsal wall, is the external spermatic nerve. Dissection.—Open the abdominal cavity by making an incision along the whole length of the linea alba, and a transverse incision on each side through the transverse muscle and the peritoneum. This having been done, a fold of peritoneum will be discovered passing along the linea alba to the urinary bladder. In this fold there is frequently a considerable accumulation of fat. DISSECTION OF THE DOG 15 Cavum ABDOMINIs.—The bony boundaries of the abdominal cavity are incomplete. Dorsally, the lumbar and sacral vertebrae with their transverse processes are covered by the thick muscles of the back. At the cranial end of the cavity some of the ribs and their cartilages form a girdle round the abdomen, while the pelvic bones serve the same purpose at the caudal end. Otherwise the lateral and ventral walls are formed by soft structures only. The ventral wall, much longer than the dorsal, slopes downwards from the pubes to the xiphoid process of the sternum, the steepness of the slope differing in different breeds. For descriptive purposes it is necessary to divide the general abdominal cavity into the pelvic cavity and the abdomen proper. The plane of separation is on a level with the promontory of the sacrum, the ilio-pectineal lines and the cranial border of pubes. It is, therefore, oblique to the long axis of the abdomen proper, inasmuch as it slopes caudalwards from the sacrum to the pubis. Though of questionable utility, the abdomen proper may be subdivided into nine regions by four imaginary planes—two transverse to the long axis of the body and two parallel to the median plane. One of the transverse planes cuts the abdomen on a level with the most caudal part of the twelfth rib. The second transverse plane is parallel to the first and is taken on a level with the lateral angle of the ilium. Thus the abdomen is divided into three transverse zones which may be named subcostal, umbilical, and hypogastric. The two longitudinal planes of subdivision are parallel to the median plane of the body and are imagined as occurring on a level with the mid-point of the distance between the lateral angle of the ilium and the symphysis pubis. The longitudinal planes subdivide each transverse zone into three regions as follows : Subcostal zone. Epigastric region. {rnin hypochondriac region. Left hypochondriac region. Umbilical zone. Umbilical region. {int lumbar region. Left lumbar region. Hypogastrie zone. Hypogastric region. {po iliac region. Left iliac region. ‘ CONTENTS OF THE ABDOMEN PRorER.—The organs lodged within the abdomen are numerous and serve widely different physiological purposes. They may be tabulated as follows : Stomach. Small intestine. (1) Alimentary organs. Large intestine. Liver. Pancreas. 16 DISSECTION OF THE DOG Fic. 3.—To show the position of the abdominal viscera in relation to the exterior. p.x., xiphoid process of sternum ; a.c., costal arch; v.f., gall-bladder; p., pylorus; h., liver; r.d., right kidney ; c.t., transverse eolon; 1., spleen; u., umbilicus; d., duodenum; o.d., right ovary; ic., cecum ; r.s., left kidney; 0.8., left ovary; ¢.d., descending colon; v.u., urinary bladder; u.t., uterus; p.r., prostate ; o.p., pubic bones. (This figure is reproduced by permission of Messrs. Bailliére, Tindall & Cox.) 17 DISSECTION OF THE DOG 18 DISSECTION OF THE DOG Ureters. Bladder. Kidneys. (2) Urinary organs. Deferent duct and prostate in the ( male. Ovaries, uterine tube of Fallopius and uterus in the female. (3) Reproductive organs. Spleen. (4) Ductless glands. { Adrenal glands. The abdominal aorta and its various (5) Arteries. branches. The caudal vena cava and its tri- ( butaries. The portal vein and its ( tributaries. The commencement of the vena azygos. (6) Veins. (7) Lymph-glands and lymphatic vessels including the cisterna chyli and the commencement of the thoracic duct. (8) The abdominal part of the sympathetic nervous system. (9) The peritoneal lining of the cavity and investment of the organs contained therein. It is of the utmost importance to the surgeon that he should be familiar with the positions of the main abdominal organs in relation to each other and to the surface of the body. In the accompanying figures (figs. 3 and +), the disposition of the muscles forming the wall of the abdomen, and the position of the underlying viscera, are indicated. Both diagrams are divided into equal squares in order that a ready comparison may be made. On first opening the abdomen only a few of the contained organs are brought to view. On turning aside the walls of the abdomen, indeed, little can be discovered but a double fat-laden membrane, the greater omentum (omentum majus), composed of four layers of peritoneum and containing, as will be explained later, a part of the peritoneal cavity called the omental bursa (bursa omentalis). On raising the omentum from the underlying coils of small intestine its connection with the greater curvature of the stomach, and its continuity with a peritoneal duplicature passing from the stomach to the spleen (ligamentum gastro-lienale), will be revealed. Projecting from under cover of the costal arch, the sharp ventral border of the liver will be visible. This border, it will be noticed, is deeply cut by fissures, in association with one of which the broad end of the gall-bladder can generally be seen. The ventral extremity of the spleen les immediately caudal to the liver DISSECTION OF THE DOG 19 on the left side of the abdomen. Not infrequently the visible part of the spleen is more caudal in position and extends farther than usual towards the middle line. This depends partly upon the size of the spleen, but largely upon the amount of distension of the stomach. lf the stomach is empty, possibly no part of it will be visible ; but when distended a considerable extent of the sac may protrude beyond the border of the liver. On turning the omentum over the ribs as far as possible, other organs will be exposed. Coils of small intestine occupy the greater part of the field of view; but close to the pubis the urinary bladder may form a conspicuous object if it happens to contain much urine. Sometimes part of the descending colon can be detected ; and, if the subject be a female, part of the uterus may also be exposed. It will be observed that the interior of the abdominal wall, as well as the surface of the organs contained therein, is smooth and shiny. This is due to the presence of a serous membrane, the peritonewm, of which the greater omentum is a part. INTESTINUM TENUE.—The small intestine is a long, smooth tube, measuring about five times the length of the body ; and extending from the pylorus, in the epigastric region—where it is continuous with the stomach—to a point slightly caudal to the umbilicus and a short distance to the right of the median plane, where it ends by joining the cecum. The small intestine is divided into: (1) The duodenum ; (2) the jejunum (intestinum jejunum), and (3). the dleum (intestinum ileum). The coils of the jejunum and ileum are freely movable, since they are suspended from the dorsal wall of the abdomen by a fold of peritoneum, known as the mesentery (mesenterium), by way of which the blood- vessels, nerves, and lymphatics gain access to the intestine. The duodenum is not so mobile, as will be seen at a later stage of the dissection. To expose this part of the alimentary tube, it may be necessary to turn the rest of the small intestine to the left. This having been done, a portion of the duodenum will be found running along the right side of the abdomen. A second part passes in the opposite direction and ends at an abrupt ventral flexure not far from the pylorus. A closer examination of the duodenum is better left until later. In association with the duodenum one of the two limbs of the pancreas will be observed. The terminal part of the ileum can generally be easily recognised as it travels cranialwards from somewhere near the pelvic inlet to its point of termination in the cecum. IntustiInuM crassuM.—lIn calibre the large intestine of the dog does not differ from the small intestine so much as it does in the other domesticated mammals. In length it falls very far short of the small intestine, measuring only about one-sixth of the length of the whole intestinal tube. For descriptive purposes, the large intestine is divided into the cecum, the colon, and the rectum. 20 DISSECTION OF THE DOG INTESTINUM caHcuUM.—The cecum is a short spiral or bent tube, the free blind end of which points towards the pelvis. There is considerable variation Costa 12 ! " s, 2 U m. longissimus dorst ! * i U m. Wo-costalis , Diaphragma « Aorta , issimus dorsi. i m., latissim i Vena cava caudalis mm. intercostales 1 m. obliquus ablominis externus a) 8 m, rectus abdominis Fie, 5.—Transverse section of the abdomen on a level with the eleventh thoracic vertebra. 7, 8, 9, 10, 11 = seventh, eighth, ninth, tenth, and eleventh ribs. in the length of this part of the intestine, and also in its form. It lies to the tight of the median plane in the umbilical region. Coton.—The colon is divided into three parts: (1) The ascending colon (colon ascendens) leaves the cecum and runs cranialwards to the right of the median plane for a short distance and then turns to the left, thus forming the DISSECTION OF THE DOG 21 right colic flexure (flexura coli dextra) ; (2) the second part of the colon, the transverse colon (colon transversum), like the first, is generally very short. It crosses the median plane immediately cranial to the root of the mesentery, and, arriving on the left side of the abdomen, curves in a caudal direction at the left colic flexure (flexura coli sinistra) ; (3) the descending colon (colon descendens) Vena cava caudalis t t ] Lymphoglandula m. longissimus dorsi, U / ~ m. tio-costalis. . m. obliquus abdominis internus m. obliquus abdominis éxternus~' m, transversus abdominis - Pancreas~~ | i i Colon ascendens ' SS ’ he 1 m. rectus abdominis i Duodenum Fic. 6.—Transverse section through the abdomen on a level with the first lumbar vertebra. Sections of small intestine are not named. forms by far the greater part of the whole tube. In order to expose it, it is neces- sary to turn the mass of small intestine over to the right. The descending colon will then be revealed to the left of the median plane in the form of a fairly straight tube running towards the pelvis from the left colic flexure. On approach- ing the pelvis, a gradual slope towards the right causes the intestine to disappear into the pelvic cavity about the middle line. In the female the colon gains the pelvis by passing dorsal to the uterus ; inthe male, dorsal to the urinary bladder. c3 22 DISSECTION OF THE DOG It will be observed that the whole length of the colon is provided with a short mesentery (mesocolon) which is not entirely independent of the mesentery of the small intestine. The cecum, unlike the rest of the large intestine, does not possess a mesentery. Peritoneum is reflected from it onto the terminal part of the ileum, to the side of which it is closely applied. Prriron=UM.—The interior of the abdominal wall and the surface of the abdominal viscera are covered more or less completely by a serous membrane m, longissimus dorst-.__ Vena cava caudalis- m. obliquus abdominis externus~ m., obliquus abdomints tnternus — m. transversus abdominis m, rectus abdominis._—-----— Fia. 7.—Transverse section of the abdomen on a level with the fifth lumbar vertebra. Sections of the small intestine are not named. known as the peritoneum. Like the other serous membranes of the body, the peritoneum is a completely closed sac in the male. In the female, a small opening at the abdominal end of the uterine tube establishes an indirect communication between the peritoneal cavity and the exterior. As in the case of serous membranes in general, the peritoneum consists of a part lining the wall of the cavity—the parietal peritoneum (peritoneum parietale)—and a part covering the viscera contained within the cavity, the visceral peritoneum (peritoneum viscerale); but, unlike other serous membranes, the visceral peritoneum and its connections with the parietal peritoneum is very complicated because of the number and disposition of the abdominal organs. DISSECTION OF THE DOG , 23 It is best to commence the examination of the peritoneum where its disposition is simplest. This will be found to be the case in the more caudal part of the abdomen. If, then, the arrangement of the peritoneum be followed in a transverse direction a short distance from the entrance to the pelvis it will be found that the ventral, lateral, and dorsal walls of the abdomen are clothed by parietal peritoneum. This is reflected from the dorsal wall along a line a little to the left of the median plane, in order that the descending colon may be encircled. The membranous double layer which connects the descending colon to the abdominal parietes is named the descending mesocolon (mesocolon descendens), and is continued as a similar duplicature associated with the transverse colon, the transverse mesocolon (mesocolon transversum), and beyond this again on to the terminal part of the ascending colon as the ascending mesocolon (mesocolon ascendens). The peritoneum should now be followed in a transverse direction at a level nearer the diaphragm ; opposite, let us say, the commencement of the ascending colon. Beginning on the left side of the abdomen, the parietal peritoneum lines the lateral and dorsal walls of the cavity as far as the margin of the kidney, where it is continuous with the base of the descending mesocolon. Here it is diverted in a ventral direction in order to enclose the spleen. The membrane from the base of the mesocolon to the spleen is a part of the greater omentum. From the right face of the descending mesocolon a double layer of peritoneum extends over to the right side of the abdomen and encloses the various parts of the duodenum, the pancreas, and the commencement of the ascending colon. The jejunum and ileum are suspended from the dorsal wall of the abdomen by the mesentery (mesenterium), a double fold of peritoneum, fan-. shaped in form. The peritoneum should now be followed in a longitudinal direction beginning with the very obvious greater omentum (omentum majus). On first opening the abdomen, the greater omentum is revealed covering the whole of the intestinal mass, and extending from the stomach and spleen, on the one hand, to the pelvic entrance on the other. A careful examination of the omentum discloses the fact that it is formed by two membranes; or, rather, by one membrane folded on itself, so that the folded margin lies in the region of the pelvis and the right side of the abdomen. Still closer examination shows that there are really four layers of peritoneum in its constitution. Two ventral layers are closely applied to each other, and two dorsal layers are similarly intimately related. Between the united layers there may be a considerable accumulation of fat. If the two ventral layers of the omentum are followed to the stomach they will be found to separate at the greater curvature of this organ so as to form serous coverings for its two surfaces. At the lesser curvature they again come together and form a thin membrane, the lesser omentum (omentum minus), which extends from the lesser curvature of the stomach to the margins of the c4 24 DISSECTION OF THE DOG porta of the liver. Here once again the layers separate, this time to form the serous investment of the liver. The continuation of the most ventral layer of. the lesser omentum follows the caudal surface of the liver, crosses its ventral] border and proceeds to the diaphragmatic surface, from which it is reflected onto the diaphragm, and thence can be followed along the ventral wall of the abdomen to the pelvis. The more dorsal of the two layers of the lesser omentum, in like manner, is continued onto the liver and from this organ to the diaphragm and the dorsal wall of the abdomen. , If the two dorsal layers of the greater omentum are followed towards the dorsal wall of the abdomen, they will be found to enclose the left limb of the pancreas before arriving at the wall itself. On the wall the two layers separate. One becomes confluent with the continuation of the more dorsal layer of the lesser omentum. The other passes into the transverse mesocolon; that is to say, it leaves the dorsal abdominal wall, surrounds the transverse colon and again returns to the wall of the abdomen. Here it is continuous with the root of the mesentery (radix mesenterii). Beyond this point the peritoneum proceeds along the abdominal wall into the pelvis and finally becomes that part of the membrane already followed along the ventral abdominal wall. From what has just been said, it is clear that a compartment of the peritoneal cavity is enclosed within the greater omentum. Consequent upon its associa- tions, this is known as the omental bursa (bursa omentalis). The communication between the main peritoneal cavity, entered when the abdomen was first opened, and the omental bursa, is by means of a comparatively small orifice, the epiploic foramen of Winslow (foramen epiploicum [Winslowi]). To find the foramen, pull the duodenum over towards the left and look for the caudate process of the liver, which will be readily recognised as that part of the liver which is in contact with the right kidney. If now the index finger of the left hand be passed along the caudate process towards the middle line it will enter the epiploic foramen. The right hand should be introduced into the omental bursa through an opening in the omentum, and the epiploic foramen sought from the left side. . The greater part of the omental bursa is contained in that part of the greater omentum which covers the intestine. The rest of it is continued dorsal to the stomach, and between this organ and those two dorsal layers of the omentum which contain the left limb of the pancreas. The boundaries of the bursa are formed as follows: Ventrally, by the ventral layers of the omentum, by the stomach, by the lesser omentum, and by the omental process of the liver; dorsally, by the dorsal layers of the greater omentum and the pancreas; on the left, by the spleen and the gastro-lienal ligament (ligamentum gastro-lienale) which connects the greater curvature of the stomach and the hilus of the spleen, and is directly continuous with the ventral layers of the greater omentum. DISSECTION OF THE DOG 25 The disposition of the peritoneum in connection with the liver cannot be adequately studied at this stage of the dissection : it will be better, therefore, to postpone its examination until the liver itself can be displayed completely. RECESSUS DUODENO-JEJUNALIS.—If the mass of small intestine be turned over towards the right and an examination be made immediately caudal and to the left of the root of the mesentery where the abrupt curve of the intestine marks the junction of duodenum and jejunum, the opening of the small duodeno- jejunal recess will be disclosed. The depth of the recess varies. It may extend along the right side of the ascending part of the duodenum to the second flexure of this tube. A. MESENTERICA CRANIALIS.—The cranial mesenteric artery should be looked for at the root of the mesentery about the point at which the transverse colon crosses the middle line. In exposing it, it will be necessary to remove a large lymph-gland, the largest in the body, which lies at the root of the mesentery and is associated with the commencement of the portal vein. This corresponds to the separate lymph-glands of the stomach, liver, and spleen, and some mesenteric glands of other animals. The commencement of the mesenteric artery has a mesenteric sympathetic ganglion (ganglion mesentericum craniale) related to it. Nerves from the ganglion form the cranial mesenteric plexus (plexus mesentericus cranialis) and accompany the branches of the artery. The cranial mesenteric artery is a branch of the abdominal aorta, It soon divides into (1) a main continuation from which arise the intestinal arteries, and (2) the common trunk of the middle colic, right colic, and ileo-cceco-colic arteries. AA. INTESTINALES.—About fifteen intestinal arteries arise at intervals from the main continuation of the mesenteric artery and run down the mesentery to the small intestine, but before reaching the alimentary tube each artery divides and the adjacent branches anastomose. Thus a series of arterial arches are produced. Along the side of the mesenteric artery opposite to that from which the intestinal arteries arise, a small group of mesenteric lymph- glands will be found. ‘The first of the intestinal arteries supplies a considerable amount of the duodenum and anastomoses with the duodenal branch of the pancreatico-duodenal artery ; and the last artery similarly anastomoses with the iliac branch of the ileo-czco-colic artery. A. cotica MEDIA.—The middle colic artery leaves the common trunk early and is the largest branch of the trunk. It supplies the transverse colon and a considerable stretch of the descending colon. A. COLICA DEXTRA.—The right colic artery, a comparatively small vessel, supplies the ascending colon. A. ILEO-ca#co-coLica.—The ileo-cxeco-colic artery divides into three branches which supply the ileum, cecum, and the commencement of the ascending colon. The iliac branch anastomoses with the last intestinal artery. 26 DISSECTION OF THE DOG It is noteworthy that there is a series of anastomoses along the course of the colon between the ileo-ceco-colic, right colic, and middle colic arteries. A. MESENTERICA CAUDALIS.—The caudal mesenteric artery can be most readily found by dissecting between the two layers of the descending mesocolon about the level at which the duodenum crosses the middle line. Very much smaller than the cranial mesenteric artery, it leaves the aorta opposite the fifth or sixth lumbar vertebra. From its point of origin the artery runs towards the pelvis in the attached border of the mesocolon and divides into two branches—left colic and cranial hemorrhoidal arteries. Colon transversum Colon ascendens } a. colica dextra I } a, colica media a. ileo-caeco-colica i u. pancreatico- ' i ' ' ! i 7 aa, intestinales i : | 4 duodenalis | & 1 t - . eee [ ee GNA: D N \Ne ae Sar Colon descendens -4—-Intestinum lenue , ‘ Duodenum y oS # , wo - Intestinum cecum get Y a ———— # =, o: - 3 - Tntestinum ileum a. colica sintstra -~ Fie. 8.—Diagram of the cranial mesenteric artery. A. colica sinistra—The left colic artery runs towards the thorax along the descending colon. This it helps to supply, and finally ends by anastomosing with the middle colic artery. A. hemorrhoidalis cranialis.—The cranial hemorrhoidal artery follows the direction of the parent vessel and ends in the pelvis where it supplies the terminal part of the colon and anastomoses with the middle hemorrhoidal artery. GANGLION MESENTERICUM CAUDALE.—As was seen to be the case with the cranial mesenteric artery, so in association with the root of the caudal vessel of the same name a small sympathetic ganglion is developed. From this filaments arise which follow the mesenteric artery, so forming the caudal mesenteric plecus (plexus mesentericus caudalis). DISSECTION OF THE DOG 27 LyMPHOGLANDULZ.—A few small lymph-glands are lodged between the two layers of the mesocolon. Dissection.— Apply a couple of ligatures a little distance apart around the colon where it is entering the pelvis, and the same at the junction of the duodenum and jejunum. Cut between the ligatures and take away all the intestines, except the duodenum and the terminal part of the colon. Remove them to a sink and slit up the entire length of the tube with scissors, cutting along the line of attachment of the mesentery. Wash away the contents, and examine the interior. INTERIOR OF THE InTESTINES.—The interior of the intestines: is lined by a soft, slimy, mucous membrane, generally more or less folded longitudinally. The folds, however, are not permanent, but may be obliterated by distension. In the small intestine, and more particularly in the ileum, elongated oval areas of mucous membrane can generally be distinguished as differing from their surroundings. These are the aggregated lymph-nodules of Peyer (noduli lymphatici aggregati [Peyeri]). Sometimes they are difficult to demonstrate, in which case they may be discovered by holding the intestine between the eye and a good light. In the cecum and in the commencement of the colon are numerous large solitary nodules (noduli lymphatici solitarii) composed, like the aggregated nodules, of lymphoid tissue. The termination of the ileum should be specially examined. The orifice by which the ileum communicates with the commencement of the colon is narrow, and, viewed from the colon side, is surrounded by a low ridge of mucous membrane, the valve of the colon (valvula coli). Immediately adjacent to the iliac aperture is a constriction marking the junction. of cecum and colon. STRUCTURE OF THE INTESTINES.—A piece of intestine should be pinned out on a sheet of cork with the mucous membrane downwards. The thin serous membrane (tunica serosa) should now be dissected off the underlying muscle. It will be noticed that the serous covering of the intestine is thinner than the membrane forming the mesentery with which it is continuous. Now turn the piece of intestine over and examine the mucous membrane (tunica mucosa). If the intestine be sunk in water and the surface thoroughly cleared of mucus, it will present a velvety appearance—most marked in the jejunum— from the presence of delicate projections, the villi. A pocket lens facilitates the examination of these. The mucous tunic should now be removed. In doing so, the connective tissue between the membrane and the muscular coat of the intestine—the submucous tissue (tela submucosa)—will be noted as being of somewhat loose texture. An examination of the muscular coat (tunica muscularis) of the tube shows that the fibres are not all disposed in the same direction. The outer, more superficial fibres run longitudinally ; whereas 28 . DISSECTION OF THE DOG the inner fibres are circular in direction. The circular fibres form a somewhat thicker layer in the ileum than in the jejunum. Dissection.—Turn to the wall of the thorax, and clean up the scalenus and ventral serratus muscles. M. scaLenus.—The scalenus muscle cannot be completely exposed at present, since its origin lies in the neck. The part which is to be examined now is applied to the chest-wall dorsal to the transverse muscle of the ribs. The scalenus is divisible into three parts, the insertions of which are as follows: (1) The most dorsal is inserted to the cranial border of the third and fourth ribs. (2) The middle part of the muscle is attached to the fifth, sixth, seventh, and eighth ribs. (3) The most ventral part of the muscle is the shortest and reaches the first rib only. It should be noted that a satisfactory differentiation of the dorsal and middle portions of the muscle is frequently impossible. Dissection.—Cut across the scalenus muscle on a level with the second rib and remove the part inserted into the ribs. M. SERRATUS VENTRALIS.—The ventral serratus muscle is extensive and powerful. Part of it is in the neck and cannot be examined at the present moment. The thoracic portion of the muscle, now exposed, arises by digita- tions from the first seven or eight ribs. The last two or three digitations are related to the origin of the external oblique muscle of the abdomen. The converging bundles of fibres are inserted into the vertebral border and a roughened area on the costal surface of the scapula. The nerve supply of the ventral serratus muscle—the long thoracic nerve (n. thoracalis longus)—is derived from the brachial plexus (from seventh and eighth cervical nerves) ; but, instead of accompanying the rest of the branches of the plexus, the nerve runs along the deep face of the scalenus muscle. Between the last cervical and the first thoracic digitations of the serratus muscle, the transverse artery of the neck (a. transversa colli) and its companion vein should be noted. The vessels disappear under cover of the serratus. Dissection.—Liberate the ventral serratus muscle from all the ribs except the first and turn it outwards. Then clean up the spaces between the ribs as far in a dorsal direction as the attachment of the dorsal serratus muscle, a level generally marked by the appearance of branches from the intercostal vessels. The external oblique abdominal muscle should be detached from the ribs. Observe the extent of the external intercostal muscles. Remove the external muscle from one or two interspaces as far as the attachment of the dorsal serratus muscle, in order to show the underlying internal intercostal muscles. DISSECTION OF THE DOG 29 Mm. INTERCOSTALES EXTERNI.—On each side of the chest there are twelve external intercostal muscles. Each muscle passes from the border of one rib to the adjacent border of the next, with fibres running in a caudo-ventral direction. It will be observed that the first seven muscles terminate about the level of union of the bony and cartilaginous segments of the ribs. Frequently the eighth muscle proceeds farther and may almost reach the sternum. Theremaining muscles are also continued between the rib-cartilages, but generally there is an interruption in their continuity at the end of the bony segment of the rib. In a dorsal direction the external intercostal muscles reach the vertebral column, but this cannot be determined as yet. Mm. INTERCOSTALES INTERNI.—The twelve internal intercostal muscles differ from the external intercostals in that their fibres pass in a ventro-cranial direction, and each muscle is continued along the entire length of an inter- costal space. In the neighbourhood of the sternum, therefore, the internal muscles can be seen before the removal of the external intercostals. Dissection.—Now carefully remove an internal intercostal muscle in order to demonstrate the intercostal vessels and nerve. These will be found lying along the caudal border of the rib, between the internal intercostal muscle and the lining membrane of the chest. AA. INTERCOSTALES.—There are twelve intercostal arteries on each side of the chest. At the present stage of dissection, the origin of the arteries cannot be determined, but will be revealed when the wall of the thorax is examined from within. Nor can the division of each artery into a dorsal and ventral branch be exposed. At the present time it will be sufficient to note that the vessel now displayed is the ventral branch (ramus ventralis) of an intercostal artery. It travels down the caudal border of the rib between the corresponding vein, which is cranial in position, and the intercostal nerve. The arteries of the first seven or eight intercostal spaces terminate by anastomosing with branches from the internal mammary artery. The rest anastomose with the musculo-phrenic artery, or are continued into the wall of the abdomen. Lateral cutaneous branches (rami cutanei laterales) of the intercostal arteries become superficial by piercing the overlying muscles about the middle of the length of each interspace. Vv. INTERCOSTALES.—Intercostal veins accompany the intercostal arteries and are to be sought between the artery and the rib. Nn. INTERCOSTALES.—Intercostal nerves course along the aboral border of the arteries of the same name. Each constitutes the ventral branch (ramus ventralis) of a thoracic spinal nerve. A lateral cutaneous branch (ramus cutaneus lateralis) leaves each nerve at the same point as does the like-named branch of the artery. 30 DISSECTION OF THE DOG The first eight nerves end a short distance from the sternum by dividing into (1) a ventral cutaneous branch (ramus cutaneus ventralis), and (2) a smaller branch for the transverse thoracic and internal intercostal muscles. The ventral cutaneous ramus becomes superficial about the termination of the external intercostal muscle. The last four intercostal nerves are continued beyond the costal arch into the wall of the abdomen, where they have already been found between the internal oblique and transverse muscles. Dissection.—Remove the intercostal muscles from all the interspaces on both sides of the chest from the sternum to about the middle of the length of the interspace. The internal intercostal muscles should be removed with the greatest care, so that the endothoracic fascia between the ribs may be preserved. In the region of the sternum a muscle, the transverse thoracic, and an artery and a vein, the internal thoracic or mammary, are exposed on each side. A. MAMMARIA INTERNA.—The internal mammary artery is a branch of the subclavian, and pursues an oblique course in the mediastinum to gain the ventral surface of the transverse thoracic muscle. Here the artery runs along in the region of the joints between the costal cartilages and the sternum, until it reaches the eighth cartilage, where it divides into the cranial epigastric and musculo-phrenic arteries. The following are the collateral branches of the artery: (1) Two inter- costal branches (rami intercostales) are distributed to the ventral end of each of the first seven intercostal spaces. These anastomose with the ter- mination of an intercostal artery. (2) Sternal and perjorating branches (rami sternales et perforantes) supply the transverse thoracic and pectoral muscles. (3) Thymic arteries (aa. thymice). (4) Anterior bronchial arteries (aa. bronchiales anteriores). (5) Anterior mediastinal arteries (aa. mediastinales anteriores. (6) Pericardiaco-phrenic artery (a. pericardiaco-phrenica). With the exception of the first two, these branches are distributed to structures within the thorax, and, consequently, are not accessible at present. A. MUSCULO-PHRENICA.—In order to examine the termination of the internal mammary artery properly, it may be necessary to cut away a part of the eighth costal cartilage. The musculo-phrenic artery runs along the border of the diaphragm, and is, at first, under cover of the eighth costal cartilage. ‘The artery supplies the diaphragm, and anastomoses with some of the intercostal arteries. A. EPIGASTRICA CRANIALIS.—The cranial epigastric artery has already been examined as it runs along the deep face of the straight muscle of the abdomen. A considerable branch has also been noted as becoming superficial, close to the xiphoid process of the sternum. There only remains to examine DISSECTION OF THE DOG 31 its origin as one of the two terminal branches of the internal mammary artery, and to note that it gains the abdominal wall by piercing the edge of the diaphragm. M. TRANSVERSUS THORACIS.—The transverse muscle of the thorax arises from the third or fourth to the seventh segments of the sternum and from the ninth costal cartilage. Its insertion is to the ribs and their cartilages from the second or third to the seventh or eighth, and to the endothoracic fascia of the intervals between the ribs. Cavum THORACIS.—It is necessary that the dissector should have, at least, some general idea of the thoracic cavity and the organs contained therein before he proceeds with their examination. The thorax is a cavity with a bony wall formed by the thoracic vertebre, the ribs, and the sternum. It resembles a laterally flattened cone, with a sloping base bounded by the diaphragm, which forms the thin, muscular partition between the thorax and the abdomen. The diaphragm being markedly concave when viewed from the abdominal side, it is important to remember that the cavity of the thorax is not so spacious as an examination of the skeleton would lead one to imagine. Between the first pair of ribs, i.e. at the apex of the cone, numerous structures pass to and from the neck. The chief organs contained within the chest are the heart and the two lungs. The heart lies between the lungs and is enclosed in a fibro-serous sac, the pericardium. The lungs are lateral to the heart and fill the greater part of the thoracic cavity. Except where it is connected with the heart by large blood-vessels, and where it is joined to the trachea by the bronchus, each lung lies free in its own side of the thorax. A serous membrane, the pleura, covers each lung and lines the corresponding part of the thoracic wall. Where the two pleure come into contact with, or approach each other in, or near the median plane, they form the mediastinal septum (septum mediastinale), a partition in which all the thoracic contents, with the exception of the lungs, are contained. Part of the septum and some features of the pleural cavity must now be displayed. That part of the pleura which covers the lungs is known as visceral: the rest of the membrane is the parietal pleura. Dissection.—Carefully free the pleura from the ribs from the second to the seventh, and divide these ribs about the middle. Cut through the sternum between the first and second and between the seventh and eighth costal cartilages. Remove the sternum and portions of ribs so isolated. The detached part of the sternum and the costal cartilages articulating therewith must be laid aside for the subsequent examination of the joints. The dissection leaves the pleural sacs intact, with the costal pleura (that part of the pleura originally attached to the ribs) lying loose upon the lungs. The interior of the pleural cavities should now be exposed by making an incision through the costal pleura on each side close to the cut ends of the ribs, and a second incision at right angles to the first. PievurA.—The disposition of the pleura must now be examined. Two 32 DISSECTION OF THE DOG incisions have just been made through that portion of the parietal pleura which is applied to the ribs and therefore called the costal pleura (pleura m. rhomboideus thoracalis m, serratus ventralis t m. subscapularis 7 if m, serratus dorsalis ¢ f m. longissimus dorsi c _m. tlio-costalis Costa 3 Vertebra horacalig Pulmon , Laterale 4 profundus\ \ \ Vena cava cranialis \ \ Sternum \ Thymus Fic. 9.—Transverse section of the thorax on a level with the third thoracic vertebra. DISSECTION OF THE DOG 33 costalis). If this be traced in a ventral direction, it will be found to reach the middle line of the sternum, where it meets its fellow membrane of the opposite side. From the sternum the two pleure are reflected dorsalwards m. longissimus dopa m. serratus dorsalis \ ‘ ! m. ilio-costalis Costa 8 *. m. latissimus dorsi ~ Aorta, Dsophagus mm. intercostales n. phrenicus\ m. obliquus A\\ abdominis eaternus “~ \ m. rectus abdominis . m. transversus thoracis A m. pectoralis profundus aX m. pectoralis superficialis__§_._ Fie. 10.—Transverse section of the thorax on a level with the eighth thoracic vertebra. 4, 5, 6, 7=fourth, fifth, sixth, and seventh ribs. as the mediastinal pleura (pleura mediastinalis). Close to the sternum the two pleure are intimately associated and a thin double membrane is produced. Soon, however, the heart intervenes between the two mediastinal pleure {here specifically known as the pericardiac pleura (pleura pericardiaca)], with . D 34 DISSECTION OF THE DOG the result that an extensive mediastinal space is formed. Followed over the heart—or more correctly, the pericardium—it will be discovered that the pleura is reflected over the root of the lung to the lung itself; that is, the parietal pleura here becomes continuous with the visceral or pulmonary pleura (pleura pulmonalis). Caudal to the root of the lung the reflection of medi- astinal pleura does not cease but is continued as far as the diaphragm in the form of the pulmonary ligament (ligamentum pulmonale). The ligament runs obliquely from the root of the lung to the dorsal border of this organ, along which it is then continued to its termination. Now trace the costal pleura in a dorsal direction. On reaching the vertebral column the membrane is reflected ventralwards as the mediastinal septum (medi- astinal pleura), in which the cesophagus and aorta are conspicuous objects. Again the mediastinal pleura is continued onto the surface of the lung by way of the root of the lung and the pulmonary ligament. Now investigate the disposition of the pleura at the apex of the chest. Here it will be found that each sac ends blindly in the cupula pleure extending for a short distance beyond the first rib, and supported, in part, by the origin of the sterno-thyroid muscle. Finally the costal pleura should be followed to the diaphragm—where it forms the diaphragmatic pleura (pleura diaphragmatica)—and from this once again to the mediastinal septum. On the right side of the thorax the arrangement of the pleura is complicated by the presence of a fold which leaves the diaphragm and passes dorsalwards to surround the caudal vena cava and the right phrenic nerve. SEPTUM MEDIASTINALE.—The mediastinal septum between the two pleural cavities is formed, as has been seen, by the apposition of the two pleural mem- branes at or about the median plane. The space between the two membranes is known as the mediastinum or mediastinal cavity, and is bounded laterally by the mediastinal pleure, ventrally by tLe sternum, and dorsally by the vertebral column. Since the unpaired, more or less median structures of the chest occupy a position therein, the greater part of the dissection of the thorax takes place within the mediastinal cavity. For convenience of description the cavity is divided into three parts: (1) A precardial mediastinum, cranial to the heart; (2) a cardial mediastinum, containing the heart and other structures occupying the same transverse zone of the chest; and (3) a postcardial mediastinum, of triangular outline and circumscribed by the heart, the diaphragm, and the vertebral column. The precardial and cardial mediastina are median ; but the postcardial mediastinum is pushed over to the left by the accessory lobe of the right lung. The precardial mediastinum contains the large blood-vessels connecting the heart with the head, neck, and thoracic limbs, the terminal part of the thoracic duct, the vagi, recurrent, phrenic, sympathetic, and cardiac nerves, the cesophagus and trachea, and the thymus and lymph-glands. The cardial mediastinum contains the heart and its enveloping pericardium, DISSECTION OF THE DOG 35 the commencement of the arterial and the end of the venous systems, the cesophagus, the termination of the trachea, the vagi, left recurrent, phrenic, cardiac, and pulmonary nerves. The postcardial mediastinum contains the aorta, the azygos vein, the vagi, left phrenic and sympathetic nerves, the thoracic duct, and the cesophagus. ARTICULATIONES STERNOCOSTALES.—Nine of the thirteen ribs on each side articulate with the sternum by means of their cartilages. The end of the rib cartilage is received into a concavity on the lateral surface of the sternum and articulates therewith by a diarthrodial joint. Each articulation is enclosed by a joint-capsule (capsula articularis) and is provided with a synovial mem- brane. On each side of the joint the capsule is thickened to form the radiate sterno-costal ligaments (ligg. sternocostalia radiata). The cartilages of the tenth, eleventh, and twelfth ribs do not reach the sternum, but each is bound by fibrous tissue to the cartilage preceding it. The cartilage of the last rib is generally quite independent of the second last. SYNCHONDROSES INTERSTERNALES.—The eight bony segments of the sternum are joined to each other by cartilage, and the union is strengthened by an internal proper sternal ligament (lig. sterni proprium internum). Strong and well marked, the ligament is narrow over the first two or three segments of the sternum, but later it increases in width. Dissection.—It will facilitate the further dissection of the thoracic contents if the fragment of the sternum and portions of the last few ribs be removed. This should be done by cutting through the ribs in a line with the level at which the other ribs were divided previously. The attachment of the diaphragm to the ribs and sternum must be noted and then severed. This dissection allows of the examination of the liver, stomach, spleen, and pancreas, which was not readily possible earlier. Hepar.—The liver is a large, solid, glandular organ of chocolate colour, closely applied to the abdominal face of the diaphragm. Approximately central in position, it lies partly under cover of the ribs, but generally a considerable proportion of its bulk projects beyond the costal arch. The liver may be described as presenting two surfaces and a circumferent edge. The cranial surface is applied closely to the diaphragm and part of the wall of the abdomen, and upon these it is moulded. It is consequently convex in all directions, and, when the organ is in situ, slopes dorso-ventrally and cranialwards in its dorsal half and caudalwards in its ventral half. The caudal surface is, in the main, concave; but, owing to the influence of adjacent hollow viscera, not regularly so. The organs with which it is in contact are the stomach, intestines, pancreas, and the right kidney. The amount of intestinal contact naturally depends largely on the degree of distension of the stomach. The margin of the liver is sharp ventrally and laterally, but blunt and D2 36 DISSECTION OF THE DOG notched dorsally. The dorsal portion of the margin cannot be satisfactorily examined at present. The ventral border, however, is readily examined, and will be seen to be interrupted by deep incisions which divide the organ into lobes. VESICA FELLEA.—A detailed examination of the gall-bladder and its associated ducts is more easily carried out after removal of the liver from the body ; but at the present time certain features should be observed. The gall-bladder is a pear-shaped sac lying in a deep depression on the caudal surface of the liver and generally in contact with the pyloric part of the stomach. The broad fundus of the sac (fundus vesice felles) is close to facies gastrica < ss N £xtremitas dorsalis, L N, ‘. Margo craniay, ts Extremitas ventralis . Facies ‘diaphragmatica us Margo caudal Fie. 11.—Outline of the spleen. the costal arch a short distance to the right of the median plane, and can mostly be seen as soon as the abdomen is opened. From the narrow dorsal end of the bladder springs a short cystic duct (ductus cysticus) which communicates with the common bile-duct (ductus choledochus). The common bile-duct should be traced to the duodenum. In order to reach the intestine it has to run between the two peritoneal layers of the lesser omentum, where it is closely related to the hepatic artery and the portal vein. Of the three struc- tures the duct is to the right, the artery to the left, and the vein intermediate and dorsal to the others. The three structures are immediately dorsal to the pylorus. Lizn.—The spleen is one of the ductless glands of the body. It is a solid organ, deeply placed within the left side of the abdomen, extending from the vertebral region to the costal arch. Generally its ventral extremity DISSECTION OF THE DOG 37 reaches. somewhat beyond the edge of the ribs, and is, therefore, visible as soon as the abdomen is opened. The spleen may be described as possessing three surfaces, three borders, and two extremities. The lateral surface is applied to the abdominal wall and is consequently convex in a dorso-ventral direction. Since the dorsal part of this surface fits into the groove formed by the vertebral bodies and the ribs, the convexity is most marked in this region. The other two surfaces look towards ~--Po----- isophagus Pylorus \ Ductus choledochus Ductus pancrealicus ' ' ' ! ‘ 1 ! t ' I pylorica Corpus venOriculs Vv “Me riculd major Ductus pancreaticus accessorius’ Fra. 12.—Outline of the stomach. the median plane and are concave. The more cranial is applied to the stomach ; whereas the more caudal is in contact with the left kidney and the intestines. The cranial and caudal borders, bounding the lateral surface, are thin. The medial border is much less prominent, but is important since it is the seat of the hilus of the spleen (hilus lienis), by which the blood-vessels and nerves enter the organ. This border, moreover, has the gastro-lienal ligament (lig. gastro-lienale) attached to it. VENTRICULUS.—The stomach is a saccular dilation of the alimentary canal D3 38 DISSECTION OF THE DOG intervening between the cesophagus on the one hand and the small intestine on the other. That part of the stomach connected with the cesophagus is distinguished as the cardia, to the left of which is the rounded fundus. When moderately distended the stomach is readily distinguished as composed of two parts: a rounded, wide body (corpus ventriculi) on the left, into which the cesophagus opens; and a narrow tubular pyloric portion (pars pylorica) com- municating with the duodenum to the right. The position of the pylorus is indicated on the exterior by a faint circular constriction at the junction of the stomach and duodenum. It is in contact with the right central lobe of the liver, and, very generally, with the gall-bladder also. Referred to the surface of the abdomen, the pylorus is immediately to the right of the median plane, between the xiphoid process of the sternum and the costal arch. The pyloric orifice looks towards the vertebral column. The two surfaces of the stomach are smooth and convex and covered by peritoneum. The borders are known as the curvatures. Of these, the dorsal or lesser curvature (curvatura ventriculi minor) is concave and from it the lesser omentum passes to the visceral surface of the liver. The ventral or greater curvature (curvatura ventriculi major) is convex and connected with the greater omentum and gastro-lienal ligament. DuopENuM.—The duodenum is the most distinctive part of the small intestine. Most of the tube lies on the right side of the abdomen, in contact with the right lateral lobe of the liver and the abdominal wall, and dorsal to the bulk of the small intestine. Commencing at the pylorus, the duodenum forms a curve dorsalwards and to the right in contact with the liver. From this, the first flexure, begins the descending part (pars descendens) which passes along the right side of the abdomen to the level of the sixth lumbar vertebra. Here a second, wider curve to the left carries the intestine across the middle line. Next succeeds the ascending part (pars ascendens) which, travelling back again towards the stomach immediately to the left of the median plane, ends in contact with the left kidney and close to the root of the mesentery, at an abrupt caudo-ventral bend, the duodeno-jejunal flexure (flexura duodeno-jejunalis). The position of the duodenum is subject to only slight variation from the fact that its ascending part is intimately connected with the descending mesocolon- The caudal limb of the pancreas is closely associated with the concave side of the descending part of the duodenum. PancrEss.—The pancreas of the dog is an elongated, lobulated gland presenting an appearance very similar to that of the salivary glands. It consists of two limbs diverging from the region of the pylorus. The left limb (cauda pancreatis) extends across the ventral aspect of the vertebral column in the dorsal wall of the omental bursa and dorsal to the stomach. Its left extremity is commonly in contact with the left kidney. The right limb (caput pancreatis) is contained within the mesentery of the duodenum and reaches from the pylorus almost to the point at which the duodenum bends to the left. DISSECTION OF THE DOG 39 The pancreas has two ducts. One, the pancreatic duct of Wirsung (ductus pancreaticus [Wirsungi]), drains the left limb of the gland, and opens into the duodenum in company with the common bile-duct. The second duct, the accessory duct of Santorini (ductus pancreaticus accessorius [Santorini], carries away the secretion from the duodenal or right limb, and opens indepen- dently into the duodenum a short distance (30 to 50mm.) caudal to the orifice of Wirsung’s duct. The two ducts intercommunicate in the interior of the gland. In order to expose the ducts of the pancreas and the termination of the bile-duct, the stomach should be pulled over to the left as far as possible and the duodenum to the right. A dissection must then be carefully conducted between the pancreas and duodenum, in the angle formed by the stomach Ramus esophageus u. gastro-lienalis ¥ ——— - --\ a. gastrica dextra \ a. gastro-duodenalis _&. pancreatico- duodenalis a, gastro-epiploica dextra t ! U I I 1 a. gastro-epiploica sinistra Fia. 18.—Ceceliac artery: semi-diagrammatic. and the adjacent part of descending duodenum. This will also expose the pancreatico-duodenal artery and vein. It will be noticed that the ductus pancreaticus is generally quite small, whereas the ductus pancreaticus accessorius is large. Occasionally the ductus pancreaticus opens into the duodenum independently, immediately distal to the orifice of the common bile-duct. A. ca@i1aca.—The cceliac artery is dorsal to the left limb of the pancreas, and is difficult of dissection from its association with the cceliac ganglia and plexus of the sympathetic nervous system. Possibly the easiest way to find the vessel is by following the already isolated hepatic artery. The cceliac artery is a very short vessel which leaves the aorta between the two lumbar parts of the diaphragm. Its branches are the hepatic, left gastric, and splenic arteries ; the two last-named generally arising at the same point or even from a short common trunk. p4 40 DISSECTION OF THE DOG A. HEPATICA.—The hepatic artery is far from being exclusively devoted to the supply of the liver. Indeed, the greater part of the blood which it carries is destined for the stomach, the duodenum, and the pancreas. Passing inacranial and ventral direction, the artery reaches the porta of the liver, where it furnishes a variable number (2-5) of proper hepatic arteries (aa. hepatice propriz) to the liver. Soon after the origin of the last branch to the liver, it contributes a right gastric artery (a. gastrica dextra), which anastomoses with the left artery of the same name on the lesser curvature of thestomach. About the same point there is commonly a pancreatic branch. The hepatic artery continues from the porta in the lesser omentum and arrives in the neighbourhood of the pylorus, where it terminates as the gastro- duodenal artery (a. gastro-duodenalis), which, in its turn, ends by dividing into right gastro-epiploic (a. gastro-epiploica dextra) and pancreatico-duodenal (a. pancreatico-duodenalis) arteries. The former supplies numerous branches in the region of the pylorus and then follows the greater curvature of the stomach in the ventral layer of the greater omentum, to anastomose with the left gastro-epiploic branch of the splenic artery. The pancreatico-duodenal artery, somewhat larger than the gastro-epiploic, follows the lesser curvature of the duodenum. It supplies the duodenum and the adjacent limb of the pancreas, and ends by anastomosing with the first intestinal artery. A. GASTRICA SINISTRA.—The left gastric artery is mainly concerned in the supply of the left part of the stomach. One of its branches follows the lesser curvature and anastomoses with the right gastric branch of the hepatic artery. Another branch passes along the cesophagus (ramus cesophageus) into the thorax. A. LIENALIS.—The splenic artery runs towards the left, dorsal to the pan- creas, and in the dorsal layer of the greater omentum. Its terminal branches supply the ventral extremity of the spleen, and its collateral branches of moment are two in number, the gastro-lienal and the left gastro-epiploic arteries. The former (a. gastro-lienalis) terminates in the dorsal end of the spleen and contributes twigs to the stomach. The left gastro-epiploic artery (a. gastro- epiploica sinistra) ramifies over the greater curvature of the stomach and finally anastomoses with the right artery of the same name. The splenic artery supplies several branches (rami pancreatici) to the left limb of the pancreas. VENA PORTH.—Since it drains the intestines, stomach, spleen, and pancreas, the portal vein is a vessel of great importance. Found ventral to and to the right of the origin of the cceliac artery, the vein is formed by the union of two venous trunks of considerable size. One of these results from the junction of the splenic and gastric veins, which, in the main, are satellites of the arteries of the same name. The other trunk is produced by the union of the cranial and caudal mesenteric veins, also satellites of the homonymous arteries. DISSECTION OF THE DOG 41 Thus constituted, the portal vein makes directly for the porta of the liver, and has been previously seen in relation to the hepatic artery and the bile- duct. Close to the porta it is joined by the gastro-duodenal vein, and generally also by a fair-sized vessel from the pancreas. Within the liver the portal vein comports itself after the manner of an artery ; that is to say, it repeatedly divides and finally ends in capillary spaces, or sinusoids, between the microscopic cells of the liver. Dissection.—Strip the mediastinal pleura from the surface of the pericardium and expose the phrenic nerves. At the same time the fat lying in the precardial mediastinum should be removed, in order that the entire length “vy. mesenterica cranialis \ . mesenterica caudalis 7 v. Kenalis . gastro-epiploica sinistra Fie. 14.—Portal vein: semi-diagrammatic. of the phrenic nerves may be demonstrated. If the subject be a young one, the thymus should he found in this position. Other structures to be sought are the pericardiaco-phrenic, thymic, and mediastinal branches of the internal mammary artery. A. MAMMARIA INTERNA.—The greater part of the internal mammary artery has already been examined. Its origin, the first part of its course, and some of its earlier branches remain for consideration. The internal mammary is a branch of the subclavian artery and arises at the first rib. From this point it runs obliquely in a caudal and ventral direction in the precardial mediastinum, and thus reaches the sternum, where its relationship to the transverse thoracic muscle begins. The collateral branches now to be dissected are as follows : (1) The pericardiaco-phrenic artery (a. pericardiaco-phrenica) is often very 42 DISSECTION OF THE DOG small. It follows the phrenic nerve as far as the pericardium. (2) Branches to the thymus (aa. thymice#) are naturally most easily demonstrated in the young. (3) Several small arteries are distributed in the precardial mediastinum (aa. mediastinales anteriores). V. MAMMARIA INTERNA.—The internal mammary vein is a satellite of the artery of the same name and receives tributaries equivalent to the branches of the artery. The mode of termination of the vein is subject to some variation. It may open into the innominate vein of its own side, or into the cranial vena cava. Apex pulmonis obus aptcalis Fic. 16.—Lateral surface of the right lung. Tuymus.—If the animal be more than two or three years old, there is little chance of seeing more than the merest trace of the thymus. When present and of good size, the organ is a greyish lobulated body lying in the precardial mediastinum, and flattened laterally in conformity with the place of its location. At its maximum development the thymus extends a little beyond the first rib on the one hand, and overlaps the heart slightly on the other. Putmones.—Each lung is a soft, spongy organ occupying a considerable part of one side of the thoracic cavity and, as has been seen, in intimate asso- ciation with the pleura of that side. With the exception of attachments by its root (radix pulmonis) and the pulmonary ligament, each lung lies free. The lungs of the dog are frequently of a greyish colour due to pigmentation. DISSECTION OF THE DOG 43 Lobi pulmonis.—Deep fissures, cutting the substance of the organ to its root, divide each lung into lobes. The left lung is completely divided into two parts, of which the more cranial is, in its turn, partially subdivided. The completely isolated lobe is basal in position and is related to the diaphragm— the diaphragmatic lobe (lobus diaphragmaticus). The imperfectly separated lobes may be known as apical (lobus apicalis) and cardiac (lobus cardiacus) from their position and relations. obbas@s eo ee ae re) rg” . t fe Lobus diaphragmaticus Apex pulmonts _ ae = Ny _/- ty a ‘. Zobus apioals << | a N = ty Incisura Fo ~~ \ \" cardiaca 1! ' f ‘ J i f ! i ' < hs ny | Fie. 16.—Medial surface of the right lung. a@, Groove for cranial vena cava ; b, groove for vena azygos ; c, groove for cesophagus ; d, groove for phrenic nerve ; e, groove for internal mammary artery ; f, depression for heart. 1, Bronchial ramus for apical lobe ; 2, bronchial ramus for diaphragmatic and intermediate lobes ; 3, bronchial ramus for cardiac lobe; 4, branch of pulmonary artery for cardiac, diaphragmatic, and intermediate lobes; 6, branch of pulmonary artery for apical lobe ; 6, 7, 8, pulmonary veins of apical lobe ; 9, pulmonary vein of cardiac lobe; 10, pulmonary vein of diaphragmatic and intermediate lobes. The right lung has apical, cardiac, and diaphragmatic lobes corhpletely separated from each other, and, in addition, an intermediate lobe (lobus inter- medius) occupying the space between the diaphragmatic lobes of the two lungs, the heart, and the diaphragm. The intermediate lobe is irregularly pyramidal, with its base applied to the diaphragm. At its ventral angle there is a deep, rounded notch, in which are lodged the caudal vena cava, the right phrenic nerve, and the fold of pleura associated with these structures. The lateral or costal surface (facies costalis) of each lung is smooth and glistening from the presence upon it of visceral pleura. Being applied closely 44 DISSECTION OF THE DOG to the wall of the chest, the surface follows the curve of the ribs and is, therefore, convex dorso-ventrally. When the lungs have been satisfactorily hardened by preservative before the chest is opened, the surface is generally marked by grooves which indicate the position of the ribs. The medial or mediastinal surface (facies mediastinalis) is of much smaller extent owing to the fact that the diaphragm encroaches more upon the thorax medially than laterally. The presence of the heart produces a deep depression on this face. Immediately dorsal to the depression for the heart is the hilus Fie. 17.—Lateral surface of the left lung. pulmonis—that is, the point of attachment of the root. Cranial to the cardiac depression, each lung, if well hardened, shows a curved groove for the internal mammary artery. Though these are the general features of the mediastinal surface, there are certain points of dissimilarity in the two lungs. In addition to the cardiac depression, the left lung possesses a deep groove, dorsal to the hilus and the pulmonary ligament, in which lies the aorta. The rest of the surface is flattened. The mediastinal surface cf the right lung has an extensive cardiac depression, in the formation of which the intermediate lobe plays a part. Running from the neighbourhood of the lung apex is a broad groove for the cranial vena cava ; and dorsal to the hilus there is a deep, slightly curved channel connected with the groove for the vena cava. The second of these grooves is produced by the vena azygos. Caudal to the lung root and partly formed by the DISSECTION OF THE DOG 45 intermediate lobe, there is a broad groove for the esophagus. The intermediate lobe itself forms a prominent, pointed projection on the surface. Sometimes the cardiac depression is crossed by a narrow, shallow groove caused by the right phrenic nerve. The dorsal or obtuse border (margo obtusus) of the lung is adapted to the channel formed by the ribs and the vertebre, and consequently is thin at the apex, but becomes thick and rounded towards the base of the organ. The ventral or acute border (margo acutus) is much shorter and thin WW Zobus S "\ \ taphragmations we 1 1 ! ! weeny ee Ze ' a — tps ee y obus-* 1! cardiacus E a, = HALL, == = ! = 1 Fe 1 igi J F ae Facies aoe Saree Marig o aculus / c Fic. 18.—Medial surface of the left lung. a, Groove for aorta; 6, groove for phrenic nerve ; c, depression for heart. 1, Bronchial rami for apical and cardiac lobes; 2, bronchial ramus for diaphragmatic lobe; 3, pulmonary artery; 4, pulmonary vein of apical lobe ; 5, pulmonary vein of cardiac lobe ; 6, pulmonary vein of diaphragmatic lobe. throughout. It occupies the narrow space (sinus costo-mediastinalis) between the ribs and the mediastinal septum, and is notched for the accommodation of the heart (incisura cardiaca). The cardiac notch is much deeper in the right lung than it is in the left; consequently more of the heart is uncovered by lung on the right side of the chest than on the left. The apex (apex pulmonis) of the lung is free, blunt, and laterally flattened. The base (basis pulmonis), being applied to the diaphragm, possesses a concave surface (facies diaphragmatica) which slopes in a caudal and lateral direction. 46 DISSECTION OF THE DOG Except medially, the border circumscribing the base is sharp and fits into the narrow space between the diaphragm and the ribs (sinus phrenico-costalis). It will be noticed that the two lungs differ in two conspicuous respects. (1) The incisura cardiaca is deeper in the right lung because the more bulky part of the heart, the base, inclines towards the right side of the body. (2) The right lung is the larger and possesses an extra (intermediate) lobe. Vena cava caudalis hy?” matcieu, i 1 Lotus W cardiacus Fig. 19.—Diaphragmatic surface of the lungs. Dissection.—The constituents of the root of the lung should now be isolated. In removing the pleura and the fatty connective tissue, great care should be exercised not to injure the pulmonary plexus of nerves which is cranial to the root. The plexus is more accessible on the left side, the vena cava interfering with its dissection on the right. The various structures should be followed into the interior of the lung, in order that their manner of branching may be observed. Rapix PULMONIS.—The root of each lung contains the following structures : (1) Several pulmonary veins conveying blood from the lung to the heart. Although it is difficult to indicate the relative position of the root constituents precisely without making the account of undue length, it may be said that, DISSECTION OF THE DOG 47 speaking generally, the pulmonary veins are most ventral. (2) Pulmonary artery distinguished from the veins by the greater thickness of its wall, and by a somewhat more dorsal position. (3) Bronchus, a large air-tube produced by the division of the trachea. The bronchus occupies the dorsal part of the lung root. (4) Bronchial vessels of small size. (5) Nerves in the form of the pulmonary plexus, which is produced by the intercommunicating branches from the vagus and sympathetic nerves, and is connected with the cardiac plexus. (6) Lymphatic vessels. Each root has certain structures closely related to it. The vena azygos curves round the root of the right lung; and the aorta is similarly disposed on the left side. Both roots are crossed by the vagus nerve ; and the phrenic nerve, though not in contact with, is ventral to the root. Both roots, moreover, have a pulmonary ligament continued towards the diaphragm. N. PHRENICUS.—The origin of the phrenic nerve by three roots from the fifth, sixth, and seventh cervical nerves has already been noted. Both right and left nerves enter the thorax ventral to a subclavian artery and pass through the precardial mediastinum. They then cross the pericardium ventral to the roots of the lungs. The left nerve continues through the post-cardial mediastinum to the diaphragm, but the right nerve courses along the lateral aspect of the caudal vena cava and is therefore not contained within the medi- astinal septum, but in the special fold of pleura provided for the caval vein. There is a further difference in the relations of the two nerves. The left crosses the commencement of the aortic arch and the pulmonary artery, the pericardium intervening ; whereas the right lies along the lateral face of the cranial vena cava. PrERIcaARDIUM.—The pericardium is a fibro-serous sac somewhat loosely enveloping the heart. It is conical in form with the base of the cone looking towards the vertebra and the entrance to the chest. The apex points towards the sternum and the diaphragm. Between the pericardium and the sternum there is no attachment; but the apex of the sac is connected with the fleshy part of the diaphragm by a strong, flattened ligament. The outer face of the pericardium is covered by pleura (pleura pericardiaca), and, as has just been noticed, is crossed by the phrenic nerves. The outer layer of the pericardium is composed of fibrous tissue, and is continued, as tubular investments, onto the large vessels associated with the base of the heart. Dissection.—Make a crucial incision through the pericardium and examine its interior. Within the fibrous pericardium is a serous membrane disposed after the customary manner of its kind: that is to say, the parietal part of the membrane lines the fibrous pericardium, and is reflected along the great vessels onto the heart itself as the visceral portion or epicardium. It will be 48 DISSECTION OF THE DOG observed that the aorta and. pulmonary artery are enclosed within a common tubular sheath of the serous pericardium. Dorsal to these vessels is a passage, the transverse sinus of the pericardium (sinus transversus pericardii). The serous membrane endows the fibrous pericardium and the surface of the heart with a smooth glistening appearance, and, like all other serous membranes, is always moist. VENA CAVA CRANIALIS.—The cranial vena cava is a large vessel formed, on about the level of the middle of the first sternal segment, by the union of the right and left innominate veins. Each innominate vein (v. anonyma) in its turn results from the junction of the jugular and subclavian veins of its own side. As collateral tributaries, each innominate vein has the internal mammary vein anda common trunk formed by the vertebral and costo-cervical veins flowing into it. Any of these, however, may join the vena cava itself. The cranial vena cava lies in the precardial mediastinum ventral to the trachea, and pierces the pericardium to the right of the aortic arch. The vein terminates by opening into the right atrium of the heart. During the process of cleaning the vena cava, several mediastinal lymph- glands (lympho-glandule mediastinales) will be revealed. Some of these are ventral to the vein: others are between the vein and the trachea. VENA AZYGOS.—-The azygos vein begins in the abdomen and drains a con- siderable proportion of the wall of the chest. At the present moment only its terminal part can be examined. The vein joins the cranial vena cava just as this is entering the heart, or it may open into the right atrium itself. VENA CAVA CAUDALIS.—The caudal vena cava begins on a level with the last lumbar vertebra, and enters the thorax by the foramen vene cave of the diaphragm. The thoracic part of the vein, which is all that should be examined now, lies in a notch in the intermediate lobe of the right lung, enclosed in a fold of pleura. The vein passes through the pericardium and opens into the right atrium of the heart. The right phrenic nerve, as has already been said, lies lateral and ventral’ to the vein. Dissection.—In order to give more room for the dissection of the heart, etc., the lungs may now be removed. Cor.—-The heart is a hollow muscular organ in the form of an irregular cone placed so obliquely in the chest that its base (basis cordis) looks slightly towards the vertebral column, but mainly towards the entrance to the thorax. The blunt apex (apex cordis), on the other hand, is directed mainly towards the diaphragm, but also towards the ventral aspect of the body and slightly towards the left. The heart of the dog is flattened in an oblique ‘dorso-ventral direction, so that it presents convex sterno-costal and diaphragmatic surfaces and thick right and left borders. DISSECTION OF THE DOG 49 The exterior of the heart is divided into areas corresponding to the four cavities in the interior. Forming the base are the two atria, separated from the much larger ventricles by the coronary sulcus (sulcus coronarius), which is continuous round the whole heart except where interrupted by the pulmonary artery. Owing to the presence of fat and blood-vessels, the depth of the sulcus cannot be properly estimated in the undissected heart. Externally the distinction between the right and left atria is very im- subclavia sinistra ena cava crantalis a. : ‘ ’ t ‘ q t Vena azygos u. brachio-cephalica.---~~ Auricula dextra----~- {/\ _on® pulmonalis , tt. pulmonales a Ventrtculus stntister Sulcus longitudinalis.- ~ Incisura [apicis] cordis Apex cordis Fic. 20.—Outline of the heart as seen from the left. perfectly marked, except on the left side. Caudally there is only a faint line of demarcation between the two chambers in the form of a shallow groove between the pulmonary veins and the caudal vena cava. Consequently the common atrial mass is crescentic in form, the horns of the crescent being two free projections, the auricule cordis, separated from each other by the pulmonary artery and the aorta. The ventricular part of the heart is responsible for the conical shape of the organ as a whole. Its base is connected with the atria, while its apex forms the apex of the entire organ. The common mass of the two ventricles presents two surfaces and two borders. The sterno-costal face (facies sterno- x 50 DISSECTION OF THE DOG costalis) is convex and is crossed obliquely by a furrow, the ventral longitudinal sulcus (sulcus longitudinalis ventralis), which commences at the root of the pulmonary artery and extends as far as the right border. The diaphragmatic surface (facies diaphragmatica) differs in being not so convex ene cava cranialis UNA AZYGOS -. 7 int: GOs oe QO « subclavia sinistra a id : » Aorla . - N > a. brachio-cephalica _ Auricula dextra Abrium dexter Ventriculus dexter Apex cordis Fig. 21.—Outline of the heart as seen from the right. and of much smaller extent. It is traversed by a dorsal longitudinal sulcus (sulcus longitudinalis dorsalis), which begins at the coronary sulcus opposite the terminal part of the caudal vena cava, and ends by joining the ventral longitudinal sulcus at the right border of the heart. The two grooves indicate the position of the septum between the two ventricles. The borders of the heart differ considerably. The right is the longer, the DISSECTION OF THE DOG 51 more convex, and much the sharper, and is notched where the two longitudinal grooves join a little to the right of the apex. Dissection.—The vessels and nerves which supply the substance of the heart should now be examined. In order to do this it is necessary to remove the epicardium and subjacent fat from the grooves. The remains of the pericardium should also be cleared away. The wall of the heart is supplied with blood by the two coronary arteries, right and left. A. CORONARIA DEXTRA.—The right coronary artery leaves the aorta behind the right valve which guards the exit of this vessel from the left ventricle. From this point it runs along the right part of the coronary sulcus to the diaphragmatic surface of the heart, where it gives off a descending branch (ramus descendens) which traverses the dorsal longitudinal sulcus. A. CORONARIA SINISTRA.— The left coronary artery is much larger than the right, and leaves the aorta behind its left valve to pass backwards to the right of the pulmonary artery. Its circumflex branch (ramus circumflexus) runs along the coronary sulcus to the diaphragmatic surface, where it ends close to the termination of the right artery. Numerous branches ramify over the surface of the left ventricle, but the largest (ramus descendens) leaves the coronary artery very early and follows the ventral longitudinal sulcus. Another descending branch of smaller size leaves the coronary artery close to its termination, and is distributed over the wall of the ventricle. Vv. corpis.—An examination of the coronary sulcus on a level with the pulmonary veins and the caudal vena cava will reveal a large venous trunk of no great length. This is the coronary sinus (sinus coronarius) in which the large vein of the heart (v. cordis major) ends. By means of the sinus the bulk of the blood from the wall of the organ is transferred to the right atrium. In addition, there are numerous small veins which open directly into the atrium. Dissection.—Now proceed to examine the interior of the heart, beginning with the right atrium. In order to open this cavity most advantageously, it is necessary to make two incisions. (1) Enter the knife at the termina- tion of the caudal vena cava and carry it in a straight line to the point at which the cranial vena cava joins the heart. (2) From the middle of the first incision carry a second to the tip of the auricle of the atrium. ATRIUM DEXTRUM.—