^ . .C'K'yy.'y'y,':- ^<,«'k'yyA<«« ^-%>%V«>V*V*\% V4'4*4*! ' T5VVt*4***4*J*4V*V4 ■y.-y. 'i<eritoneuin in the fen)ale pelvis ......... 80. DLssection of the female reproductive organs .... 81. Dissection of female pelvis ....... 82. Viscera of the female pelvis, with reproductive and urinary organs in longitudinal section ....... 83. Fecial circulation in the foal ...... 84. Diagram of the fecial circulation ...... 85. The fcetal heart viewed from the left ..... 91 92 93 95 97 99 102 104 IG6 108 109 111 113 114 116 119 121 124 126 127 133 135 137 138 140 141 148 154 157 158 160 162 163 165 168 172 173 174 177 181 183 184 186 192 193 194 THE THORAX. The dissection of the thorax cannot be begun until the thoracic limbs have been removed. Along with the thorax it is convenient to combine the examination of the muscles, &c., of the back and loins. Dissection. — The dissector's first duty is the removal of the remains of muscles left behind on the separation of the thoracic limbs from the trunk. In doing this care should be taken not to destroy the various branches of the intercostal and lumbar nerves and vessels. The dissection of the aponeurotic tendon of origin of the latissimus dorsi muscle demands special care if the lumbo-dorsal fascia (and origin of the dorsal serratus muscle) is not to be injured. The lumbo-dorsal fascia (Fascia lumbodorsalis). — This extensive sheet of deep fascia invests the muscles of the back and loins, but may be easily raised from the underlying longissimus dorsi. Medially it is connected with the supra-spinous ligament and the extremities of the spinous processes of the vertebrae. Caudally it is continuous with the gluteal fascia. In the region of the scapula it develops into a strong sheet, firmly attached to the spinous processes of the third, fourth and fifth thoracic vertebrae, affording origin to the rhomboid and splenius muscles and connected with the scapular attachment of the ventral serratus muscle. Later in the dissection it will be found that the lumbo-dorsal fascia sends a strong septum from its deep face between the longissimus dorsi and ilio-costalis muscles, and a weaker septum between the longissimus dorsi and spinalis muscles. Laterally the lumbo-dorsal fascia splits into two layers, the more superficial of which constitutes the aponeurotic origin of the latissimus dorsi muscle. The deeper layer affords origin to the dorsal serratus and ilio-costalis and muscles of the abdominal wall. Mm. serrati dorsales. — The two dorsal serratus^ muscles form together a thin sheet extending along the greater part of the dorsal wall of the thorax. Their origin is from the lumbo-dorsal fascia, and the muscular bundles succeeding this extensive aponeurotic origin run in different directions in the two muscles. ^ Serratus (from serra, a saw) [L.], toothed or notched like the edge of a saw. 1 2 TOPOGRAPHICAL ANATOMY OF THE M. serratus dorsalis caudalis. — The fibres of the caudal portion of the dorsal serratus are arranged in an oblique cranial and ventral direction. They terminate in very definite slips or digitations that are inserted into the caudal border of the last eight or nine ribs. M. serratus dorsalis cranialis. — The fibres of the cranial muscle run obliquely in a ventral and caudal direction, and end in digitations that are attached to the cranial border of the ribs, from the fifth or M. M. trapezius thoracalis. M. latissimus dorsi. ' Dorsal cutaneous nerves (thoracic). | M. obliquus externus abdominis. Dorsal cutaneous nerves (lumbar). , N. iliohypogastricus. , i i N. ilioinguinalis. ] | [ triceps brachii. M. deltoideus. Lateral cutaneous nerves. M. pectoralis profundus. Fig. 1. — Superficial dissection of the Thorax and Abdomen before removal of the Limb. sixth to the eleventh or twelfth, covered by the caudal muscle. Two or three of these digitations are Dissection. — Iveflect both serratus muscles bv making a longitudinal incision through their aponeurotic tendon of origin. In reflecting these muscles it will be found that the hmibo-dorsal fascia detaches a partition that passes inwards between the adjacent borders of the longissimus dorsi and ilio-costalis muscles. This will require to be cut through before the ilio-costalis can be thoroughly exposed. 'i'he reflection of the dorsal serratus reveals three deeper muscles. The largest and longest of these is the longissimus dorsi. It is the chief occupant of the groove foi'med by the ribs and the spinous processes of the vertebrae. The narrow lateral muscle, m. ilio-costalis, is readily distinguished by its numerous tendinous attachments to the ribs. The third muscle is the shortest and lies medial to the longissimus, with which it is continuous, where the thoracic spinous processes are longest. The dissector will observe a series of cutaneous nerves piercing the THORAX AND ABDOMEN OF THE HORSE 3 longissimus dorsi muscle close to its lateral border or emerging from the interval between this muscle and the ilio-costalis. These are derived from the dorsal branches of the thoracic nerves. That part of the longissimus dorsi which lies over the lumbar vertebrae is pierced by cutaneous branches of the lumbar spinal nerves. Small blood vessels, branches of the intei'costal and lumbar vessels, accompany these nerves. M. ILIOCOSTALIS.^ — The composite ilio-costalis muscle has origin from the lumbo-dorsal fascia, the first two or three (or more) lumbar transverse processes, and the cranial border and lateral surface of the last fifteen ribs. The oblique bundles of fibres of which the muscle is composed, more or less blended with each other, terminate in long, narrow, flattened tendons. The tendons of the more superficial and lateral bundles cross three or four ribs, with a slight lateral inclination, to end on the caudal border of the first fourteen or fifteen ribs, close to their angles, and the transverse process of the seventh (possibly also of the sixth) cervical vertebra. The slenderer tendons of the deeper and medial bundles are directed in a caudal direction. They cross one or two ribs and end on the cranial border of the ribs from the fourth to the last. Small synovial bursse may intervene between some of the tendons and the ribs over which they pass. M. LONGISSIMUS DORSI. — This very long and powerful muscle is of great complexity, and extends from the sacrum to the last cervical vertebra. Close to the ilium it forms a shoaling depression into which the middle gluteal muscle extends, and from which a considerable number of the fibres of the muscle take origin. In the loins the longissimus is at its thickest, and entirely fills the space between the spinous and transverse processes of the vertebrae. Indeed, it overhangs the tips of the lumbar transverse processes. In the thoracic region the muscle gradually becomes narrower and of less thickness, until it terminates at the last cervical vertebra as a single flattened tendon. A strong, glistening tendinous sheet, particularly noticeable in the lumbar region, covers the surface of the muscle, blends with the supraspinous and sacro-iliac ligaments, and is firmly attached to the crest and sacral tuber of the ilium and the spinous processes of the first two segments of the sacrum. The longissimus dorsi has, naturally, extensive attachments. It arises from the crest and the coxal and sacral tubers of the ilium, the transverse and spinous processes of the first and second sacral vertebra, and the inner surface of the ilium between these points. It is attached to the spinous, transverse, articular and mammillary processes of the lumbar vertebrae, the transverse processes of the thoracic vertebrae, the 1 It has been suggested that this muscle should be called longissimus costarum. TOPOGRAPHICAL ANATOMY OF THE lateral surface of the ribs as far as their angles, and, finally, to the transverse process of the last cervical vertebra. M. LONGissiMUS CERVicis. — If the dissection of the neck has been M. intercostalis externus. M. spinalis et semi spinalis dorsi et cervicis M. longissimus dorsi. M. iliocostalis. M. intercostalis internus. i A. transversa colli. M. longissimus cervicis. Ligamentum nuchae. A. cervicalis profunda. . cervicalis ascendens. V cephalica. transversa scapuli M. scalenus. A. axillaris. A. thoracica externa. M. obliquus externus abdominis. m. rectus abdominis M. transversus costarum. A. intercostalis. Lateral cutaneous nerve. Fig. 2. — Dissection of the Chest Wall, seen from the right. carried far enough, the dissector of the muscles of the back should examine the longissimus cervicis. This muscle appears to be a con- tinuation of the longissimus dorsi, from which it is only separated with THORAX AND ABDOMEN OF THE HORSE 5 difficulty. Its fibres take origin from the transverse processes of the first six thoracic vertebra? ; and its insertion is to the transverse processes of the last four cervical vertebrae. M. SPINALIS (et semispinalis) doksi et cervicis. — The spinal muscle of the back and neck appears to be an off-shoot from the longissimus dorsi with which its caudal portion is connected in the most intimate fashion. Careful dissection shows that the muscle has tendinous connections with the spinous processes of the lumbar and the last five or six thoracic vertebrge ; but it is here that the identity of the muscle is confused with that of its much larger neighbour. About the eleventh or twelfth thoracic vertebra the spinal muscle begins to become more independent. Its more cranial attachments are to the caudal border of the first five or six thoracic spinous processes and the rudimentary spinous processes of the last four or five cervical vertebrae. Dissection. — Remove the longissimus dorsi and spinalis muscles. In doing so, observe the muscular branches of the dorsal rami of the intercostal and lumbar arteries. M. MULTIFIDUS ^ DOESI. — A series of small muscles lie close against the vertebral column. They arise from the sacrum, the articular and mammillary processes of the lumbar and the last two or three thoracic vertebrae, and from the transverse processes of the other thoracic vertebrae. Each muscular bundle crosses from two to four spinous processes in an oblique cranial and dorsal direction to its insertion into the spinous process of a lumbar or thoracic vertebra. In the lumbar region the insertion is close to the free end of the spinous process, and this is also the case from the last to about the tenth thoracic vertebra. Thereafter, however, the insertion is further and further removed from the extremity of the process. Mm. levatores cost arum. — The levators of the ribs form a series of small muscles arising from the transverse processes of the thoracic vertebrae. The fibres of each muscle run in a caudal, ventral and lateral direction to be inserted into the cranial border of the rib succeeding the vertebra from which they have taken origin. The first rib is not provided with a levator muscle. Mm. intertransversales lumborum. — Thin and weak inter- transverse muscles, containing a considerable proportion of tendinous tissue, occupy the intervals between the transverse processes of the lumbar vertebrae. Generally there is no muscle between the processes of the fifth and sixth vertebrae. ^ Multifidus (from multus, ma.ny +Jindere, to cleave or split) [L.], cleft or divided into many parts. 6 TOPOGRAPHICAL ANATOMY OF THE Dissection. — Clean the surface of the muscles that lie between the ribs, and of the transverse costal muscle that runs obliquely over the sternal end of the first three ribs. M. TRANSVERSUS COSTARUM. — The transverse costal muscle is flat and th*n, and arises from the first rib ventral to the scalene tubercle. It passes obliquely in a caudal and ventral direction and ends on the Dorsal ramus of thoracic nerve. M. niultifidus. Dorsal branch of intercostal artery. M. longissimus dorsi. M. levator costae M. iliocostalis. Dorsal cutaneous nerve. Fig. 3. — Deep dissection of the Back in the Thoracic Region. extremities of the second, third and fourth costal cartilages and the adjacent part of the sternum. Mm. intercostales externi. — On each side of the chest there are seventeen external intercostal muscles. Each passes from the caudal border of one rib to the cranial border of the next, its fibres sloping in a caudal and ventral direction. Ventrally the muscles end at the junction of the bony segment of the rib and the costal cartilage. Dorsally each muscle comes into contact with a levator of the rib, from THORAX AND ABDOMEN OF THE HORSE 7 which it is distinguished with difficulty ; indeed, the levator may be regarded as a strong and specialised part of the intercostal. It should be noted that the degree of development of the external intercostal muscle varies greatly in different parts of the chest as well as in different individuals. In some intercostal spaces the muscular fibres may fail to reach from one rib to the next, a thin aponeurosis taking their place. In the ventral part of the last few intercostal spaces also the muscle may be very defective. Dissection. — Remove the external intercostal muscles from several interspaces in order to show the underlying internal muscles. Take care not to injure the intercostal vessels and nerves that, near the vertebral column, will be found lying between the two muscles and about midway between two ribs. Mm. INTERCOSTALES INTERNI. — The seventeen internal intercostal muscles differ from the external intercostals in that their fibres run with a ventro-cranial slope, and each muscle is continued between the costal cartilages. In the neighbourhood of the sternum and costal arch, therefore, the internal muscles can be seen before the external inter- costals have been removed. Near the vertebral column the internal intercostal muscles become thin and may disappear. Aa. INTERCOSTALES. — The origin of the seventeen intercostal arteries cannot be determined at the present time. When the interior of the thorax is being examined at a later stage of the dissection, it will be found that the first intercostal arises from the deep cervical artery ; the second, third and fourth (possibly also the fifth) from the supreme intercostal artery ; while all the rest spring from the thoracic portion of the aorta. Each intercostal artery divides into a small dorsal and a larger ventral branch. The dorsal branch (ramus dorsalis) divides into (1) a ramus spinalis that enters the vertebi'al canal by an intervertebral foramen, supplies small twigs to the membranes enclosing the spinal cord, and ends by joining the ventral spinal artery ; and (2) a ramus muscularis that pierces the dorsal end of an intercostal space to supply the muscles and skin dorsal to the plane on a level with the bodies of the thoracic vertebrse. The ventral branch (ramus ventralis) is now under examination. At first it lies between the external and internal intercostal muscles about midway between two ribs. Later it runs between the internal intercostal muscle and the pleura in a groove on the caudal border of a rib. Each intercostal artery ends at the ventral extremity of an inter- 8 TOPOGRAPHICAL ANATOMY OF THE space by anastomosing with the intercostal branches of the internal thoracic and musculo-phrenic arteries. In its course, an intercostal artery furnishes branches to the pleura, ribs, muscles and overlying skin. Vv. INTERCOSTALES. — Intercostal veins accompany the intercostal arteries and are to be sought between the artery and the rib. Their termination will be revealed when the interior of the thorax is being examined. Nn. INTERCOSTALES. — Intercostal nerves follow the caudal border of the arteries of the same name, and are derived from the thoracic spinal nerves. Eighteen pairs of thoracic nerves (nn. thoracales) leave the vertebral canal by the intervertebral foramina, each nerve receiving a numerical designation in accordance with the thoracic vertebra behind which it leaves the canal. That is, the first thoracic nerve passes between the first and second thoracic vertebrae : the eighteenth thoracic nerve leaves the vertebral canal between the eighteenth thoracic and the first lumbar vertebra. Within the intervertebral foramen, or immediately on its exit therefrom, each thoracic nerve divides into a dorsal and a ventral branch. The smaller dorsal branch (ramus dorsalis) crosses the medial border of the levator muscle of the rib, and thereupon divides into a medial and a lateral ramus. The medial ramus runs upwards on the surface of the multifidus and supplies the deep muscles of the back. The lateral ramus passes in a lateral direction under the longissimus dorsi, and has already been noted as piercing the edge of this muscle or emerging between it and the ilio-costalis. It furnishes twigs to these muscles, and ends, as a dorsal cutaneous nerve, in the skin of the back. In the scapular region it supplies the dorsal serratus and rhomboid muscles and ends in a cutaneous nerve in the skin over the scapular cartilage and the ligamentum nuchse. The larger ventral branches (rami ventrales) of the thoracic nerves form the intercostal nerves at present under examination. The first thoracic nerve contributes to the brachial plexus nearly all the fibres composing its ventral branch. Consequently, the first inter- costal nerve is very small. The second intercostal is also relatively small, because many of the fibres of the ventral branch of the second thoracic nerve go to the brachial plexus. The other intercostal nerves are about equal in size but of varying length in accordance with the intercostal space in which they are placed. They accompany the intercostal vessels, lying at first between the external and internal intercostal muscles, and later between the internal muscle and the pleura. The second to the sixth nerves end in THORAX AND ABDOMEN OF THE HORSE 9 the pectoral muscles. The second to the eighth furnish branches to the transverse thoracic muscle. The seventh to the seventeenth pass beyond the costal arch and are found during the dissection of the wall of the abdomen between the transverse and internal oblique muscles. They end in the rectus abdominis muscle. Small branches are supplied to the diaphragm by the intercostal nerves from the eighth to the last. At a distance from the vertebral column that becomes greater with Dorsal root. Spinal ganglion. Mixed spinal nerve. Dorsal ramus. Dorsal cutaneous nerve. Ventral root. Recurrent nerve Sympathetic ganglion. Ramus communicans Ventral ramus. Lateral cutaneous nerve. Fig. 4. — Diagram of a typical Thoracic Nerve. each succeeding nerve, the intercostal nerves (with the exception of the first) give off a large lateral cutaneous branch (ramus cutaneus lateralis), which passes down the intercostal space between the external and internal intercostal muscles. The earlier lateral cutaneous rami pierce the external intercostal and end in the ventral serratus and latissimus dorsi muscles and the skin, and one or two of them are connected with the caudal pectoral nerves. The middle members of the series of cutaneous branches pierce the external intercostal muscles close to the attachment of the digitations of the external oblique ab- 10 TOPOGRAPHICAL ANATOMY OF THE dominal muscle, aud each divides into a small dorsal and a larger ventral branch. The latter supply filaments to the skin and the external oblique muscle. The lateral cutaneous branches of some of the last intercostal nerves are distributed in the wall of the abdomen. Aa. lumbales. — Lumbar arteries behave, in the main, in the same manner as do the intercostal arteries. In the present dissection it is only necessary to note that their dorsal branches supply the muscles and skin of the back in the lumbar region. Nn. lumbales. — Like the thoracic nerves, these divide into dorsal and ventral branches. They are examined in detail during the dis- section of the abdomen. It has already been noted that the dorsal branches furnish cutaneous nerves that pierce the lumbar portion of the longissimus dorsi muscle. Dissecfio7i. — The intercostal muscles should be carefully removed from one or two interspaces. This will reveal that part of the endothoracic fascia that lines the lateral wall of the chest. The endothoeacic fascia (Fascia endothoracica). — The endothor- acic ^ fascia forms a thin elastic lining to the wall of the chest. Its inner face is covered by and gives support to the parietal part of the pleura. In the mid-dorsal line the fascia is connected with the aorta and the other larwe blood vessels in this neis^hbourhood, while in the sternal region it is continuous with the fibrous layer of the pericardium. At the present moment, only those portions of the endothoracic fascia that bridge the intervals between the ribs are exposed. An examination here will show that the fascial fibres run, for the most part, from the border of one rib to the adjacent margin of the next ; and later, when portions of the ribs are removed, it will be possible to determine that the fascia is stronger where it crosses the intercostal spaces than it is where it is applied to the inner surface of the ribs themselves. And at a still later stage of the dissection, it will be found that the fascia is very thin where it covers the cranial surface of the diaphragm. The thoracic cavity (Cavum thoracis). — It is necessary that the dissector should have some general idea of the thoracic cavity and the organs contained therein before he proceeds to their examination. The thorax is a cavity with a bony wall formed by the thoracic vertebrae, the ribs and the sternum. In form it resembles a laterally flattened cone, with a sloping base bounded by the diaphragm, which forms a thin muscular and tendinous partition between the cavity of the thorax and that of the abdomen. It is important to remember that, the diaphragm being markedly concave when viewed from the abdominal aspect, the 1 Ivdov (endon) [Gr.], within. Ouipa^ (thorax) [Gr.], a breastplate, cuirass, corslet, the part covered by the breastplate, the chest. THORAX AND ABDOMEN OF THE HORSE 11 cavity of the thorax is not so spacious as an examination of the skeleton would lead one to imagine. This may be realised by reference to Fio-s. 39-42, in which it is indicated that, at the end of expiration, the curve of the diaphragm, about the median plane of the body reaches the level of the sixth rib. The undissected wall of the thorax may be described as containing the following six layers : — (1) The skin. (2) The cutaneous muscle and fascia. (3) Several strata of muscles applied to the outer surface of the ribs, vertebrae and sternum, (4) A bony skeleton consisting of the thoracic vertebrae, the ribs (including bony and cartilaginous segments), and the sternum. Between the ribs are the intercostal spaces (spatia iutercostales) occupied by the intercostal muscles. (5) The endothor- acic fascia, with which are associated the longus colli and transverse thoracic muscles. (6) The pleura. The entrance to the thorax (apertura thoracis cranialis), ovoid in outline and flattened laterally, is bounded by the first thoracic vertebra, the first pair of ribs and the cranial end of the sternum. It is occupied by the longus colli muscle, trachea, oesophagus, nerves, large blood vessels proceeding to and from the neck and the thoracic limbs, lymphatic vessels and lymph glands, and (in the young) the thymus. The opposite extremity of the thoracic cavity (apertura thoracis caudalis) is bounded by the last thoracic vertebra and the costal arch, and is closed by the diaphragm. It is convenient to consider that the thorax possesses a dorsal, a ventral and right and left lateral walls, though the demarcation of these is largely arbitrary. The dorsal wall may be held to correspond to the thoracic vertebrae and the ribs as far as their angles, with the ligaments and muscles connected with these bones. The ventral wall is formed by the sternum and the cartilaginous segments of the sternal ribs, with their associated muscles and ligaments. The lateral walls consist of the bony segments of the sternal ribs from their angles onwards, the corresponding part of the asternal ribs and their cartilaginous segments and the intercostal muscles. The limits of the lateral wall of the thorax, as distinct from the wall of the abdomen, are marked by the border of the last rib and the gently curved line, the costal arch (arcus costalis), with a concavity looking in a dorsal and cranial direction, formed by the overlapping cartilages of the asternal ribs. Although the cavity of the thorax has been described above as having the form of a flattened cone with a sloping base, this description requires amplification, as can best be done by a consideration of the various diameters of the cavity. Because the distance between the sternum and the vertebral column increases from the level of the first 12 TOPOGRAPHICAL ANATOMY OF THE rib to that of the sixth, and because of the cranial and ventral slope of the diaphragm, the greatest dorso-ventral (vertical) diameter of the cavity occurs on a level with the diaphragmatic attachment to the sternum. The slope of the diaphragm is also responsible for the fact that the dorsal wall is almost twice as long as the ventral wall (sternum). The degree of curvature of the ribs causes the transverse diameter to increase from the first rib to the last ; though, in some animals, this diameter remains fairly constant from the eleventh rib to the last. The chief organs contained within the thorax 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 fill the greater part of the thoracic cavity, and, except where it is connected with the heart by large vessels, and where it is joined to the trachea by a bronchus, each lung lies free in its own side of the thorax. A serous membrane, the pleura, covers each lung and lines the con-esponding part of the thoracic wall. Where the two pleurae come into contact with or approach each other in or near the median plane, they form the Tried last inal septum, a partition in which all the thoracic contents, with the exception of the lungs, the caudal vena cava and the right phrenic nerve, are contained. Part of the septum and some of the features of the pleural cavity must now be displayed. That part of the pleura that covers the lung is known as visceral ; the rest of the membrane is the parietal pleura. Dissection. — On both sides of the body the ribs from the second to the fourteenth inclusive must be cut through with bone forceps about their angles. The same ribs are, in like manner, to be separated from their cartilages. Then, with a knife, remove en masse the portion of the chest wall thus indicated. The greatest care is necessary in con- ducting this dissection, otherwise the lung, which, in a properly hardened subject, is closely applied to the inner surface of the chest wall, will be injured. The pleura. — Before any further dissection is undertaken, the disposition of the pleura^ must be ascertained. That part of the membrane which is applied to the ribs, and in which a large window has just been made, is known as the costal pleura (pleura costalis). If this be traced in a ventral direction it will be found to approach or reach the middle line of the sternum, where it meets its fellow membrane of the opposite side of the thorax. From the sternum each membrane is reflected dorsalwards as the mediastinal pleura (pleura mediastinalis). In some regions the mediastinal pleurae are intimately related to each other, and a thin double membrane, the mediastinal septum (septum mediastinale), is produced. In other regions, however, 1 TrXevpd (pleura) [Gr.], the side, rib, the membrane that lines the chest. THORAX AND ABDOMEN OF THE HORSE 13 M. longus colli CEsophagus, Lung. Trachea. Tnincus brachiocephalicus. Vena cava cranialis. Mediastinum Aorta. (Esophagus. Lung. Vena cava caudalis. Cava! fold of right pleura. Mediastinum. Diaphragm. CEsophagus. Bronchi. A. pulmonalis. Heart. Pericardium. Fig. 5. — Diagrams to illustrate the arrangement of the Pleurae in transverse sections of the Thorax. ^ = Cranial to the Heart. fi = On a level with the Heart. C = Caudal to the Heart. The Pleurae are represented by the red lines. 14 TOPOGRAPHICAL ANATOMY OF THE structures of considerable size intervene between the two mediastinal pleurae, with the result that a more or less extensive mediastinal space is formed. The heart is the largest of these structures, and it is customary to designate that part of the mediastinal pleura that is applied to the membranous sac surrounding this organ by the specific name of 'pericardiac pleura (pleura pericardiaca). On following the pleura over the heart — or, more correctly, over the pericardium — it will be discovered that it is reflected over the root of the limg on to the lung itself; that is, the parietal pleura here becomes continuous with the visceral or pulmonary pleura (pleura pulmonalis). It should be further noticed that caudal to the root of the lung the reflection of the mediastinal pleura is carried backwards as far as the diaphragm in the form of the pulmonary ligament (ligamentum pulmonale). During the foregoing examination it will have been observed that the mediastinal and costal pleurae meet at the sternum at a very acute angle. Thus the most ventral part of the pleural cavity is very narrow, and is distinguished as the costo-mediastinal sinus (sinus costo- mediastinalis). Now trace the costal pleura in a dorsal direction. On reaching the vertebral column the membrane is reflected ventral wards as the mediastinal septum, in which the oesophagus and aorta are conspicuous objects. Again the mediastinal pleurals continuous with the pulmonary pleura by way of the root of the lung and the pulmonary ligament. Next investigate the disposition of the pleura at the apex of the chest. Here each sac ends blindly in the cupula ^ pleurce, which, on the left side, generally lies within the first rib, but on the right side may extend beyond the rib and come into contact with the scalenus muscle. Finally, the costal pleura should be followed to the diaphragm — where it forms the diaphragmatic pleura (pleura diaphragmatica) — and thence again to the mediastinal septum. Owing to the bulging of the diaphragm towards the cavity of the chest, the part of the muscle close to its costal attachment is applied to the inner surface of the ribs. Consequently, in this region — the phrenico-costal sinus (sinus phrenicocostalis) — there is a certain amount of the pleural cavity into which the thin edge of the lung does not extend even during inspiration. Naturally, during expiration and when the hollow abdominal organs are distended, the area of contact of diaphragmatic and costal pleurae is greater. The diaphragmatic line of pleural reflection, that is, the line along 1 Cupula [L.], a cup, dim. of cupa, a tub. THORAX AND ABDOMEN OF THE HORSE 15 which the pleura passes from the ribs on to the diaphragm, should be examined with care, since it is of clinical importance. The line runs over the union of the seventh and eighth ribs and their cartilages, crosses the sternal end of the ninth rib close to the costo- cartilaginous articulation, and then sweeps over the ribs, with a gentle curve, the convexity of which looks downwards, in such a manner as to recede gradually from the sternal end of the successive ribs until the second last of these is crossed scarcely ventral to the middle of its length. About the middle of the cranial border of the last rib the line turns towards the middle line, and, with a slight forward inclination, ends opposite the middle of the width of the last intercostal space. On the right side of the thorax the arrangement of the pleura is complicated by the presence of a fold, the fold of the vena cava (plica venae cavse), that leaves the diaphragm and passes dorsalwards to' surround the caudal vena cava as this travels from its point of perfora- tion of the diaphragm to the heart. The right phrenic nerve is enclosed in a secondary fold that springs from the right face of the main fold. The plica venae cavse is delicate and lace-like in the adult horse, and occupies a deep fissure between the intermediate lobe and the main mass of the right \nn