Book VII. PHYSIOLOGY OF THE HORSE. 9^3 



pipe (fig- 833. g), is a large canal for the transmission of air, formed by alternate rings of membrane and 

 segments of cartilage, rendering it at once flexible and cylindrically hollow. The oesophagus (figs. #31. h 

 & 833. s) is the continuation of the funnel-like cavity of the pharynx. It is externally muscular, and in- 

 t( rnally membranous and cuticular, by which formation it is elastic, to allow of distention in the act of 

 swallowing. The oesophagus penetrates the chest within the mediastinum, and passing along the spine 

 (fig- 833. i;, through an opening in the diaphragm, terminates in the stomach. 



Subsect. 12. The Thorax or Chest. 



6391. The chest of the horse is bounded anteriorly by the matters filling up the space between the two 

 first ribs, posteriorly by the diaphragm, laterally by the ribs, above by the vertebra?, and below by the 

 sternum or breast bone. In dissecting the horse, after the interior membranes, muscles, &c. are thrown 

 back (fig.831.bbbb), there appear the lobes of the lungs (ficcc) j the heart (d) ; mediastinum or mem. 

 branous division of the chest (ee); the sternum or breast-bone (/) ; the ensiform cartilage (g) ; and 

 tendinous centre of the diaphragm (A, j). 



6392. When the chest is opened a smooth polished membrane is seen, which covers the surface, and then 

 is reflected over its contents ; this is called the pleura ; and by a junction of the two pleiira', a division 

 of the chest into two nearly equal portions is effected, which membranous division is called the nie- 

 diasfmum. By this division of the chest into two parts, very important benefits arise; as when one 

 cavity is opened the lungs immediately collapse, but the respiration may be carried on by the other. In a 

 similar manner ulceration may proceed to destroy the lobes of one side of the chest, as in glanders, but 

 may be checked bv the mediastinum from proceeding to the other. The pleura does not, as in man, ap- 

 pear to take on inflammation independently of the substance of the lungs ; thus the horse is not subject 

 to pleurisy. The thymus gland, which is a considerable body in the colt, and which forms the sweetbread 

 in calves, is hardly discernible in the old horse. It is situated between the folds of the mediastinum, but 

 its uses are unknown. • 



6393. The diaphragm or midriff (fig. 831. i, h) is a very important part of the body of the horse, dividing 

 the chest from the belly by its disk, hut which is far from elliptical, extending much further backwards 

 than forwards. Its fibres radiate from their origins to unite in one tendinous centre (A). In a state ot rest 

 it is anteriorly convex, and posteriorly concave ; but at each inspiration these appearances are nearly re. 

 versed (6398.) It is perforated for the passage of the vena cava, the a6ita, the vena azygos, thoracic duct, 

 and o?sophagus, all which pass through it by means of three openings. It has been found ruptured in some 

 desperate cases of broken wind. 



6394. The heart (fig. 831. d) is the great agent of circulation, and is made independent of the will; were 

 it otherwise, man and other animals might cease to live at their own discretion. The pericardium is 

 first seen surrounding the heart so completely, that it swims within it by means of a little fluid termed 

 liquor pericardii. The heart is a composition of membranous and muscular fibres, having four principal 

 cavities, and several openings. It is situated within the mediastinum, so as to occupy a cavity ot its 

 own, distinct from either side of the chest. Its base is in a line with the dorsal vertebra?, and its apex is 

 directed to the left of the sternum, between the eighth and ninth ribs. Its two ventricles are imme- 

 diately within its body, and its two auricles are rather without, appended to it. The left ventricle con- 

 tains arterial blood, and from it originates all the arteries except the pulmonary. The right ventricle is 

 the reservoir of the venous blood, and it receives all the veins except the pulmonary. Within the ventri- 

 cles are valves to prevent the return of the blood. The auricles are less muscular than the ventricles : 

 the left, or pulmonary, opens into the left ventricle; and the right communicates with the right ventricle. 

 Into the right and larger auricle the anterior and posterior cavas enter by two openings, and into the leit, 

 the pulmonary veins pass. . . . 



6395. The circulation of the blood may be shortly described as originating with the left ventricle of the 

 heart, which sends its blood, by means of the great vessel called the aorta, to all parts ot the body. 1 he 

 blood thus distributed is collected again by the veins from all parts, and is by them returned into the heart 

 by means of the two cavas, which pour their contents into the right auricle, which immediately lories it 

 into the right ventricle. From the right ventricle it is again forced out into the pulmonary artery, 

 which carries it throughout the lungs to undergo a change, and to be finally returned by eight 

 trunks into the left auricle, which immediately empties it into the left ventricle to renew the process 

 described. .... „ . , , T , . 



6396 The lungs are s/xmgy masses divided into right and left, with less divisions called lobes. Ineir 

 colour varies according to age: thus, in the colt they are of a light lively pink ; in the full grown horse 

 they approach to a grever tint ; and in the very old subject they are of a still deeper tone. The bronchia 

 are continuations of the trachea or windpipe, which, dividing on its entrance into the chest, ramifies 

 throughout the sul stance of the lungs, giving these masses their spongy cellular structure, m which dis- 

 tribution the air vessels are accompanied by ramifications of the pulmonary artery and veins. Prom the 

 extreme vascularity of these parts they are very liable to inflammation. 



6397. The theory of respiration. By some extraordinary sympathy, the colt at birth gasps, and air 

 rushes into the lungs before collapsed : having once felt this stimulus, by a common consent between 

 the diaphragm and intercostal muscles, the cavity of the chest is diminished to expel the air received, and 

 to inspire a fresh quantity ; and which process is then continued through life. 'Ihe body appears vitally 

 nourished by two sources : the one through the medium of digestion ; the other by means ot the blood 

 itself, which, in its progress through the body, gives out its vital principles of heat to the mass, and 

 vitality to the muscular fibre, for unless the blood effect its part in the contractile phenomena it will be 

 in vain for nervous influence to exert its power. Having given out these principles, it is returned by the 



make room for a fresh inhalation, to oxygenate a fresh quantity of blood, and thus to renovate afresh the 

 vital powers subservient to its influence. 



Subsect. 13. The Abdomen. 



6398. The viscera of the abdomen include the stomach (fig. 833. a) ; lobes of the liver (bb) ; omentum 

 or caul attached to the whole inferior curvature of the stomach (c) ; the spleen (d) ; the kidneys (ee) ; 

 the rectum (/) ; the ovaria (g g) ; the uterus (A) ; the bladder distended with urine (i) ; the diaphragm 

 or muscular' partition dividing the belly from the chest (k k) ; oesophagus or gullet proceeding to the 

 stomach (I); trachea (m) ; vena cava ascendens (n) ; a6rta descenriens («), which passes through tne 

 abd,~;men (a a), as does the cava descendens (b) ; the aorta ascendens (p) ; carotid arteries (q) ; J ug i . r 

 veinsM; oesophagus (s) ; and maxillary artery, forming the most convenient situation tor ieeling the 



, ?.'*.. K ° i '. * - . i ~c *u« i i. I.... lain nnpn 



puis 



!C organs separately, .„ 



and very elastic, as we perceive by the effects of dropsy, great fatness, and likewise by the '""ease in 

 pregnancy. The omentum or caul (c) is the tatty apron which first presents itself on opening an animal i 



