904 PRACTICE OF AGRICULTURE. Part III. 



the act of swallowing. The oesophagus penetrates the chest between the mediastinum, and passing along 

 the spine {fig. 618 I), through an opening in the diaphragm, terminates in the stomach. 



SuBSECT. 12. The Thorax or Chest. 

 5718. The chest of the horse is bounded anteriorly by the matters fiUing up the 

 space between the two first ribs, posteriorly by the diaphragm, laterally by the ribs, 

 above by the vertebrce, and below by the sternum or breast bone. In dissecting the 

 horse, after the interior membranes, muscles, &c. are thrown back {Jig. 616 bbbb), 

 there appear the lobes of the lungs {cccc)'j the heart {d) ; mediastinum or membran- 

 ous division of the chest {ee)', the sternum or breast-bone {f) ; the ensiform cartilage 

 {g) ; tendinous centre of the diaphragm (/i, i). 



5719. 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 pluree, a 

 division of the Ichest into two nearly equal portions is effected, which membranous division is called 

 the mediastinum. 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 by the mediastinum from proceeding to the other. The pleura does not, as in 

 man, appear 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. 



5720. The diaphragm or midriff {fig.&\& i, h) is a very important part of the body of the horse, dividing 

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

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

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

 reversed. (5724.) It is perforated for the passage of the vena cava, the aorta, the vena azygos, thoracic 

 duct, and oesophagus, all which pass through it by means of three openings. 



5721. The heart {fig. 616 d) is the great agent of circulation, and thus 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 between the mediastinum, so as to occupy a cavity of its own, dis- 

 tinct 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 immediately within 

 its body, and its two auricles are rather without, appended to it. The left ventricle contains 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 ventricles 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 left, the 

 pulmonary veins pass. 



5722. 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 of the body. It is 

 received by the veins in like manner from ail parts, and proceeds to return its blood into the heart by 

 means of the two great veins called cavas, which pour the returned blood into the right auricle, which 

 immediately forces 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. 



5723. The lungs are spongy masses divided into right and left, with lesser divisions called lobes. 

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

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

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

 ramifies throughout the substance of the lungs, giving these masses their spongy cellular structure, 

 in which distribution the air vessels are accompanied by ramifications of the pulmonary artery and 

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



5724. The theory of respiration. By some extraordinary sympathy, the colt at birth gasps, and air 

 ruslies 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. The body appears 

 vitally nourished by two sources : the one through the medium of digestion, the other by means of 

 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 phe- 

 nomena, it will be invain for nervous influence to exert its power. Having given out these principles, 

 it is returned by the veins, and is passed forwards into the lungs, circulating throughout their substance, 

 and imbibing by their contiguity or continuity with the air vessels, oxygen gas from the atmosj)heric 

 air contained in them. In return for the oxygen received, carbon is given out, which passes off in the 

 form of aqueous vapor. As the blood is renovated, so the air it acted on is deteriorated, and is therefore 

 expired from the chest to 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. 



57Q5. The viscera of the abdomen includes the stomach (fig. 618 a); lobes of the 

 liver (b b) ; omentum or cawl attached to the whole inferior curvature of the stomach (c) ; 

 the spleen (rf) ; the kidneys (ee) ; the rectum (/) ; the ovaria (gg) ; the uterus (h) ; 

 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 (/) ; 

 trachea (m) ; vena cava ascendens {n) ; aorta descendens (o), which passes through' 

 the abdomen (a a) , as does the cava descendens (b) ; the aorta ascendens (;j) ; carotid' 

 arteries ($-) ; jugular veins (r)) oesophagus (.s) ; and maxillary artery, forming the 

 most convenient situation for feeling the pulse (t), complete the viscera and general 

 appearances of the horse when laid open. 



5726. The abdomen or cavity of the belly is the largest cavity of the body, and forms an extensive 

 oval _ vault, containing very important viscera, which may be considered as , the chylopoetic, the" 



