44 



VETERINARY STUDIES 



Heart. — The heart is enclosed by a serous sac, the pericardium, 

 which supports it in place beneath the third, fourth and fifth 

 dorsal vertebrae by attaching to the large blood vessels at its 

 base, to the diaphragm behind, and to the sternum below. The 

 horse's heart measures about 10.5 inches in length by 7.5 wide 

 at the base, and is cone-shaped ; the weight is variable but aver- 



Fig. 24. — Circulation. Diagrammatic. 



1. Heart and Blood Vessels. A, Heart; B, pericardium; C, anterior vena 

 cava; D, posterior vena cava; E, anterior aorta; F, posterior aorta. 



2. The Circulation in Diagram. A, A, auricle-ventricle valves; B, B, 

 semilunar valves. 



ages about nine pounds. The heart contains four cavities which 

 are easily seen when it is cut open. The two located at the base 

 are called auricles; and the two at the apex, ventricles. 



The auricles are much alike ; as are also the ventricles, except 

 that the left ventricle is larger and has a wall which is more than 

 twice as thick as the right. The heart is covered by the peri- 

 cardium and lined with the endocardium. Its muscle fibers are 

 involuntary so far as control is concerned, but are striped. 



