ANATOMY. 31 



diately subjected to the chemical action of the bile aiul j)niu'reatic 

 fluid. Chyme, acted upon by these juices, becomes ch/le. Passinoj 

 through the <ireat length of the small intestines, nearly all of the 

 nourishing parts of the chyle are absorbed, and the residue (remain- 

 der) enters the cjecum, which is the water reservoir. (Water re- 

 mains in the stomach of the horse only a short time and then passes 

 promi)tly through the small intestines into the csecum.) 



The residue, soaked in water, gives up, in its ])assiige through 

 to the rectum, the small amount of nutritive matter that has not pre- 

 viously been absorbed. By means of muscular cross ridges in the 

 floating colon the effete material is mechanically molded into pellets 

 of dung, which are stored in the rectum, whence they are ejected, at 

 intervals, through the anus. The ejection is called defecation. 



URINARY SYSTEM. 



(Plate VI.) 



The organs of this system secrete effete material in the form 

 of a watery fluid, called urine, and expel it from the body (unnation). 

 They are the kidneys, ureters, bladder, and urethra, all lined with 

 mucous membrane. 



The Tcidneys are two in number, right and left, situated on either 

 side of the spine, immediately below the lumbar vertebne. Their 

 action is to secrete the urine from the blood by a process of filtering. 

 Each kidney has a tube or duct, called the ureter, which carries the 

 urine to the storage reservoir, the bladder. This muscular organ, by 

 contraction, discharges the urine, at intervals, through a tube called 

 the urethra, which extends to the head of the fenis. 



The normal amount of urine secreted in tw^enty-four hours and 

 expelled through the penis varies from 3 to 6 quarts. The color 

 in health is yellowish. 



CIRCULATION. 



(I'late vn.) 



The organs which convey the blood throughout the body are 

 the heart, arteries, capillaries, and veins. 



The heart is a hollow organ, made up of involuntary muscles, and 

 inclosed in a serous sac called pericardium; it is situated between the 

 lungs, in the thoracic cavity, and averages about 6^ pounds in weight. 

 It is divided into two parts, right and left, separated by a muscular 

 wall. Each part contains two cavities, one above the other, the 

 upper called the auricle and the lower the ventricle. The cavities are 

 connected by openings, which are guarded by valves to prevent a 

 back flow of the blood. 



