318 THE LIVER. 



dung until it is evacuated. It 1ms none of these bands, because, all tbe 

 nutnment being extracted, the passage of the excrement that remains 

 should be hastened and not retarded ; still, however, it would be very dis- 

 aoTeeable were the horse to void his faeces in the same fluid state as does 

 the cow. To prevent this the inner coat of the rectum and a portion of the 

 colon form a series of cells, in which the excrementitious portion of the 

 food lodo-es until it becomes drier and more compact, and it is then eva- 

 cuated in a much less ofiensive form. The faeces descend to the rectum, 

 which somewhat enlarges to receive them ; and when they have accu- 

 mulated to a certain extent, the animal, by the aid of the diaphragm and 

 the muscles of the belly, presses upon them, and they are evacuated. A 

 curious circular muscle, and always in action, and called the spTunder 

 (constrictor muscle), is placed at the anus, to prevent the constant and 

 unpleasant dropping of the faeces, and to retain them until the horse is 

 disposed voluntarily to expel them. This is effected by the efforts of the 

 animal, assisted by the muscular coat of the rectum, which is stronger than 

 that of any of the other intestines, and aided by the compression of the 

 internal obhque and transverse muscles. 



The larger intestines derive their blood from the posterior mesenteric 

 artery. Then- veins terminate in the vena portae. 



THE LIVER 

 Is situated principally in the right side of the abdominal cavity, but 

 extending partially into the left, and is surrounded by the ribs, diaphragm, 

 and stomach, its right lobe or division in contact with the diaphragm, 

 the duodenum and the right kidney, and the middle and left divisions 

 with the stomach. It is an irregularly shaped, reddish-brown substance, 

 of considerable bulk, and performs a very singular and important office. 



It has been already stated (p. 303) that the blood, which has been con- 

 veyed to the different parts of the body by the arteries, is brought back 

 to the heart by the veins ; but that which is returned from the stomach 

 and intestines, and spleen, and pancreas, and mesentery, instead of flowing 

 directly to the heart, as the blood from every other part of the body does, 

 has an extra duty to perform before again entering into the general circu- 

 lation — to secrete the bile : to effect tliis, it is collected in a large but 

 short vein, called the vena port^, which enters the liver by two large 

 branches, that spread by means of innumerable minute branches through 

 every part of it. As the blood traverses this organ, a fluid is separated 

 from it, called the hile, which answers an important purpose, for the pro- 

 gress of digestion is promoted by the bile changing the nutritive portion 

 of the food from chyme into chyle, and separating it from that which, con- 

 taining little or no nutriment, is voided as excrement. Having performed 

 this additional duty the fluid is returned from one vein into another, that 

 is, from the vena port», which conveys it to the Hver for the secretion of 

 bile, to the hepatic veins, which return it to the general circulation. 



Almost every part of the hver is closely invested by the peritoneum, which 

 seems to discharge the office of a capsule to this viscus. Its arteries are 

 very small, considering the bxdk of the Hver, to nourish which, however, 

 is their only duty ; their place in the secretion of bile is cui-iously 

 supphed by a vein — the vena portce — a vessel formed by the union of the 

 splenic and mesenteric veins, and which seems, if it does not quite usurp the 

 office and discharge the duty of the artery, to be alone concerned in the 

 secretion of the bile. There is a free intercourse between the vessels of 

 the two. 



The bile, thus formed, is in most animals received into a reservoir, the 

 gall-bladder, whence it is conveyed into the duodenum (g, p. 313) at the 



