230 THE ABDOMEN AND ITS CONTENTS. 



taken ip by the lacteals or absorbent vessels of the small intestines, passes thiough 

 this \alvular opening of the ileum, and a part of it enters the colon, while the 

 remainder flows into the caecum. Then, from this being a blind pouch, and from the 

 rellular structure of this pouch, the food must be detained in it a very long time ; and 

 in order that, during this detention, all the nutriment may be extracted, the caecum 

 and its cells are largely supplied with blood-vessels and absorbents. It is principally 

 the fluid part of the food that seems to enter the caecum. A horse will drink at one 

 time a great deal more than his stomach will contain ; or even if he drinks a less 

 quantity, it remains not in the stomach or small intestines, but passes on to thf 

 caecum, and there is retained, as in a reservoir, to supply the wants of the system 

 In his state of servitude, the horse does not often drink more than twice or thrice in s 

 day, and the food of the stabled horse being chiefly dry, this water stomach is mosi 

 useful to him. The caecum will hold four gallons. 



The colon is an intestine of exceedingly large dimensions, and is capable of con- 

 taining no less than twelve gallons of liquid or pulpy food. At its union with the 

 caecum and the ileum, although larger than the latter intestine (y), it is of com- 

 paratively small bulk ; but it soon swells out to an enormous extent. It has likewise, 

 in the greater part of its course, three bands like the caecum, which also divide it, 

 internally, into the same description of cells. The intention of this is evident, to 

 retard the progress of the food, and to give a more extensive surface on which the 

 vessels of the lacteals may open : and therefore, in the colon, all the chyle is finally 

 separated and taken up. When this is nearly accomplished, the construction of the 

 colon is somewhat changed : we find but two bands towards the rectum, and these 

 not puckering the intestine so much, or forming such numerous or deep cells. The 

 food does not require to be much longer detained, and the mechanism for detaining it 

 is gradually disappearing. The blood-vessels and absorbents are likewise rapidly 

 diminishing. The colon, also, once more contracts in size, and the chyle having been 

 all absorbed, the remaining mass, being of a harder consistence, is moulded into 

 pellets or balls in its passage through these shallower cells. 



At the termination of the colon, the rectum (straight gut) commences. It is smallei 

 in circumference and capacity than the colon, although it will contain at least three 

 gallons of water. It serves as a reservoir for the dung until it is evacuated. It has 

 none of these bands, because, all the nutriment being extracted, the passage of the 

 excrement that remains should be hastened and not retarded. 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, called the sphincter (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 oblique and 

 transverse muscles. 



The larger intestines derive their blood from the posterior mesenteric artery. Theli 

 veins terminate in the vena portae. 



THE LIVER. 



Between the stomach and the diaphragm 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 is the liver. 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. 163) that the blood, which has been conveyed 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, passes first through the liver. 

 It enters by two large vessels that spread by means of innumerable minute branches 

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

 froc i it, called the bile. It is probably a kind of excrement, the continuance of which 

 in Lie blood would be injurious ; but while it is thrown off, another important purpose 

 r * answered the process of digestion is promoted, by the bile changing the nutritive 



