308 THE ANATOMY OF THE HORSE. 



the small intestine, of which about thirty feet is arbitrarily apportioned 

 to the former, and the remainder (about forty feet) to the latter. They 

 are arranged in the form of numerous coils, which occupy the iliac, 

 umbilical, and hypogastric regions. The coils are attached to the free 

 edge of a fold of peritoneum called the great mesentery, and, inasmuch 

 as this mesentery is of considerable breadth, they may move from place 

 to place within the above-mentjpned areas. When distended, they have 

 not a puckered, but a smooth, surface. The terminal part of the ileum 

 joins the crook of the caecum, into which it projects for a little distance, 

 after the manner of a tap into a barrel ; and at the point of entrance 

 there is a valvular arrangement — the ileo-ccecal valve, to prevent 

 regurgitation from the csecum into the ileum. 



The Peritoneum is the lining membrane of the abdominal and pelvic 

 cavities. It belongs to the class of serous membranes, and, like all such 

 membranes, it consists of & parietal and a visceral division, these being 

 portions of one great sac. The parietal part is that which lines the 

 abdominal walls, or parietes ; the visceral part invests the solid and 

 hollow organs, or viscera, of the abdominal cavity. In virtue of this 

 membrane, all the free surfaces that present themselves when the 

 abdominal wall is removed have a smooth and shining appearance. 

 The surface of the membrane is covered by a layer of endothelial cells, 

 and these rest upon a layer of vascular connective-tissue. The object 

 of the membrane is to facilitate the movements of the different ab- 

 dominal organs on each other and on the walls of the cavity, and 

 especially to facilitate the vermicular or peristaltic movements of the 

 intestines. For this purpose the surface of the membrane is kept moist 

 by a sparing amount of serous fluid, which gives to the membrane its 

 glistening aspect. 



To trace the exact disposition of the peritoneum in the horse is very 

 difficult, in consequence of the unwieldy character of the intestines. 

 When the student has the opportunity he should examine the mem- 

 brane in a foal, in which the different organs can be manipulated with 

 ease. 



The parietal and visceral peritoneum, as has already been stated, 

 form portions of one great sac, and the various abdominal viscera are 

 external to this sac. The sac of the peritoneum, it must be observed, 

 encloses not an actual, but merely a potential, cavity ; the inner surface 

 of every portion of the sac being in contact with the same surface of 

 another portion. To facilitate the understanding of this, let the student 

 imagine the cavity of the abdomen (including the pelvis) as having its 

 natural form, but deprived of all its contents, and completely lined by 

 peritoneum, which, for simplicity's sake, he may suppose to be elastic. 

 The continuity of the membrane, and the fact that it formed a closed sac 

 would then be apparent. Now let him imagine a simple tube of intestine 



