DISSECTION OF THE ABDOMKN. 295 



regions are separated from the three posterior regions by another trans- 

 verse vertical plane ' passing through the external angle of the ilium 

 (angle of the haimch). Again, each of these three regions — anterior, 

 middle, and posterior — is further subdivided into a central and two 

 lateral regions, this subdivision being effected by two vertical and 

 parallel longitiidinal planes, each passing through the centre of Poupart's 

 ligament. 



Directions. — The intestines of the horse, owing to their unwieldy size, 

 and generally also to the weight of their contents, are extremely incon- 

 venient to dissect. From the following description and the accompany- 

 ing plates, the student should first learn how the intestinal tube is 

 divided. He should then, with as little disturbance of the different 

 intestines as possible, observe how they are disposed within the abdo- 

 minal cavity. 



The Intestines (Plates 41 and 42). The intestinal tube begins at the 

 pyloric orifice of the stomach, and it •terminates on the surface of the 

 body, at the anus. It is primarily divided into small and large intestines, 

 and each of these is naturally or arbitrarily divided into segments. 



The Small Intestine comprises the first portion of the tube, and in 

 a horse of medium size it measures about seventy-two feet in length. As 

 is expressed by its name, it is of smaller calibre than the large intestine. 

 Moreover, it is distinguished from nearly every part of the large intestine 

 by having a smooth and regular contour when distended. The first two 

 feet of the tube occupies a fixed position, and is termed the duodenum. 

 It received this name because in man its length is about equal to the 

 breadth of twelve fingers. The remainder of the small intestine has a 

 comparatively loose mode of suspension ; and it is arbitrarily divided 

 into jejunum and ileum, the former succeeding the duodenum, and 

 measuring about thirty feet, the latter comprising the remainder of the 

 tube — about forty feet. These terms are borrowed from human anatomy, 

 where the term jejunum was applied in consequence of that portion of 

 the intestine being generally found empty in the dead body, while the 

 ileum was so designated on account of its convoluted disposition. 



The Large Intestine is, for the most part, of vastly greater calibre than 

 the small ; and, unlike the latter, it has when distended, not a smooth, 

 but a bosselated, surface. In a medium-sized animal it is about twenty- 

 five feet in length. It is subdivided — and in a much more natural 

 fashion than the small intestine — into caicum, colon, and rectum, the 

 colon being further subdivided into double and single colon. 



When the muscles which enclose the abdomen below and on each side 

 have been removed, it most commonly happens that only the large 

 intestines are exposed, and consequently their examination must precede 

 that of the small intestines. 



The CjECUM is the first of the large intestines. In an animal of 



