682 ESSAY ON THE DESCRIPTIVE ANATOMY OF THE 
large intestine at the junction of the caecum and colon, by an 
orifice provided with a valve. 
The first portion of the ileum is simply attached by mesen- 
tery to the spine ; but in addition to this, in the last part of its 
course, the gut is connected with the caecum by a fold of peri- 
toneum, which is not large enough to prevent them deviating 
more than an acute angle from each other. 
The ileum is, on the whole, the narrowest portion of the small 
intestine, but the thickest in its coats. 
Having now specially to describe the structure of the small 
intestine, it may be taken as a whole, merely alluding to local 
peculiarities. 
This portion of the alimentary canal has four coats, to be 
described in the same order as those of the stomach, i. e. peri- 
toneal, muscular, cellular, and internal mucous. 
The first, or peritoneal, has nothing of peculiar, beyond its 
enclosing a little triangular space all along the upper attached 
border of the gut. The looseness of the peritoneal folds attach- 
ing the small intestine is very marked ; and Colin (Soc. cit.) 
notes, that the mesentery is proportionately larger in young than 
in adult quadrupeds, so that the gradual shortening of this 
explains the spontaneous reduction of exomphalus or umbilical 
hernia. 
The second, or muscular, coat is mostly developed at the com- 
mencement of the duodenum and terminating portion of the 
ileum. It consists of white involuntary fibres, arranged so as to 
form an outer longitudinal layer and an inner circular one, both 
of which completely encircle the gut. 
The third, or cellular, coat is similar to that of the stomach, 
in being disposed in two layers, so as to connect the three coats 
together. It is especially condensed on the inner surface of the 
muscular coat, so as to take the appearance of a fibrous tunic 
attached to the mucous lining by loose cellular tissue. 
The fourth, or mucous, coat is thin, having a velvety appear- 
ance, due to villi, peculiarly small in the intestines of the horse, 
but remarkably developed in other animals, especially carni- 
vora and fishes. The villi may be seen by a pocket lens, on 
a well-washed piece of intestinal mucous membrane, and 
between them are seen numerous foramina, which are the open- 
ings of tubular glands, known as the crypts of Lieberkuehn. 
In addition to the tubular glands, by dissecting, from without, 
the muscular from the mucous coat, lining the commencement 
of the duodenum, we find clusters of vesicles similar to the 
vesicular structure of the salivary and pancreatic glands. 
These form distinct layers provided with ducts which open 
