308 THE ANATOMY OF THE HORSE. 



his memory regarding their form and connections with one another, he 

 must proceed to examine their structure. This should be done by taking 

 a short piece of the gut, slitting it up, and pinning it with its mucous 

 surface downwards on a block of wood. 



Structure op the Small Intestine. The wall of the bowel is made 

 up of four layers, viz., serous, muscular, submucous, and mucous. 



1. The Serous Layer, the most external, is a part of the visceral 

 peritoneum. It reaches the bowel by the mesentery, whose two layers 

 separate at the concave border of the intestine, and pass round each side 

 to meet and become continuous on its convex or free border. It is 

 closel^ adherent to the subjacent muscular layer, which it completely 

 covers except at the line of separation of the two layers of the mesentery, 

 where the vessels enter. It must be stripped off to expose the next coat. 



2. The Muscular Goat is made up of two distinct sets of fibres: 

 1. Longitudinal fibres, which are most external, and form a thin layer 

 uniformly spread along the wall. 2. Circular fibres, thicker than the 

 preceding, and also spread over every part of the wall. These fibres are 

 of the pale, non-striated variety. 



3. The Submucous Coat is composed of loose areolar tissue uniting 

 the muscular and mucous layers. In the duodenum it contains the 

 glands of Brunner, which have the racemose type of structure, and are 

 about the size of a hemp seed. Their ducts pass through the mucous 

 membrane, and open on its free surface. 



Directions. — A few feet of the jejunum and about the same length of 

 the ileum should be taken and slit up along the line of attachment of 

 the mesentery. After the pieces have been gently washed, they should 

 be spread on a flat surface with the peritoneal coat downwards. 



4. The Mucous Membrane forms an inner lining to the intestine. It is 

 a soft, velvety -looking membrane which, when healthy and fresh, has a 

 pinkish-yellow colour. When a piece of intestine is floated in water, the 

 mucous membrane is seen to be studded with short, thread-like projec- 

 tions, to which the velvety appearance of the membrane is due. These 

 are the intestinal villi. Each of them may be regarded as an upheaval 

 of the mucous membrane, containing in its interior microscopic blood 

 and lymph vessels, some non-striped muscular fibres, and a framework 

 of lymphoid tissue. The villi are important agents in the absorption 

 of nutrient particles from the contents of the bowel. They are found 

 throughout the whole of the small intestine, but are more numerous in 

 the jejunum than in the ileum. The free surface of the mucous mem- 

 brane, including the villi, is formed by a single layer of columnar epi- 

 thelium with goblet cells interspersed. Contained within the substance 

 of the membrane are numerous microscopic tubular glands- — the glands 

 fo Lieberkuhn — whose mouths open on the free surface. The solitary 



