402 THE DIGESTIVE APPARATUS IN MAMMALIA. 



possess all the characters of the valvulse conniventes (valves of Kerkring) of 

 Man ; they resist the effects of traction exercised on the intestinal membranes, 

 and are formed by two mucous layers laid together, with a plentiful supply 

 of connective tissue between them. 



The internal surface of the small intestine also offers for study a mul- 

 titude of villosities and glandular orifices, or follicles, which will be noticed 

 hereafter. It communicates with the inner surface of the stomach by the 

 pyloric orifice, and with that of the caecum by means of an opening which 

 projects into the interior of that reservoir, like a tap into a barrel. This 

 projection, which is not very marked, is formed by a circular mucous fold, 

 strengthened externally by muscular fibres, and is named the ilio-ccecal 

 valve or valvula Bauhini. Two additional orifices open on the surface of the 

 small intestine in its duodenal portion, from 5 to 8 inches from the pylorus : 

 one is the orifice common to the biliary and principal pancreatic duct, the 

 other that of the accessory pancreatic duct. 



STRUCTURE. The wall of the small intestine, like that of the other 

 hollow viscera in the abdominal cavity, is composed of three tunics : 



1. Serous membrane. -This envelops the organ everywhere, except at its 

 small curvature, which receives the insertion of the mesentery. 



2. Muscular coat. Covered internally by a layer of condensed connective 

 tissue (which is sometimes designated as a fourth tunic) this middle mem- 

 brane has two planes of fibres : one, superficial, is formed of longitudinal 

 fibres uniformly spread over the whole surface of the viscus ; the other, 

 deep-seated, is composed of circular fibres, which are a continuation of those 

 of the pyloric ring. 



3. Mucous membrane. This tunic, extremely interesting to study, is soft, 

 spongy, highly vascular, very delicate, and of a reddish-yellow colour. Its 

 external face is loosely adherent to the muscular layer, and its free 

 aspect exhibits the villi, and the glandular or follicular orifices already 

 noticed. 



It comprises, in its structure, an epithelial covering, and a mucous 

 derm or corium. 



The epithelial layer is formed by a single row of cylindrical (or columnar) 

 cells, implanted, by their summit, on the surface of the derm, and lining the 

 interior of the orifices which open on the inner face of the membrane. The 

 base of these cells has an amorphous cushion, which, when they are all 

 united, appears like a thin layer spread on the inner surface of the 

 intestine. 



The mucous derm is thick and loose in its deeper portion, and is con- 

 stituted by fasciculi of connective tissue mixed with elastic fibres, and 

 lymphoid elements. On its free surface it exhibits villosities and depressions, 

 which correspond to the glands. It has a muscular layer, whose unstriped 

 fibres are arranged in a similar manner to those of the muscular coat of the 

 intestine. Lastly, it contains follicles, and vascular and nervous reticulations. 

 We will study each of these. 



The villi are the foliated or conical appendages which are found to be 

 most developed in the shortest portions of the intestine. In Birds and the 

 Carnivora they attain their maximum length ; while in Ruminants they 

 are in a rudimentary state, though, whatever may be their dimensions, 

 they are always visible to the naked eye. Their number is considerable, 

 and they have been justly compared to the pile of velvet. In structure, they 

 are formed of a small spongy mass of embryonic connective tissue, in the 

 centre of which are one or more lymphatic (or lacteal) vessels, with a 



