THE INTESTINES. 473 



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 multitude 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 taj) into a barrel. This projection, whicli is not very 

 marked, is formed by a circular mucous fold, strengthened externally by masculur 

 fibres, and is named the ilio-cmcal valve or valvula Bauliini. 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 membram. — This envelops the organ everywhere, except at its 

 small curvature, which receives the insertion of the mesentery ; it adheres 

 closely to its surfaces and the great curvature. 



2. Muscular membrane. — Covered internally by a layer of condensed connec- 

 tive tissue (which is sometimes considered a fourth tunic), this middle membrane 

 has two planes of fibres — one, superficial, is formed of longitudinal fibres 

 uniformly spread over the whole surface of the viscus ; the other, deep, 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 membrane, and its free face 

 exhibits the villi, and the glandular or follicular orifices already noticed. 



It comprises, in its structure, an epitheKal lining, and a mucous dermis or 

 coriiun. 



The epithelial layer is formed of a single row of spheroidal cells, implanted, 

 by their summit, which is often ramifying, in the surface of the dermis, 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. 

 Their protoplasm is frequently fenestrated or reticulated (Renaut) ; and among 

 the ordinary spheroidal cells, some calyciform cells are met with here and there. 



The miu'ous dermis is thick and loose in its deeper portion, and is constituted 

 by fasciculi of connective tissue, mixed with elastic fibres, and lymphoid elements. 

 On its free surface it exhibits projections {villosities) and depressions (follicles), 

 which correspond to the (/lands. It has a muscular layer, the unstriped fibres of 

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

 intestine. Lastly, it contains follicles, and vascular and nervous networks. We 

 will study each of these. 



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

 most developed in the shortest portion 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 each formed by a small 

 mas3 of connective tissue, in the centre of which is a chyliferous vessel or several 



