iv DIGESTION IN THE INTESTINE 233 



seen in another connection probably in relation with the pre- 

 dominatingly vegetable or animal character of the usual diet. 



The structure of the walls of the small intestine differs in no 

 essential from that of the oesophagus and stomach. The muscular 

 coat is composed of plain muscle. The longitudinal fibres form a 

 comparatively thin layer which becomes denser along the free 

 border of the intestine, the transverse or circular fibres are thicker 

 and more distinct : the first in contracting can only dilate and 

 shorten the intestinal tube (Exner) ; the second, on the contrary, 

 constrict and lengthen it. The longitudinal cells which thicken 

 along the free border of the intestine must stretch and distend 

 the numerous folds or convolutions. 



FIG. 76. Meissner's plexus, from submucous layer of intestine. Gold chloride method. (Cadiat.) 

 a, a, ganglia ; b, b, cords of plexus ; c, small blood-vessel ; d, nerve filaments that accompany 

 the small artery. 



A ganglia ted plexus lies between the two muscular coats, and 

 is in relation with the caeliac plexus, and branches of the vagus 

 and great splanchnic, and is known as the plexus myentericus or 

 plexus of Auerbach. It is principally composed of non-medullated 

 fibres, which give off a number of fine branches to the longitudinal 

 and circular muscle-cells (Fig. 75). 



Other, larger branches pass through the circular bundles of fibres 

 to reach the submucous layer, where they form a second gangliated 

 plexus, the filaments of which are much finer than those of the 

 preceding ; this is called the plexus of Meissner (Fig. 76). From 

 this plexus, nerve fibres pass into the muscular layer of the 

 mucous membrane, breaking up into fibrils that ramify in the 

 proper tissue of the mucous coat and villi, and terminate, accord- 

 ing to Berkley, in small pear-shaped or globular dilatations. 



