858 



STRUCTUEE OF THE SMALL INTESTINE. 



comparing the upper and lower ends of the entire tube, which will 

 be pointed out after it has been described as a whole. 



Fig. 252. 



STRUCTURE OF THE 

 INTESTINE. 



SMALL 



Fig. 252. — Diagram of the Abdominal 



PART OF THE ALIMENTARY CaNAL (Brintonj. 



C, the cardiac, and p, the pyloric end of 

 the stomach ; d, the duodenum ; j, i, con- 

 vohitions of the small intestine ; cc, 

 ciECum, with the vermiform j^rocess ; AC, 

 ascending, tc, transverse, and DC, de- 

 scending colon ; SF, sigmoid flexure ; R, 

 rectum ; A, anus. 



The small intestine, like the 

 stomach, is composed of four 

 coats, viz., the serous or peri- 

 toneal, muscular, areolar, and 

 mucous. 



The external or serous coat 

 almost entirely surrounds the in- 

 testinal tube in the whole extent 

 of the jejunum and ileum, leav- 

 ing only a narrow interval be- 

 hind, where it passes off and be- 

 comes continuous with the two 

 layers of the mesentery. The 

 line at which this takes place is 

 named the aftached or mesenteric 

 border of the intestine. The 

 duodenum, however, is but 

 partially covered by the peri- 

 toneum. 



The muscular coat consists of 

 two layers of fibres ; an outer 

 longitudinal, and an inner or 

 circular set. The longitudinal 

 fibres constitute an entire but 

 comparatively thin layer, and are 

 most obvious along the free bor- 

 der of the intestine. The cir- 

 cular layer is thicker and more 

 distinct and its fibres are placed 

 more closely together, clefts how- 

 ever being left here and there 

 between the bundles. 



The muscular tunic becomes 

 gradually thinner towards the 

 lower part of the small intestine. 



It is pale in colour, and is com- 

 posed of plain muscular tissue, the cells of which are of considerable 

 length. The progressive contraction of these fibres, commencing in 

 any part of the intestine, and advancing in a downward direction, 

 produces the peculiar vermicular or peristaltic movement by which 

 the contents are forced onwards through the canal. In this move- 

 ment the circular fibres are mainly concerned ; but the longitudinal 

 fibres also aid in it ; and those found along the free border of the 

 intestine may have the effect of straightening or unfolding, as it were, 

 its successive convolutions. 



_ The areolar or submucous coat of the small intestine is a tolerably 

 distinct and whitish layer, of a loose texture, which is connected more 

 firmly with the mucous than with the muscular coat, between which two 



