STRUCTURE OF THE SMALL IXTESTIXE. 841 



ner and paler ; the valvulse conniventes are small, and gradually dis- 

 appear towards its lower end ; the villi are shorter ; and the groups of 

 Peyer's glands are larger and more numerous. The diameter of the je- 

 junum is about one inch and a half, that of the ileum about one inch 

 and a quarter. A given length of the jejunum weighs more than the 

 same of the ileum. 



At a point in the lower part of the ileum it is not very uncommon to find a cul-de- 

 sac or diver ticulum given off from the main tube. The origin of these diverticula is 

 explained by reference to the history of development, from which it appears that they 

 arise in connection with the ductus vitello-intestinalis, uniting the intestine with 

 the umbilical vesicle. They are not to be confounded with hernial protrusions of 

 the mucous membrane, which may occur at any point. (See Meckel's Handbook of 

 Anatomy, French edition, vol. ii. p. 431.) 



Structure of the Small Intestine. 



Structure. The walls of the small intestine, like those of the stomach, 

 are composed of four coats, viz., the serous, muscular, areolar, and mucous. 



The external or serous coat, derived from the peritoneum, almost en- 

 tirely surrounds the intestinal tube in the whole extent of the jejunum 

 and ileum, leaving only a narrow interval along one border of the intes- 

 tine, where it is reflected from it and becomes continuous with the two 

 layers of the peritoneal duplicature named the mesentery. The line at 

 which this reflexion takes place is named the attached or mesenteric border 

 of the intestine. The opposite border and sides of the tube, which are 

 covered by the peritoneum, are quite free and movable upon the adjacent 

 parts. The upper part, however, of the small intestine, named the duode- 

 num, is but partially covered by the peritoneum, as has been already more 

 particularly described. 



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 border 

 of the intestine. The circular layer is much thicker and more distinct ; its 

 fibres are placed closely together, and run in a circular direction around the 

 bowel, but it does not appear that they individually form perfect rings. 



This muscular tunic becomes gradually thinner towards the lower part 

 of the small intestine. It is pale in colour, and is composed of plain mus- 

 cular fibres. 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 digestive mass 

 is forced onwards through the canal. In this movement 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 straighten- 

 ing 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 it is placed. 

 By turning a portion of the intestine inside out, and then blowing forcibly 

 into the cavity, the areolar tunic may be inflated, the air being driven into 

 its areolar tissue, through the part at which the peritoneal investment is 

 wanting. It supports the mucous membrane, and forms a layer of loose 

 substance in which the vessels divide and subdivide into smaller branches, 

 preparatory to entering the mucous tissue. It consists of filamentous areolar 

 tissue, mixed with fine elastic fibres. 



