232 



PHYSIOLOGY 



CHAP. 



or alimentary pulp, in order to mix it with the digestive secretions, 

 to facilitate the absorption of the products of digestion, and finally 

 to expel the undigested and unabsorbed residues of food, along 

 with the excreta that collect in the gut. 



The several parts of the intestine, morphologically speaking, 

 show a gradual differentiation, from which a correlative differentia- 

 tion of function may be argued. The duodenum is distinguished 

 not only by the absence of a mesentery, and by being only partially 

 covered with peritoneum, but also by presenting the widest and 

 most muscular part of the small intestine. Its total length varies 

 between 25 and 30 cm., its breadth from approximately 35 to 50 mm. 



FIG. 75. Auerbach's plexus between the two muscular coats of the intestine. Gold chloride 



method. (Cadiat.) 



The remainder of the small intestine, which has a mesentery, is 

 arbitrarily divided into jejunum (upper -) and ileum (lower f ), 

 which have no distinct morphological boundaries. But the portion 

 between the commencement of the jejunum and the end of the 

 ileum (where the ileo-caecal valve is situated) gradually alters in 

 structure and appearance, so that the two ends of the segment 

 can be readily distinguished. The jejunum is larger and more 

 muscular ; the ileum is narrower with thinner and paler coats, the 

 valvulae conniventes are smaller and gradually disappear at the 

 lower end, the villi are shorter, the groups of Peyer's patches 

 larger and more numerous. Owing to all these differential 

 characters, any part of the jejunum weighs more than a corre- 

 sponding portion of the ileum. The diameter of the jejunum is 

 about 3*25 cm., that of the ileum 2'60 cm. They vary consider- 

 ably in length, with age and with different individuals, as we have 



