368 



PHYSIOLOGY 



CHAP. 



and to favour absorption of the soluble substances and concentra- 

 tion of the faeces. Another peculiar feature of the large intestine 

 is the so-called appendices epiploicae, which, however, are of little 

 physiological importance. 



In the rectum the longitudinal muscle fibres are reduced to 

 two bundles, one anterior, the other posterior. They do not give 

 rise to any formation of ridges and sacculi. In the rectal ampulla 

 the mucous coat alone exhibits longitudinal folds, which are 



FIG. 103. (Left.) Outline sketch of a section of the ascending colon. (Allen Thomson.) s, 

 serous covering ; s 1 , s', reflection of this at attached border forming a short wide mesocolon 

 between the folds of which the blood-vessels are seen passing to the colon ; a, one of the 

 appendices epiploicae hanging from the inner border ; lm, indicates at free border one of the 

 three bands formed by the thickening of the longitudinal muscular coat ; the dotted line 

 continued from the margins of these bands represents the remainder of the longitudinal 

 muscular coat, and the thick line within it, marked cm, represents the circular muscular 

 layer ; m, the mucous membrane at the flattened part ; r, the crescentic bands of indentations 

 which divide the sacculi. 



Fio. 104. (Right.) Diagram of last part of rectum, with the sphincters and muscles of the anal 

 region. (Testut) R, rectum ; f.c, circular coat ; f.l, longitudinal coat ; Si, internal sphincter 

 of plain muscle ; S.e, external sphincter of striated muscle ; m.e, levator ani ; m.o.i, obturatoriu 

 internus ; /, os ischiadicum. 



obliterated when it is distended by the faecal masses. At the 

 level of the anal canal, the longitudinal fibres completely surround 

 the rectum, and the circular fibres thicken and form the so-called 

 internal sphincter. At the end of this layer of plain muscle a 

 firm ring of striated muscle forms the external sphincter. Beyond 

 this last is another striated muscle, the levator ani, which forr 

 a sort of diaphragm, the concavity being turned upward anc 

 forward (Fig. 104). 



With these anatomical premises, it is easy to understand the 

 mechanism of defaecation, which, as distinguished from that 

 which takes place through the ileocaecal valve, may be termed 



