370 



PHYSIOLOGY 



CHAP. 



a while, probably because the faeces which have descended into 

 the rectum are partly pushed back into the colon by the action of 

 these muscles. But when the tension of the walls of the colon 

 and rectum exceeds certain limits, this voluntary mechanism not 

 only fails to remove the sensation of desire but even increases it, 

 by reflexly promoting the active movements of the lower part of 

 the rectum, which makes the need to defaecate more urgent, and 

 aids evacuation where it is sluggish. 



A spurious desire to defaecate may be aroused by pressure 

 exerted ab extrinseco on the walls of the rectum from a large 



calculus in the bladder, 

 a tumour of the prostate, 

 or the presence of the 

 head of the foetus in the 

 pelvis. The same effect 

 may be produced by in- 

 ternal haemorrhoids, or 

 an inflammation of the 

 rectal mucosa such as 

 is commonly associated 

 with dysentery. The 

 term tenesmus is em- 

 ployed in medicine for 

 this spurious desire to 

 evacuate. 



Three mechanical 

 factors normally co- 

 operate in the act of 



FIG. 105. Schema of dog's hypogastric plexus, with afferent j - / N 



nerves, and efferent nerves to rectum, anus, and bladder. CieiaecatlOn : (a) active 



(FranQOis-Franck.) G.m.i, inferior mesenteric ganglion; -np-piafolQie nf fViP Trmor>lp 



AT.c, nervous erigens from 1 and 2 sacral nerves (IS, SS) ; F C1 



N.pud.int, nervous pudendus internus ; P. i, hypogastric of the sioilioid Colon and 



plexus,- which with the nervous pudendus innervates the ^7 \ i -i , 



. rectum, anus, and bladder. rectum ; (O) inhibition OI 



the tone of the sphincters, 



aided by the contraction of the levator ani ; (c) active intervention 

 of abdominal compression, by contraction of the diaphragm and 

 forcible and prolonged contraction of the abdominal muscles with 

 closure of the glottis. The association, succession, and co-ordination 

 of these factors varies in different cases, according as there is a 

 tendency to diarrhoea or to constipation. 



It is certain that of the three mechanical factors which thus 

 take part in the act of defaecation, the first is the most essential. 

 In fact (as we shall see), under certain experimental or pathological 

 conditions the entire process of evacuation may take place as a 

 pure reflex, independent of any active intervention of the voluntary 

 muscles. 



VIII. The nerves of the large, like those of the small, 

 intestine come from the cerebrospinal and from the sympathetic 



