44 THE INVOL UNTAR Y NER VO US S YSTEM 



with the central nervous system by way of the vagus nerve, while 

 the corresponding cells and nerves for the cloacal region are con- 

 nected with the central nervous system by way of the pelvic 

 nerve (Fig. 5). The small intestine is usually described as 

 separated from the large by a valve, the ileo-ccecal valve; but, 

 as Elliott has shown, there is in reality no valve here but a 

 strong sphincter muscle, the ileo-colic sphincter, which terminates 

 the intestine proper. 



The hind gut originally formed a tube composed of three 

 chambers, to which Gadow gave the names Coprodceum, Uro- 

 dceum, and Proctodceum. The coprodceum was separated from 

 the urodceum by a sphincter, and the urodceum from the proc- 

 todceum by another sphincter ; thus relaxation of the urinary 

 sphincter allowed the urine collected in the urodceum alone to 

 pass out, while relaxation of the internal sphincter of the rectum 

 as well as of the urinary sphincter allowed the faeces to pass out 

 (Fig. 7). The coprodceum has in the higher vertebrates separated 

 entirely from the urodceum and opens directly to the exterior, still 

 retaining its sphincter muscle, the internal sphincter of the anus. 

 The urodceum or bladder opens to the exterior by way of the 

 urethra and still retains its original sphincter muscle in the shape 

 of the urethral muscles and the unstriped sphincter muscle. 



Thus there are the following sphincter muscles which termin- 

 ate the different regions of the gut : 



1. The ileo-colic sphincter at the end of the small intestine. 



2. The internal anal sphincter at the end of the coprodceum. 



3. The internal vesical sphincter and the urethral muscles at 

 the end of the urodceum. 



The motor nerve supply to all these muscles is arranged on 

 a uniform plan (Fig. 7). Thus Elliott has shown that the 

 ileo-colic sphincter contracts upon stimulation of the superior 

 splanchnic nerves, and that its motor nerve cells are situated in 

 the superior mesenteric ganglion. Langley and Anderson have 

 shown that the internal anal sphincter contracts on stimulation of 

 the inferior splanchnics and its motor nerve cells are situated in 

 the inferior mesenteric ganglion. Finally Elliott has shown that 

 the internal vesical sphincter and the muscles of the urethra con- 

 tract on stimulation of the inferior splanchnics, and their motor 

 nerve cells are situated in the inferior mesenteric ganglion. 



The motor fibres of these sphincter muscles thus arise from 



