THE NOTOCHORD AND ALIMENTARY CANAL 449 



with the supply of motor nerves to the alimentary canal ; they form 

 essentially an efferent gut-system in contradistinction to the sympa- 

 thetic or skin-system. 



A marked distinction exists between these cranial and sacral 

 nerves. The vagus never supplies the large intestine, the sacral 

 nerves never supply the small intestine. Associated with the large 

 intestine is the bladder, the whole system arising from the original 

 cloacal region ; the vagus never supplies the bladder, its motor 

 nerves belong to the sacral outflow. The motor nerves to the 

 ureters, to the urethra, and to the trigonal portion of the bladder 

 between the ureters and the urethra, do not arise from the sacral 

 outflow, but from the thoraeico-lumbar. These muscles belong really 

 to the muscles in connection with the Miillerian and "Wolffian ducts 

 and skin, not to the cloacal region. 



The motor innervation then of the alimentary caual reveals this 

 striking and suggestive state of affairs. The motor innervation of 



O CO 



the whole of the small intestine arises from the cranial region, and 

 is immediately followed by an innervation from the sacral region for 

 the whole of the muscles of the cloaca. It thus indicates a head- 

 region and a tail-region in close contiguity, the whole of the spinal 

 cord region between these two extremes being apparently unrepre- 

 sented. Xot, however, quite unrepresented, for Elliott has shown 

 recently that the ileo-colic valve at the junction of the small and 

 large intestine is in reality an ileo-colic sphincter muscle, and that 

 this muscle receives its motor nerves neither from the vagus nor 

 from the sacral nerves, but from the thoraeico-lumbar outflow or 

 sympathetic system. This may mean one of two things, either that 

 a band of fibres belonging to the skin-system has been added to the 

 gut-musculature, for the purpose of forming a sphincter at this spot, 

 or that the region between the vagus territory and the cloaca is repre- 

 sented by this small band of muscle. The second explanation seems 

 to me the more probable of the two. Between the mesosomatic 

 region represented by the vagus, and the cloacal region, there existed 

 a small metasomatic region, represented by the pronephros, with its 

 segmental duct, as already discussed in Chapter XII. That part of 

 the new alimentary canal which belonged to this region is the short 

 piece indicated by the ileo-colic sphincter, and innervated, therefore, 

 from the same region as the organs derived from the segmental duct. 

 Such innervation seems to me to suggest that originally the 



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