1 5 8 THE IN VOL UNTAR Y NER VO US S YSTEM 



and so account for the innervation of the endodermal muscles of 

 the whole of the small intestine by nerve cells connected with 

 the vagus nerve. 



The innervation of the endodermal muscles of the cloacal 

 region of the gut is so similar to that of the endodermal muscles 

 of the rest of the gut, with the substitution of the connector fibres 

 of the pelvic nerve for those of the vagus nerve, that I am 

 strongly inclined to attribute the formation of the cloacal region 

 to the same kind of agency as has been already argued for the 

 front part of the gut. The difficulty is that there is no evidence 

 here of any lateral root system like that of the bulbar nerves, or 

 of a double segmentation as is implied by the presence of a 

 lateral root ; but if, as is most probable, the muscles of the ap- 

 pendages in this sacral region were all unstriped, then, as already 

 argued for the vagus region, the lateral roots to such appendages 

 would have arisen originally from nerve cells in the sacral region 

 of the cord, and when those nerve cells become peripheral, there 

 would be left only their connector nerves remaining the pelvic 

 nerve to indicate their original position. On this most prob- 

 able supposition, that in this region the appendages of the an- 

 cestral animal possessed only unstriped muscles, the formation 

 of the cloacal portion of the alimentary canal is in complete 

 harmony with that of the rest of it. 



It is not so easy to imagine the genesis of the ectodermal 

 musculature which belongs to the somatic segmentation ; it can 

 hardly have arisen from the longitudinal body muscles of the 

 annelid, for although unstriped they resemble the body muscles 

 of Ammoccetes in general structure so closely as to make it most 

 improbable that they have given origin to muscles so different 

 in character as the dermal musculature : indeed it seems to me 

 that the longitudinal body muscles have remained practically 

 unchanged through the annelids, arthropods and vertebrates. 

 There are left the circular muscles and the dorso-ventral somatic 

 muscles ; it is possible to conceive their origin from the circular 

 muscle group, but there is no evidence that adrenaline will cause 

 contraction of any member of this group, and the characteristic 

 of the dermal musculature, which they share with the vascular 

 musculature, is their response by contraction to adrenaline or to 

 stimulation of motor nerves belonging to the sympathetic nervous 

 system. This fact suggests that we must look for the origin of 



