984 MUS. O. A. MEllUITT HAWKKS ON THE [Dec. 11, 



(h) The remaining riband of the Vagus is the ramus intestincdis. 

 After passing backwards for about 1"5 cm. it divides into three 

 main branches (A, B, C), which are distributed to the heart 

 and viscera. The most dorsal of the branches (0) soon divides 

 into two, which, after a sinuous course around the first spinal 

 nerves, pass on to the stomach. Of these two nerves the ventral 

 again divides into two, one passing caudad on to the stomach, 

 the other foi-ward to the heart. The intestinal nerves soon break 

 up into a fine plexus, immediately under the peritoneum. This 

 plexus can only be traced a short distance. 



At the present time there is a general tendency to regai-d the 

 Yagus as a composite nerve, the component parts being a nerve 

 or part of a nei-ve for each branchial arch and cleft and one or 

 more nerves or parts of nerves to the intestines. Therefore in a 

 primitive Vagus there would be found a series of ganglionated 

 nerves, each equivalent to the Glossopharyngeus, plus the ramus 

 intestinalis, wlaich is probably " the collector of all branchial 

 nei'ves which may at one time have existed behind the present 

 gill-region" (14. p. 228). The dorsal rami of the collected nerves 

 probably disappeared eai'ly, owing to the reduction of the dorsal 

 area. The primitive Vagus of this series of primitive branchial 

 and intestinal nerves perhaps alone retained its dorsal ramus to 

 supply the reduced cutaneous areas of its own and the immediately 

 following segments. Tlie most primitive Vagus yet found is in 

 Chimcera. In Ghlmnydoselachus also one would have expected 

 to find the Vagus not one nerve but a series of nerves. This 

 expectation is, however, by no means fulfilled, as appears by the 

 foregoing description. The condition here is intermediate between 

 that of Chimcera and ScyUium. In Chlamydoselachus there is a 

 series of roots which have the appearance of being somewhat 

 pulled forward in order to emerge somewhat anteriorly to their 

 region of origin. This direction in which the roots pass, no doubt 

 indicates a comparatively recent union of these roots into one 

 nerve. In higher forms we find the vagal complex passing back- 

 wards out of the cranium, whilst the comparatively new spinal 

 accessory roots are first pulled foi-ward befoi-e they can pass back- 

 wards and out of the cranium. How the condition in Chlamydo- 

 selachus may have arisen from a simple GhlmceraAike condition is 

 best expressed in Johnston's words (14. p. 226) : — " It appears that 

 the visceral sensory and motor fibres of the caudal branchial 

 segments, instead of continuing to reach the brain by way of the 

 dorsal roots pi-oper to their segments, have pi'Ogressively changed 

 their course so as to run to the brain through the root of the next 

 more cephalic segment. This process may have begun as a 

 result of the expansion of the gill-sacs and growth of bianchial 

 arch muscles which served to crowd the ventral rami of the nerves 

 at the caudad end of the gill-region. When the shifting had 

 brought the penultimate gill back to the level of the ultimate 

 nerve root, the fibres of the ultimate ventral rami shifted their 

 course to the root of the penultimate nerve As the process 



