1906.] NERVES OF CHLAMYDOSELACHUS ANGUIJ^EUS. 985 



went on progressively the bi-ancliial and pharyngeal rami became 

 united by the common branchio-infcestinal trunk which enters the 

 brain by way of the Vagus root " (14. p. 229). '' The motor roots 

 must have been collected under the influence of the sensory com- 

 ponents As the motor fibres grow out from their nuclei 



in the brain tliey must follow some path of low resistance in 

 travelling to their muscles. Since the motor fibres develop late, 

 they find such a path already provided in the neai--by sensory 

 root. The motor fibres follow this and a mixed trunk is formed. 

 When the sensory fibres of a given root shift their course .... 

 to the root next cephalad, the motor fibres on issuing from the 

 brain find no path in that segment, but must turn forwards to 

 the next cephalic sensory root and follow it. As this goes on 

 gradually from segment to segment there are formed a number 

 of roots emerging from the coi-d or brain caudad to the complex 

 and running alongside the brain to join it." 



The Lateralis root of the Vagus originates, as is to be expected, 

 from a segment anterior to the vagal roots proper. It has an 

 extra-cerebral course backwards for a considerable distance before 

 it joins the branchio-intestinal Vagus. This indicates a compara- 

 tively primitive condition, for, as specialisation proceeds, the root 

 would run intra- cerebi-ally until nearer the origin of the Vagus 

 proper. The loose union of the constituent nerve-strands of the 

 composite Vagus, added to the presence of at least two separate 

 branchial ganglia, show also that the condition, although not 

 primitive, is not highly specialised. 



H. The Spinal Nerves. 



Following the Vagus there are four of the so-called spino- 

 occipital nerves, which pass out of the cranium by four separate 

 foramina. Two of these roots in Ghlamydoselachus^ are placed 

 completely undei-, the third partly under, the covei- of the vagal 

 roots. This origin is not to be explained, Johnston thinks, as due 

 to " a shifting through the long branchial region," and a conse- 

 quent crowding in the vagal region. He says (14. p. 231) : " The 

 dorsal and ventral hypoglossal roots need not be considered as 

 spinalartige nerves. They probably ai'e not equivalent to spinal 

 nerves at all, but are only the general cutaneous and somatic 

 motor components of nerves of the vagus region, the visceral and 

 motor components of which have been collected into the single 

 large vagus root. The presence of these nerves in the vagus 

 region, then, does not require the hypothesis that they have 

 shifted forward from the postbranchial region, but is directly 

 opposed to such an hypothesis." If this view, founded upon the 

 study of nerve components, be true, it will to some extent be in 

 opposition to the conclusions of Fiirbi-inger on the metamerism of 

 the head, for his argument is entirely dependent upon the spinal 

 character of such incomplete nerves as the above. 



The third and fourth spino- occipital nerves have each a dorsal 



