GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 



23 



the vertebrate series because of the constancy of the sensory area 

 supplied by it, the skin of the anterior part of the head. 



Two ventral nerves are connected with the myelencephalon, 

 the so-called hypoglossiis and the abducens. The hypoglossus 

 or twelfth cranial nerve arises by a variable number of roots in 

 cephalo- caudal succession in about the region of junction of brain 

 and spinal cord. These roots unite into a common trunk or 

 plexus which supplies the muscles of the tongue. In lower ver- 

 tebrates it is evident that the roots are the equivalent of several 

 ventral segmental nerv^es and form a simple continuation forward 

 of the series of ventral nerves of the trunk. The number of such 

 nerves present is greater in the more primitive forms. While 

 in higher vertebrates an interval representing several segments 



V I VII 

 Vill N hypoglossus N. spin. ven'. 

 Saccus vasculosus 



Fig. 7. — A. sketch of the brain of Chimaera monsirosa from the left side to show 

 especially the position of the nerve roots. The roots of the nerves were blackened 

 by osmic acid and in this way one or more roots of the abducens and hypoglossus, 

 not to be seen in ordinary dissections, were brought to view. The nerve roots are 

 indicated by the usual Roman numerals. The hypoglossal and spinal roots are 

 numbered w, x, y, z, a, b, c, 4, 5, 6, after Fiirbringer's scheme. 



intervenes between the roots of the hypoglossus and abducens, 

 in such forms as Chimaera (Fig. 7), Heptanchus and related forms 

 (Fig. 2), and in Petromyzon dorsatus (Fig. 51) only one or two 

 ventral nerves are wanting in the series between the abducens 

 and the ventral spinal nerves. In Bdellostoma the series of 

 ventral nerves is quite complete except the eye-muscle nerves, 

 which are wanting. The abducens or sixth cranial nerve also 

 arises by several rootlets which may extend from the level of the 

 VII nerve root back nearly to the level of the IX nerve. The 



