378 FISHES 



CHAP. 



nervous dorsally and medianly, the roof and inner wall of each 

 being formed by an extension of the thick, glandular choroid 

 plexus which forms the roof of the thalamencephalon. 



The Spinal Nerves. The spinal nerves of Cyclostomes (e.g. 

 Petromyzon) consist of a series of dorsal nerves arising on each 

 side from the dorsal surface of the spinal cord, and of a similar 

 double series arising from the ventral surface, the dorsal nerves 

 regularly alternating with the ventral nerves. Each myotome is 

 supplied by a dorsal and a ventral nerve which pass separately to 

 their peripheral distribution in the skin and muscles. In Fishes, 

 as in the higher Vertebrates, each dorsal nerve, now termed a 

 dorsal root, enlarges into a ganglion and then unites, either before 

 or directly after issuing from the neural canal, with the next 

 ventral nerve or ventral root in front to form a main spinal 

 nerve. At the same time the spinal nerves of opposite sides 

 tend to form pairs in the same transverse plane. After the 

 union of the two roots the spinal nerve divides into three typical 

 branches : a dorsal nerve (ramus dorsalis), and a ventral nerve 

 (ramus ventralis), both of which include somatic sensory or 

 afferent fibres, and somatic motor or efferent fibres, for the 

 innervation of the skin and muscles of the dorsal and lateral 

 portions of a myotome ; and a visceral branch (ramus visceralis), 

 composed of afferent and efferent visceral fibres, which supplies 

 the adjacent viscera (alimentary canal and its glands and blood- 

 vessels), and helps to form the sympathetic nervous system. 1 The 

 somatic afferent and the visceral afferent fibres enter the spinal 

 cord by the dorsal roots, the somatic efferent leaving the cord 

 through the ventral roots, although the visceral efferent fibres 

 traverse both roots. In the region of the paired fins more or fewer 

 of the rami ventrales unite to form a plexus, the brachial or the 

 pelvic plexus, from which the nerves to the fins take their 

 origin. 



The Cranial Nerves. It is usual to describe the cranial 

 nerves of Cyclostomes and Fishes as consisting of ten serially 

 disposed pairs, viz. : the olfactory (i.), optic (ii.), oculomotor (iii.), 

 trochlear (iv.), trigeminal (v.), abducens (vi.), facial (vii.), auditory 

 (viii.), glossopharyngeal (ix.), and the vagus (x.) Like the spinal 

 nerves, the cranial nerves collectively include somatic sensory 

 (general cutaneous) and motor fibres, and also visceral sensory 



1 See Gaskell's important paper, Journ. Physiol. vii. 1886, p. 1. 



