78 FISHES CHAP, 
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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.’ 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 (1.), optic (i1.), oculomotor (111.), 
trochlear (iv.), trigeminal (v.), abducens (vi.), facial (vil.), auditory 
(vul.), 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 
' See Gaskell’s important paper, Journ. Physiol. vii. 1886, p. 1. 
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