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MANUAL OF ELEMENTARY ZOOLOGY 



The ten pairs of spinal nerves pass out between the 

 vertebrae to be distributed over the body. 



Spinal Nerves. „ , , , .. . 1 „/.. 



Each nerve is surrounded as it issues by a soft, 

 white calcareous concretion. Every nerve arises by two roots, 

 a dorsal and a ventral, and the dorsal root bears a small 

 swelling, the dorsal root ganglion. Just outside the back- 

 bone the two roots join, and the nerve thus formed 

 proceeds at once to divide, giving rise to (a) a short dorsal 

 branch to the muscles and skin of the back, (b) a long 

 and conspicuous ventral branch to the muscles and skin of 

 the sides and ventral surface of the trunk, and in some 



cases to the limbs, 

 and (c) a small ramus 

 communicans to the 

 sympathetic system. 

 The dorsal root is 

 also called sensory 

 or afferent because 

 along it impulses 

 pass inwards to the 

 spinal cord and pro- 

 duce, among other 

 effects, sensation, 

 and the ventral is 

 called similarly 

 motor or efferent 

 because along it 

 impulses pass outward and produce, among other effects, 

 contraction of muscles. This is shown by the fact that 

 cutting the dorsal root deprives of sensation the parts sup- 

 plied by its nerve, while cutting the ventral root paralyses 

 the same parts. Each of the branches contains elements 

 derived from both dorsal and ventral roots. The course of 

 the dorsal branches and rami communicantes is much the 

 same in all cases, but that of the ventral branches differs 

 greatly in different nerves and must now be followed. 

 The first spinal nerve 1 is known as the hypoglossal. 



1 The nerve which is counted as the first spinal nerve in the frog is 

 in reality the second. The true first spinal nerve, which should issue 

 between the skull and the first vertebra, appears in the embryo, but is 

 lost later on. 



cm.. Centrum ; d.br., dorsal branch of the nerve ; 

 d.r., dorsal root; d.r.g., dorsal root ganglion; 

 n.a., neural arch; r.c, ramus communicans; 

 v.dr., ventral branch ; v.c, vertebral canal ; 

 z>.r., ventral root. 



