THE SPIXAL XERVE 73 



The larger perivascular spaces acquire a thin lining of tissue from 

 the arachnoid and pia mater and open into the subarachnoid space 

 so that the tissue fluid mingles with the cerebrospinal fluid there. 

 This fluid is mostly secreted by the chorioid plexuses (p. 83) and is 

 eventually filtered into the venous sinuses which carry the blood 

 from the brain. 



Composition of a Spinal Nerve 

 The most typical of the structural arrangements of the nervous 

 system may be made out from a study of the connections of any 

 one of the paired nerves of the spinal series (Fig. 39). In the 

 spinal cord the difference in appearance as between the white and 

 the grey matter has already been described (p. 32). A spinal 

 nerve arises by two roots, one of which is dorsal and bears a small 

 ganglion containing nerve-cell bodies, the other ventral and with- 

 out a ganglion. Impulses passing through the dorsal root are 

 centripetal or afferent in that they pass only in the direction of 

 the central nervous system and they are also in many cases sensory 

 in that their effects may be consciously experienced. The most 

 characteristic sensory impulses are those which come from the 

 skin. Many afferent impulses do not enter consciousness and a 

 majority of these come from deeper parts. In a similar fashion 

 the impulses of the ventral root are centrifugal or efferent, in that 

 they pass only in a direction away from the central nervous system, 

 and are in most cases motor in that their effects are commonly 

 observed as muscular contraction. The two roots, however, unite 

 immediately outside the spinal cord, and subsequently redivide 

 into a dorsal ramus, a ventral ramus, and either a ramus com- 

 municans or two rami communicantes. Each spinal nerve has a 

 grey ramus communicans and the thoracic, the first five lumbar 

 (in the rabbit), and the second to fourth sacral nerves (in the 

 rabbit) have also a white ramus communicans. The dorsal and 

 ventral rami are then distributed as somatic nerves to the body 

 wall, each of them containing fibres from both dorsal and ventral 

 roots and also fibres (for the blood-vessels) which have come from 

 the sympathetic ganglia (see below) through the grey communi- 

 cating rami. The white communicating ramus is a visceral nerve 

 containing fibres derived from both dorsal and ventral roots and 

 connecting through autonomic ganglia with the visceral organs. 



