THE NERvousS SYSTEM. 67 
dura mater, forms a tough external investment, while another, 
the pia mater, is a more delicate, highly vascular membrane lying 
next the nervous matter, and amply supplied with blood-vessels tor 
the supply of nourishment. The nerves, however, are distributed 
freely throughout the body, and though not so adequately pro- 
tected are more capable of withstanding mechanical stress, and are 
commonly found in connective tissue situations where mechanical 
injury is not likely to occur. 

Fic. 36. Plan of the central and peripheral connections 
of a spinal nerve: an, afferent (sensory) neurone; asn, 
afferent sympathetic neurone; ca, cp, anterior (ventral) and 
posterior (dorsal) columns of grey matter; en, efferent 
(motor) neurone; esn, efferent sympathetic neurone; grp, 
dorsal root ganglion; i, intestine (visceral. organs); m, 
skeletal muscle; na, np, anterior (ventral) and posterior 
(dorsal) rami of spinal nerve; ra, rp, anterior and posterior 
roots of spinal nerve; rc, ramuscommunicans (sympathetic) ; 
sk, skin; sp, white matter of cord; ts, ganglion of sym- 
pathetic trunk. Slightly modified, from Herrick. 
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. 36). In the spinal 
cord the difference in appearance as between the white and the 
grey matter has already been described (p. 39). A spinal nerve 
arises by two roots, one of which is dorsal and bears a small 
ganglion of cellular material, the other ventral and without a 
