The Nervous 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. 



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Fig. 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 

 rootsof spinal nerve; re, ramus communicans (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 



