( EN Til 1 1 /, NE RVOl r S 8 I r S TEM. 



•_'< i.j 



by His into the ventral and dorsal laminae during development. 'Flic ventral 

 horns of the gray substance form pari of the ventral laminae. 



The cells of the sympathetic system which are interpolated in one portion 

 of the pathway formed by the efferent elements lie, of course, entirely out- 

 side of the central system. (See Fig. 87.) The central neurone- (2) occupy 

 all parts of the central system, and hence where the bodies or branches of the 

 first two groups are absent, the system is composed of central neurone- only. 



Arrangement of the Cells Forming- the Several Groups. — All three 

 groups of elements are grossly arranged so as to be bilaterally symmetrical 

 with reference to the dorso-ventral median plane of the body. There are 

 some minor exceptions to this general statement, but these are not known to 

 have any physiological significance. 



Fig. 89.— Schema showing the encephalon and cord; the unshaded portion is that derived from the 

 dorsal lamina; the shaded that from the ventral (from Mi not) : C, cerebrum ; Cb, cerebellum ; F, foramen 

 of Monro; /, infundibulum ; .1/, bull.; 0, olfactory lube; P, pons; Q, quadrigemina ; Sp.c, spinal cord; 

 ///, third ventricle; IV, fourth ventricle. 



The main axones of the afferent elements are distributed almost entirely 



to the dorsal columns, and to the gray matter of that side of the cord on 

 which they enter, though some crossing occurs in the dorsal commissure. In 

 the cord, the efferent elements have their cell-bodies mainly on the side ol 

 the cord from which the efferent fibres emerge. In the case of some cranial 

 efferent nerves the arrangement is different. There is found, for instance, a 

 partial decussation of the fibre- of the oculo-motorius ; complete decussation 

 in the case of the patheticus, and no decussation in the case of the abducens. 

 It is the central cells which furnish almost all the axones forming the com- 

 missures, the decussating bundles ami the projection systems, while the 

 association tracts arise entirely from them. 



Segmentation. — The grouping of the cell-bodies of the afferent fibres i- 

 originally segmental, one spinal ganglion corresponding to each segment of 

 the trunk. Tn the brain, the original segmental arrangement has been 

 greatly modified. In the trunk, too, the distribution of the distal portion of 



