ORIGIN OF THE CEREBRAL NERVES. 487 



of the spinal cord, enveloped by the ascending root of the 

 fifth nerve. 



The innermost clusters of these masses that are approximated 

 to the motor root of the fifth, present cell elements of a larger 

 calibre, having a length of 39 /z, and a thickness of 12 /n; so that 

 from the motor nucleus of the fifth, as far as to the external 

 clusters of the just-described masses, we have a repetition of 

 the three different-sized nerve corpuscles that are contained 

 respectively in the anterior cornu, in the root, and in the caput 

 of the posterior cornu of the spinal cord. 



2. Roots of the fifth, the nuclei of origin of which are 

 situated above the plane of emergence. Descending Roots. 

 The descending roots of the fifth are divisible into three 

 portions. 



The external descending root of the fifth (figs, 249, V; 250, 



251, 5; 252 and 253, 5 c) arises from the region of the nates, as 

 far down as to the point of emergence of the fifth, out of those 

 large vesicular cells, from which, on the other hand, the already 

 described (p. 452) interlacement of the tegmentum proceeds. 

 It descends outside the central grey matter around the aquae- 

 duct, and, continuously augmenting in size, is enclosed, while 

 in the pons, in that lateral region of the grey floor which 

 creeps along the internal surface of the superior peduncle. 

 Upon the inner border of its semilunar area, which Stilling 

 and Deiters considered to be an ascending root of the fourth, 

 grape-like clusters of corpuscles are situated, forming a con- 

 tinuous chain, the elements of which are distinguished from 

 those of the substantia ferruginea by their spheroidal form and 

 the absence of pigment (p. 447). 



The middle portion of the descending roots of the fifth pro- 

 ceeds from the cells of the substantia ferruginea. This elon- 

 gated cluster of pigmented fusiform (60 JJL long, 20 30 p thick) 

 nerve corpuscles, which in the Locus cseruleus glimmers through 

 the grey floor, commences in the lower half of the testes, and 

 extends downwards for more than a centimeter to just above 

 the emergence of the motor roots of the fifth, being always 

 situated internally to the outer descending roots (figs. 250, 251, 



252, F). Scattered fragments of this compact formation, how- 

 ever, extend into the lateral regions of the posterior division 



