486 THE BEAIN OF MAMMALS, BY TH. MEYNERT. 



the other hand, such reflex influences may undoubtedly occur 

 through root-fasciculi proceeding from some other of the mani- 

 fold masses of origin of the fifth pair. To institute a compari- 

 son between the whole series of these masses, and the relations 

 presented by the origins of the posterior roots of the spinal 

 cord, and to refer it to the type of the cord, is an important 

 and still unaccomplished work that awaits monographic inves- 

 tigation. 



The reflectorial relations of the motor nucleus of the fifth, 

 as we may conclude from the phenomenon of trismus, must 

 embrace the sensory region of the spinal cord, and it is on this 

 account probable that certain portions of the fibrse arcuatse 

 of these planes, notwithstanding that we are unable to trace 

 them into transverse sections of downward tending masses, 

 may still turn out to be tracts of origin of the posterior 

 columns. Possibly we must regard the large scattered nerve 

 corpuscles in the motor area, which are here somewhat more 

 numerous than at higher levels, as outlying portions of the 

 superior nucleus of the fifth, which connect it with the fasciculi 

 of the posterior column, here appearing as fibrse areuatse. 



The multiple origin of the external large sensory root of the 

 fifth will best be understood by recognizing four separate and 

 principal forms of origin. 



1. Roots of the fifth, the nuclei of origin of which are situ- 

 ated within, or not far distant from, the plane of emergence. 

 The nucleus of origin of this very remarkable tract embraces 

 in longitudinal section the outer side of the motor root of the 

 fifth in a semilunar manner. In Man it possesses a vertical 

 length of 4'4 millimeters, with a transverse diameter of 1'2 mil- 

 limeters, and an antero-posterior diameter of 2 '8 millimeters. 



It is composed of small clusters that are separated from one 

 another by the delicate fasciculi of origin of the nerve which 

 arise in a plexiform manner from the nerve corpuscles (fig. 

 253 g, 5 d). The matrix in which these clusters lie consists of 

 finely granular connective tissue, and the nerve corpuscles for 

 the most part have a length of 18 to 24 ju, and a thickness 

 of 6 to 9 p. Stilling correctly regards the whole formation 

 as a continuation of the gelatinous substance which extends 

 down continuously as far as the caput of the posterior cornu 



