COURSE OF THE TRACTS FOR CONSCIOUS SENSATION. 



745 



upon the same side. These, however, in their further course through 

 the spinal cord likewise cross over to the opposite side through the an- 

 terior white commissure. A portion of these fibers, at first undecus- 

 sated, appear, however, to remain upon the same side. It is perhaps 

 their function to innervate those muscles of the trunk that, like the 

 respiratory, the abdominal and the 

 perineal muscles, are generally made 

 to contract together on both sides. 



With reference to the relations of the 

 crossed and uncrossed fibers individual varia- 

 tions occur. In isolated cases the con- 

 ditions are reversed, and in rare instances 

 the pyramidal tracts remain upon the 

 same side from the brain downward. In 

 this way are to be explained the exceedingly 

 rare cases in which paralysis of voluntary 

 movement has been observed on the same 

 side as the lesion of the cerebral cortex. 

 Cases are uncommon also in which the 

 muscles of the trunk and the lower 

 extremities are moved bilaterally on volun- 

 tary efforts at unilateral movement. Finally, 

 it should be mentioned that Unverricht 

 occasionally observed in the dog a double 

 decussation of the tracts: once in the pyra- 

 mids and again in the spinal cord. Pyram- 

 idal tracts are present only in mammals. 



The cerebral motor nerves naturally 

 have their center for voluntary stimu- 

 lation in the cortex of the cerebral 

 hemisphere. From this point the fibers 

 that transmit voluntary impulses pass 

 through the internal capsule and the 

 crusta of the cerebral peduncle, where 

 they lie in front of and internal to the 

 pyramidal tracts. Then their course 

 is directed to their nuclei of origin. 

 In Fig. 255, c represents the course of 

 the facial nerve to its nucleus of origin. 

 The hypoglossal nerve passes with the 

 pyramidal tract and behaves like the 

 anterior root of a spinal nerve. 



Course of the Tracts for Conscious 

 Sensation. From the cortical area in 

 which is situated the center for sensi- 

 bility, which is designated the sensory 

 sphere and is fully described on p. 785, 

 the conducting tracts pass through 



the posterior third of the posterior limb of the internal capsule (Figs. 

 263, 264). The fibers for the transmission of the muscle-sense pass 

 through the middle, those for the sensations of pressure, temperature, 

 and pain through the inner half of the posterior third of the internal 

 capsule. The tracts then pass through the tegmentum of the cerebral 

 peduncle and their continuation through the pons and further to 

 the medulla oblongata. The fibers for the transmission of cutaneous 



A R 



FIG. 255. Course of the Paths for Voluntary 

 . Movement: a, b, Paths for the cerebral 

 motor nerves; c, path for the facial nerve; 

 5, corpus callosum; N.c, caudate nucleus; 

 G.i, internal capsule; N.I, lenticular nu- 

 cleus; P, pons; A". /, nucleus of origin 

 of the facial nerve; Py, pyramid, with 

 decussation; O.I, olivary body; G.r, 

 restiform body; P.R, posterior root; 

 A.R, anterior root; x, lateral pyramidal 

 tract; z, anterior pyramidal tract. 



