COURSE OF THE TRACTS FOR CONSCIOUS SENSATION. 



747 



fibers are concerned in the regulation of equilibrium, and their superior 

 path of conduction is situated in the cerebellum. In cases of tabes these 

 tracts and the columns of Stilling and Clarke are often degenerated. 



In the human brain the sensory conducting path for the tactile and the mus- 

 cular sense (continuation of the posterior roots and the direct cerebellar tract) 

 develops first. This passes from the nuclei of the posterior columns (Fig. 257) 

 through the thalamus and the lenticular nucleus to the central convolutions, 

 especially the posterior. 



The circumstance that a portion of the sensory cutaneous nerves 

 cross in the spinal cord to the opposite side explains the fact that uni- 

 lateral division of the spinal cord in man, and in apes, abolishes cutaneous 

 sensation upon the opposite side of the 

 body below the level of the lesion, while 

 the muscular sense is preserved. Upon 

 the side of the injury hyperesthesia is 

 present below the level of the section. 



From experiments on mammals Brown- 

 Se"quard concluded that the decussating 

 sensory nerve-fibers cross in the spinal cord 

 to the opposite side at various levels the 

 fibers that transmit tactile sensibility at the 

 lowest level, then those that transmit sensa- 

 tions of tickling and pain, and at the highest 

 level those that transmit thermal impressions. 



All of the fibers that, pursuing a 

 longitudinal course, connect the spinal 

 cord with the medullary mass of the 

 cerebrum, thus, as a rule, undergo com- 

 plete decussation in their course. There- 

 fore, in man the result of a destructive 

 lesion of one cerebral hemisphere is gen- 

 erally complete paralysis and abolition 

 of sensation upon the opposite side of 

 the body. Also the fibers arising from 

 the nuclei of origin of the cerebral 

 nerves decussate within the brain. 



tffunic. 

 grac. 



G.S. 



A.R. 



FIG. 257. Course of the Sensory Fibers 

 from the Posterior Roots through the 

 Spinal Cord upward to the Cerebrum. 

 The explanation of the fibers will be 

 clear from the description in the text, 

 and with it also Fig. 256 should be 

 compared: A,R, anterior root; P.R, 

 posterior roof, V.G, ground-bundle of 

 the anterior column; Py.V, anterior 

 pyramidal tract; Py.S, lateral pyram- 

 idal tract; G.S, ground-bundle of 

 the lateral column; Kl.S, cerebellar 

 tract of the lateral column; G, column 

 of Golj; B, column of Burdach; Py, 

 pyramid; O/, olive; L, fillet or inter- 

 mediary layer of the olive; 



fibre 



arcuatae; restiform body; nucleus of 

 the slender column and nucleus of the 

 cuneate column of the medulla ob- 

 longata. 



Only in those cases, not rare it is true, in 

 which the lesion, as from pressure, inflamma- 

 tion, etc., involves the cerebral nerves situated 

 at the base, are paralysis and anesthesia ob- 

 served on the same side of the head. 



Particulars as to the site of decussation 

 have already been stated. Decussation takes 

 place (i) in the spinal cord, (2) in the medulla 

 oblongata, and finally (3) in the pons. Decus- 

 sation is already complete in the peduncles. Gubler observed, in cases of 

 unilateral injury of the pons, paralysis of the facial nerve on the same side, but 

 paralysis of the body on the opposite side. From this he concluded that the 

 nerves from the trunk must undergo decussation below the pons, the facial 

 fibers within the pons. Such rare instances are designated alternating hemiplegia. 

 The conditions are made clear in Fig. 255. 



Exceptions to decussation are formed by the olfactory nerves, which do not 

 decussate at all (?), and the optic nerves, which decussate only partially in the 

 chiasm. 



