168 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



designated by Head * as deep pressure sensations. This author has 

 shown that the latter are mediated through the afferent fibers to 

 the muscles and constitute apparently a part of the group of sensa- 

 tions described as muscle sense. The path for this deep sensibility 

 to touch probably lies in the posterior columns. 



Homolateral and Contralateral Conduction of the Cuta- 

 neous Impulses. Great interest, from the medical side, has 

 been shown in the question of the crossed or uncrossed conduc- 

 tion of the cutaneous impulses in the cord. The matter is nat- 

 urally one of importance in diagnosis. In human beings it was 

 pointed out by Brown-Sequard f that unilateral lesions of the 

 cord are followed by muscular paralysis below on the same side 

 and loss of cutaneous sensibility on the opposite side. This syn- 

 drome has been held clinically to establish the diagnosis of a uni- 

 lateral lesion, and has led to the view that, while the conduction 

 of the motor impulses is homolateral, that of the sensory impulses 

 is contralateral. Experimental work on lower animals, on the con- 

 trary, has not supported this view. While results in this direction 

 have varied, as would be expected from the intrinsic difficulties 

 connected with the interpretation of the sensations of an animal, the 

 general outcome has been to show that the sensory conduction is 

 bilateral, but mainly on the same side. That is, if the cord is 

 cut on one side only (hemisected) in the thoracic region, the sensi- 

 bility of the parts supplied below the lesion is impaired upon the 

 same side, but not completely abolished, showing that some 

 crossing has taken place.J It is possible that this crossing is more 

 complete in man than in the lower animals, although later studies 

 in man of unilateral lesions of the cord (Brown-Sequard paralysis) 

 indicate that the contralateral loss of cutaneous sensibility affects 

 chiefly the senses of pain and temperature; the loss of touch is not 

 complete, and muscular sensibility is affected only on the same 

 side. On the whole, it would seem that the crossing, of the sensory 

 fibers in the cord is only partial, and is more extensive in man 

 than in the lower animals. This partial crossing is probably com- 

 pleted in the brain, especially in the great sensory decussation in 

 the medulla. 



The Descending (Efferent or Motor) Paths in the Antero- 

 lateral Column. The main descending path in the cord is the 

 pyramidal system of fibers. In man, as shown in Fig. 70, there 

 are two fasciculi belonging to this system, the direct and the 

 crossed pyramidal tracts. Both tracts arise from the anterior 

 pyramids on the ventral face of the medulla, whence the name of 



* Head and Rivers, "Brain," 1905, 99. 



t Brown-Sequard, "Journal de Physiologic, " 6, 124, 232, 581, 1863. 



j Mott, "Brain," 1895, 1, and Bertholet, "Le NeVraxe," 1906, vii., 283. 



