CENTRAL NERVOUS SYSTEM. 233 



argued that the mode of the sensation was independent of the kind of stimu- 

 lus, but dependent on the nature of the central cells among which the afferent 

 fibres terminated. It will be seen, however, that this argument does not 

 touch the character of the nerve-impulses in any two sets of nerves, and we 

 have no observations by which to decide whether the nerve-impulses passing 

 along the optic nerve-fibres arc, for example, similar or dissimilar to those 

 which pass along the auditory fibres. 



If the nerve-impulses are always all alike, there seems no escape from the 

 inference that separate nerve-fibres convey the impulses destined to give rise 

 to different sensations. At the same time, it is just possible that the nature 

 of the impulses and of the resultant sensation is, in the nerves of cutaneous 

 sensibility, determined by the form of the peripheral stimulus, and that, as a 

 consequence, different branches of the same nerve-fibres may be conceived of 

 as susceptible to different forms of stimulation, and thus the two different 

 sensations follow from the partial stimulation of the same nerve-fibres. 



The second possibility, that the nerve impulse has different characters in 

 different afferent nerves, and further may be modified by the nature of the 

 normal stimulus (pressure or temperature), is not to be too readily rejected, as 

 Hering at least argues in favor of such a view'. 1 



Pathway of Impulses in the Spinal Cord. — The question arises bow 

 these impulses are distributed among the afferent tracts which are recognized 

 in the cord, and whether these tracts form special paths for the impulses that 

 rouse the several sensations of pressure, temperature (heat and cold), and 

 pain. Since it is necessary to know the sensations of the subject, this prob- 

 lem can be, in some ways, best studied in man. Here, owing to wounds or 

 disease, it may so happen that some of these sensations are lost or greatly 

 diminished, and it is to be determined whether this loss is constantly associ- 

 ated with the interruption of definite tracts. Unfortunately, however, the 

 material for such a study is very meagre. 



In man the typical group of symptoms following hemisection of the spinal 

 cord above the lumbar region has long been known as Brown-Sequard's 

 paralysis. The clinical observations on cases suffering from such a lesion 

 have been recently summarized by Oppenheim 2 as follows : 



1. A paralysis of the homo-lateral muscles. In the ease of the leg. the 

 effects are most intense and persistent in the flexors of the thigh and shank, 

 and the extensors of the foot. 



2. When the two sides of the body are contrasted, there appears to be a 

 homo-lateral hyperesthesia, accompanied by contra-lateral anaesthesia. 



3. As to the several forms of sensation, the following may be stated : 



(«) The muscle sensations (Bathyasthesia, Oppenheim): the defect is 

 never contra-lateral ; sometimes, however, it is bilateral, but in mosl cases is 

 homo-lateral. 



(b) The contact sensations are very often not affected at all — sometimes 



'Hering: "Zur Theorie der Nerventhatigkeit," Akademischer Vortrag, Leipzig, 1S99. 

 'Oppenheim: ArcMvfur Physiologic, Physiol. Al.tlil., Suppl. Bd., 1 licit. .Inly. 1899. 



