398 THE NERVOUS SYSTEM. 



varies in effect according to its situation. If seated in the spinal 

 cord, it produces paralysis on the same side of the body ; if above the 

 decussation of the pyramids, in the medulla, tuber annulare, crus cere- 

 bri, or cerebral hemisphere, it produces paralysis on the opposite side ; 

 and, finally, a lesion involving the decussation of the pyramids causes 

 paralysis on both sides of the body at once. 



These are the general results of both experiment and observation, 

 and they express the most habitual and important conditions of uni- 

 lateral and bilateral paralysis. But there are certain variations from 

 the type of simple and complete decussation which have some influence 

 on the phenomena. 



First, the study of descending degenerations of the pyramidal tract 

 shows that, beside the principal mass of fibres in this tract which cross 

 to the opposite side of the cord at the decussation of the pyramids, 

 there are a certain number w^hich continue downward on the same 

 side, forming in the cervical region the "columns of Tiirck " (Fig. 

 106). These direct fibres are in small proportion, representing, on the 

 average, considerably less than ten per cent, of the whole pyramidal 

 tract, and in man they do not extend, as a rule, below the cervical 

 region. What becomes of them here is unknown ; but it is evident 

 that their destination may be twofold. They may terminate in the 

 anterior horns of gray substance ; in which case the decussation of the 

 pyramidal tracts would be partial, and the upper limb would receive 

 some motor power from the same side of the brain. Or they may 

 finally cross, through the white commissure, to the opposite side of 

 the cord ; in which case the decussation would be complete, a part of 

 it taking place below the pyramids, in the cervical region. This would 

 explain the results obtained by various experimenters (Van Kempen, 

 Brown-Se'quard, Yulpian), who have found that in animals a division 

 of one lateral half of the spinal cord in its upper portion is followed by 

 a certain degree of paralysis on the opposite side. All, however, are 

 agreed, that this effect is not produced by a similar section in the lum- 

 bar region, but slightly or not at all in the dorsal portion, and is only 

 pronounced after a section in the cervical region. 



Secondly, The proportion between the direct and crossed fibres of the 

 pyramidal tract, in man, may vary in exceptional cases, so that the 

 majority of these fibres may be direct, and only the minority decussate. 

 Under these conditions, a lesion in the brain, contrary to the general 

 rule, would cause paralysis on the same side of the body. According 

 to Charcot* such instances exist, although their occurrence is extremely 

 infrequent. Similar exceptional variations have been recorded in regard 

 to other decussating tracts in the nervous system. 



Decussation of the Sensitive Tracts. Sensitive impressions, passing 

 from the integument to the nervous centres, undergo, like the motor 



* Lecons sur les Localisations dans les Maladies du Cerveau et de la Moelle pi- 

 ni&re. Deuxieme Partie. Paris, 1880, p. 195. 



