342 PHYSIOLOGY CHAP. 



through their collaterals with the cells of the ventral roots, are 

 excito-motor, and belong to the spinal reflex arcs ; and by sending 

 axons cerebralwards, act as sensory nerves, and are part of a cere- 

 bral reflex arc. On the other hand, the peripheral motor neurones 

 function as involuntary or reflex fibres when they react to stimuli 

 received through the dorsal roots, and as voluntary motor nerves 

 when excited by the pyramidal tracts. 



Thus the cells of the spinal mechanisms are not merely in 

 relation with local functions of the cord, but also send impulses to 

 the cerebral nerve-cells and receive others from them in turn. 

 The phylogenetic evolution of the nervous system goes pari passu 

 with an ever-increasing development of the long spino-cerebral 

 and cerebro-spinal conducting paths. The pyramidal tract, which in 

 the higher vertebrates represents the complex of the long cerebro- 

 spinal motor conducting paths, increases gradully in bulk and 

 attains its maximal development in man. The direct ventral 

 pyramidal tract only appears in man, and, according to Sherrington, 

 in the ape. In rats and guinea-pigs, according to Lenhossek, the 

 pyramidal tracts are small and run in the dorsal columns : while 

 in rabbits, cats, dogs, they pass, according to Spitzka, through the 

 lateral columns, after decussating in the medulla oblongata. In 

 the cat (Boyer), in the dog (Muratoff), in the monkey (Mellus and 

 Sherrington), and sometimes in man also (Pitres), there is a small 

 direct lateral pyramidal tract, as the bulbar decussation is not 

 always complete. In the lower vertebrates (amphibia, reptiles, and 

 also birds), it is probable that there are no cortico-spinal nor long 

 centripetal tracts, such as are present in the higher vertebrates 

 with a well-developed cerebral cortex. 



It is essential to bear in mind the varying development and 

 course of the cerebro-spinal and spino-cerebral conducting tracts 

 in different classes of vertebrates, in order to avoid the error which 

 the older physiologists fell into, when they applied the data 

 obtained from the physiological effects of partial transections of 

 the cord in the lower vertebrates to human physiology. 



We have seen that complete transection of the cord produces 

 paraplegia by interrupting all the conduction paths. We must, 

 therefore, confine ourselves to studying the effects of partial 

 spinal transection upon the motility and sensibility of the more 

 caudal parts, by experiments on the vertebrates nearest to man, 

 as well as from the simpler and least equivocal clinical 

 observations. 



Clinical cases of cerebral lesions taught us long since that the 

 motor and sensory paths decussate in the cerebro-spinal axis. 

 Haemorrhage in the right brain causes motor and sensory 

 paralysis of the left half, and when in the left brain, of the right 

 half of the body. Brown-Se'quard, however, records certain 

 exceptions to this rule, which can be explained either by an 



