722 D 



PHYSIOLOGY OF THE NERVOUS SYSTEM. 



of fibres in each muscle, which, to be effective, 

 must have the same relation to the component 

 elements of the muscle. It is impossible to 

 imagine how each order of fibre should comport 

 itself with reference to the other two, so that 

 their actions may not interfere. Nor can any 

 one fail to perceive that the emotional fibres 

 must be infinitely less frequently employed 

 than the others, and in some individuals so 

 seldom called into action as to be greatly ex- 

 posed to the risk of atrophy for want of use. 



Another phenomenon, which this hypothesis 

 fails to explain, is the paralysis of the sphincter 

 ani muscle which accompanies certain lesions 

 of the brain, generally of grave import. Such le- 

 sions are almost always accompanied by para- 

 lysis, chiefly of the hemiplegic kind, but not ne- 

 cessarily complete. On the contrary, in several 

 such cases distinct reflex actions exist, indicating 

 that, although the brain's influence is withheld 

 from the limbs, that of the cord is* not. If, 

 then, the cord be sufficiently free from morbid 

 depression to allow of reflex movements taking 

 place in the inferior limbs, why is the sphincter 

 ani (the actions of which according to Dr. 

 Hall are eminently reflex) so completely para- 

 lysed that it offers not the slightest resistance 

 to the introduction of the finger into the anus ? 

 So long as the cord is free from lesion and so 

 capable of performing its functions that the 

 lower limbs exhibit reflex movements, the 

 sphincter ani muscle ought not to be paralysed, 

 if the excito-motor hypothesis be true. For, 

 admitting that this muscle has sensori-volitional 

 fibres 'which are paralysed by the cerebral le- 

 sion, it should have excito-motor fibres likewise 

 which ought to enable the muscle to resist the 

 entrance of the finger into the rectum. Such 

 resistance, however, it certainly does not make, 

 for the muscle is completely paralysed in 

 the cases referred to. And it is plain that, ac- 

 cording to the excito-motory hypothesis, a cere- 

 bral lesion ought not to afteot the sphincter ani 

 further than to destroy the control of the will 

 over it, unless the depressing influence of the 

 lesion extend to the whole cord, and in such a 

 case there ought to be complete paralysis of the 

 limbs likewise. 



In fine, it cannot be denied that the excito- 

 motor hypothesis takes a narrow and confined 

 view of that power of the nervous centres which 

 it professes to elucidate. As I have before 

 remarked, it limits this power to the excitation 

 of motion, and it confines the exciting agency 

 to nerves which naturally propagate centrad, 

 and which only propagate such impressions as 

 may excite movements. 



Now it admits of unquestionable proof that 

 impressions on sensitive nerves may, by a pro- 

 cess of reflexion, excite other sensitive nerves. 

 Are we to suppose the existence of a special 

 series of fibres for such phenomena? Such 

 a supposition would involve the most palpable 

 contradictions, and is wholly inadmissible. 



The second hypothesis, which accords with 

 the views of Mailer, is just as competent to 

 explain the phenomena of decapitated animals, 

 and of limbs paralysed to cerebral influence, 

 as that of Dr. Hall. It receives considerable 



support from the universal concurrence of sen- 

 sation or mental perception with those normal 

 actions which Dr. Hall would attribute to excito- 

 motory fibres. If it be supposed that these 

 fibres have a certain relation to the vesicular 

 matter of the cord, there are as good grounds 

 for the further supposition that they may con- 

 tinue to be affected by it after the brain has 

 been separated from the cord. This hypothesis, 

 however, is as inadequate as that of excito- 

 motory fibres to explain the influence of emotion 

 on paralysed limbs; and it likewise fails to 

 explain the paralysis of the sphincter, which, 

 under this hypothesis, ought to occur in every 

 case of cerebral disease. The chief objection, 

 however, to this hypothesis is anatomical ; for 

 it is far from being proved that the fibres of the 

 spinal nerves are continued upwards through 

 the cord into the brain. For instance, what 

 evidence have we that the fibres of the lumbar 

 region of the cord pass into the brain ? The 

 fibres of the anterior pyramids, no doubt, are 

 true cerebro-spinal fibres, because they com- 

 municate equally with brain and cord, and 

 distinctly pass from the one to the other; but it 

 cannot be shown that they have any continuity 

 with the fibres of any of the spinal nerves. 

 Much less can it be shown that they contain 

 the fibres which are continued up from, to say 

 the least, the anterior roots of ALL the spinal 

 nerves, which ought to be the case if this 

 hypothesis be correct. The bulk of the pyra- 

 mids is very much opposed to this view. It is 

 most probable that the pyramids are cerebro- 

 spinul commissures. The apparent longitudinal 

 course of the fibres in the spinal cord affords no 

 indication that they pass into the brain, for 

 it is well known that many of the fibres forming 

 the roots of spinal nerves take a very oblique 

 course from their point of separation from the 

 cord to their emergence from the spinal canal ; 

 and it is probable that this obliquity is con- 

 tinued in the cord itself, so thnt their real 

 origin would be much higher up than their appa- 

 rent one. This great length of oblique course 

 gives to the fibre the appearance of being strictly 

 longitudinal, whereas it may be implanted in 

 the vesicular matter of the cord. 



The third hypothesis is more consonant than 

 either of the others with what appears to be the 

 true anatomy of the spinal cord namely, that 

 each segment has its proper nerves implanted 

 in it, that it is connected with adjacent seg- 

 ments by commissural fibres, and that the 

 whole cord is connected with the cerebrum 

 and cerebellum by commissural fibres; by the 

 anterior pyramids and olivary columns with 

 ihe former, and by the restiform bodies with 

 the latter. 



This hypothesis, the reader will bear in mind, 

 assumes that mental and physical actions are 

 performed through the same fibres affected by 

 a mental stimulus in the one case, and a physical 

 stimulus in the other the change produced by 

 the physical stimulus being, in the case of 

 reflex actions, reflected at the centre. The 

 same afferent and efferent fibres are excited in 

 the one case as in the other, the former acting 

 as sensitive or excitor, or both; the latter as 



