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 g,rave 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 seveml 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. Ilall 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 tbe 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 affect 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 Midler, 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 ease of cerebml 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 bmin 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 au. 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 spinal connnissure,s. 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 that their real origin would be much higher up than their appa rent one. This great length of oblique course g,ives to the fibre the appearance of being stricdy longitudinal, whereas it inay be implanted 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, thari each segment has its proper nerves implante4 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 commissuml fibres; by the anterior pyramids and olivary columns with. ihe former, and by the restiforna 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 e.ase as in the other, the former acting' as sensitive or excitor, or both ; the latter as4 channels for voluntary, emotional, or strictly physical impulses to motion.