I am not aware that this view of the me chanism of the various actions of the nervous system had been ever distinctly enunciated before it had been stated by Mr. Bowman and myself in onr work on the Physiological Ana tomy and Physiology of Man, in 1845.* There is nothing, however, in this hypothesis at variance or inconsistent with the views of Prochaska ; for this physiologist seems to have held the opinion that the nerves are im planted in the segment of the cerebro-spinal axis into which they sink, and do not pass beyond it.
I shall now examine into the merits of each of these hypotheses, and, first, of the excito !notary hypothesis.
It is unnecessary to repeat the objections already stated (p. 721n) to the use of the term I shall only remark that some of these objections are equally opposed to the hypothesis as to its name.
Nevertheless this hypothesis has much to commend it : and not the least argument in its favour is that drawn from the compound nature of spinal nerves, as proved by Bell, in which nerve-fibres of different endowments are bound together in the same sheath. If it be proved (as it has been) that fibres of sensation and of motion may be thus placed in juxtaposition in the same nervous trunk, it seems not an unreasonable conjecture that fibres of other function ( excitors and their corresponding ;Ro tors) might be enclosed in the same sheath vvith them.
Both anatomy and experiment, however, unite to prove the existence of sensitive fibres distinct from motor fibres; they are found separate in the roots of the nerves, and combined in the nervous cords : but neither anatomy nor experiment fa vours the existence of a distinct series of excitor and of corresponding motor nerves. Anatomical research affords not the slightest indication of such a series of nerves. And experiments on the roots of the nerves, where it might reason ably be expected that the excitors would be separated from the motors (following the ana logy of the motor and sensitive fibres), are by no means favourable to the existence of such fibres in the roots. The failure of experi menters to excite motion by irritation of the posterior roots of the spinal nerves has been already referred to. A new and extensive series of experiments is much needed to settle this question. I would remark that galvanism should not be used in them, as the results of stimulation by that agent are extremely fal lacious, from its liability to extend beyond the parts included between the electrodes.
Other very serious anatomical objections may be urged to this hypothesis. It supposes
the existence of two sets of fibres in the spinal cord. Evidence in favour of these is wanting just as much as in favour of those in the roots of the nerves. Many facts favour the conclu sion that the fibres which constitute the roots of the nerves of any segment of the cerebro-spinal centre are implanted in the grey matter of that segment, and that none of them are continued beyond that segment up into the brain. They penetrate the spinal cord more or less obliquely, and form their connection with the grey matter a little higher up than the point of penetration ; but there is no evidence to show that they assume a completely vertical direction to pass up to the brain.
The form and varying dimensions of the spinal cord in its several regions are opposed to this view. If the sensori-volitional fibres are all continued up into the brain, and the (so called) excito-motor fibres are implanted in the cord, that segment of the cord should be the largest in which the greatest number of these fibres is to be found. Now the great extent of excitor surface in the lower extremities, the inagnitude of their muscles, the importance of their movements, and, at the same time, the great developement of reflex actions in them, would lead most reasonably to the expectation that the lumbar segment of the cord to which these nerves belong should exceed considerably in size the cervical segment which gives nerves to the upper extremities, where the excitor sur face is of less extent, where the muscles are less powerful,and the reflex actions considerably less conspicuous. Moreover,the lumbar region of the cord would be, if Dr. Hall's views were coirect, the centre of those excito-motor acts connected with dercation, micturition,partmition, &c., of which he speaks so much, and on this account might fairly claim a greater amount of substance. But the fact is, that the lumbar swelling of the cord is smaller than the cervical ; and that while it contains, and owes its bulk mainly to, a large quantity of vesicular matter, but a small pro portion of fibrous matter is found in it. More over, it is impossible to understand the great superiority of size of the lumbar portion over the dorsal segment of the cord, if we are to admit that this latter segment contains in addi tion to its own fibres (sensori-volitional and excito-motory) the sensori-volitional fibres of the lumbar. swelling also, which ought to be very numerous.