Of the functions of the roots of spinal nerves. —The greatest part of the bedy is supplied with nerves which are implanted in the spinal cord, or which, in anatomical language, have their origin in that nervous centre. As these nerves present very definite and constant cha racters as regards the manner in which they are connected with the centre, characters which are not limited to the human subject, but which be long to all classes of vertebrate animals, it was a point of prin-Jary importance to discover the object of an arrangement so peculiar as regards its anatomical characters, and so universal. To our countryman, Sir C. Bell, belongs the great merit of having seen the importance of deter mining this point as a preliminary step in the investigations into the nervous system ; arid to him must be awarded the credit of having achieved the discovery of the difference in the endowment of the anterior and of the posterior roots of these nerves. Ile experimented on young rabbits, by removing the posterior wall of the spinal column. " Ott laying bare the roots of the spinal nerves," says Sir C. Bell, "I found that I could cut across the posterior fasciculus of nerves, which took its origin from the posterior portion of the spinal marrow, without convulsing the muscles of the back ; but that, on touching the anterior fascictilus with the point of the knife, the muscles of the back were immerliately convulsed." 4' Numerous experimenters, subsequent to Bell, obtained precisely similar results. Muller, however, obtained the most decisive evidence of the proper functions of the roots of the nerves, by experimenting on frogs instead of on mam inalia ; in the former the spinal canal is of great width, especially at its lower part, and the roots of the nerves can be exposed with great facility, whilst in the latter the operation is tedious, painful, and bloody, the spinal canal narrow, and the roots of the nerves small and difficult to get at. Moreover, the excita bility of the nerves lasts very much longer in frogs than in mammalia, and on this account the former animals are well adapted for dis playing the effects of section of the roots and the influence of mechanical and other stimuli upon them.
In these experiments, (which I have frequently repeated with similar results,) irritation, mecha nical orgalvanic, of the anterior root of the spinal nerve always provokes muscular contraction. No such effect follows irritation of the posterior root. Section of the anterior root causes para lysis of motion ; section of the posterior root, paralysis of sensation. This latter effect is shown by the entire insensibility to pain evinced on pinching a toe, whilst in the limb of which the posterior roots of the nerves remained en tire such irritation is evidently felt acutely. If the anterior roots of the nerves which are dis tributed to the lower extremities be cut on one side, and the posterior roots on the other, voluntary power svithout sensation will remain in the latter, and sensation without voluntary power in the former.
Valentin, Seubert, Panizza, and Longet have performed similar experiments on mammiferous animals with precisely similar effects.
I have never seen motion produced by irri tation of one of the posterior roots of the spinal nerves still in connexion with the cord, except ing when the galvanic stimulus has been ap plied, and too strong a current has been em ployed. Valentin states that he has observed
motions so produced in rabbits, but not in frogs and tortoises. Dr. Hall has seen thern in the turtle and skate. Van Deen speaks of them as constantly occurring. But Muller denies the power of the posterior roots to excite motion, except by " traction on the cord itself." No such effect ever follows any kind of stimulation of the posterior root when it has been separated from the cord.
The conclusion which inevitably follows from these experiments is that the anterior root of each spinal nerve is motor, and the posterior sensitive.
Comparative anatomy confirms this conclu sion, by showing that a similar arrangement of the roots of spinal nerves prevails among all classes of vertebrate animals, and that if in any particular class either the motor or sensitive power predominate, there is in correspondence with it a marked developement of the anterior or posterior roots. The frequent occurrence, likewise, of paralysis of sensation and motion, as a consequence of disease within the spinal canal, also tends to the same inference.
Kronenberg finds a small nem of commu nication between the posterior and the antenor root, which is looked upon by some as beinj the means of giving to the anterior root the slight degree of sensitive power which Majendie attributes to it.
From the determination of the office of each root of the spinal nerves we obtain the further important result, that the nerve, which is formed by the junction of these two roots, is sensitive and motor, and that nervous fibres of different en dowments may be bound together in the same sheath constituting one nerve, whtch is com pound in its functions. And the anatomical distribution of spinal nerves, both in man and the inferior animals, to the muscles and sen sitive surfaces of the trunk and extremities, is entirely confirmatory of the results thus derived from experiment.
By the use of the various means for deter mining the functions of nerves, above detailed, and aided by the determination of the law dis covered and developed by Bell and others, as to the motor nature of the anterior and the sen sitive endowment of the posterior roots, and the subsequent binding together of these fibres in one sheath to form a compound nerve, physiologists have made great advances in determining the functions of the various encephalic nerves, and our knowledge on this subject may be said to have approached more to perfection than that of any other physiological questions. The main facts connected with the anatomy and physio logy of each of these nerves will be found under the articles headed by their names.
Of the fiatctions of the nervous centres.— In examining into the functions of the va rious parts of the cerebro-spinal axis I shall adhere to the definitions already adopted in the previous part of this article, and use the term spinal cord as denoting the nervous cylinder within the spinal canal, and the encephalon as the intra-cranial mass, consisting of medulla oblongata, mesocephale, cerebellum, and cere brum.