« PreviousContinue »
tions of the roots of the Spinal nerves. By the experiments on the latter, it was indisputably shown that the "anterior" roots were those which bestowed motor power. It was also observed that they were destitute of ganglions: and as both the Portiodura, proved experimentally to be a nerve of motion, and the lesser root of the Fifth, likewise proved to give motor power, were unprovided with ganglions, it was seen to be characteristic of nerves of motion that they should be without these appendages. Again, as to the "posterior" roots, on which the ganglions are formed, it has been said that, owing to the violence unavoidably inflicted in experimenting on the Spinal nerves, evidence to be relied on could not be obtained to prove that these were the roots of Sensation. But when the experi ments on the roots of the Fifth showed, in an unerring manner, that the smaller, non-ganglionic root, analogous to the anterior roots of the Spinal nerves, gave motor power alone, and that the larger, ganglionic root, analogous to the posterior roots, bestowed sensation, all doubt on the matter was removed; it was concluded, most decidedly, that the function of the posterior roots of the Spinal nerves was to confer Sensation.
GENERAL CLASSIFICATION OF THE NERVES.
Having succeeded in establishing on a firm foundation the important physiological truth-That the nerves of Motion are distinct from those of Sensation-the author had made a most valuable advance in our knowledge of the nervous system. But he did not cease his labours at that point. By surveying the nerves of the body generally, and observing the different modes in which they arose from the subdivisions of the brain and spinal cord, on the one hand, and the appropriation of particular kinds of nerves to distinct organs, on the other, he was led to believe that such peculiarities of origin and distribution had an important significance,-that they indicated distinctions in the functions of the nerves additional to those which he had already ascertained.
In taking that extended view, two principal objects attracted his notice, First, he was particularly struck by the remarkable manner in which the large series of Spinal nerves, with their
analogous nerve of the brain, the Fifth, arose from the central organs, and passed to their destinations; secondly, by the mode in which another series, comparatively small, and formerly adverted to as represented by the Portio-dura, came off from a limited portion of the brain, and was distributed to its appropriate parts.
Attending to the series of Spinal nerves and Fifth. The chief distinguishing characters of these nerves were, first, that they all arose from the spinal cord and brain by two distinct roots, one of motion, the other of sensation. Secondly, that, with an exception to be noticed presently, they were distributed generally and promiscuously over the whole body. Thus, with the reservation alluded to, these combined nerves furnished to all members and regions of the frame the two properties most essential for a nervous system to give. They bestowed Sensation on the integuments and every other sensitive surface from the crown of the head to the sole of the foot, together with Taste: they gave also motor power to the trunk, the neck, and the upper and lower extremities. But here the exception must be specified. It relates to the power of the Fifth in giving motion. That nerve distributes its branches which bestow sensation freely and without bounds to every part, internal and external, of all the head; but as a motor nerve, it is confined exclusively to particular muscles-to those which move the jaw in the act of Mastication. The author, as already stated, had applied to it the name, Nerve of Sensation and Mastication. Accordingly, the general character to be assigned to the series of Spinal nerves and Fifth was, first, that they bestowed Sensation or Touch over the whole extent of the body, without limit, and the sense of Taste in addition; secondly, that they bestowed Motor power, also without limit, upon every region and member of the frame apart from the head, and upon the muscles of mastication exclusively, in the head.
Next, attending to the smaller series of nerves, observed to be opposed in various respects to those just mentioned. The characters which chiefly distinguished them were, first, that they arose, by single roots, from a defined and limited portion of the brain, near its junction with the spinal cord, and were capable of bestowing motor power alone; secondly, that they were distributed solely to a particular region of the body, in
cluding the face, throat, neck, and chest. Again, it was remarked that, instead of coursing to their respective destinations, like the Spinal nerves and Fifth, in a symmetrical manner, by the shortest and straightest route, those nerves proceeded in a devious and divergent way, crossing the paths of the others, and terminating in organs already supplied with nerves. In reference to the Portio-dura, the most conspicuous of them, some peculiarities of a special kind were noticed. The principal was the exclusiveness of its distribution to those muscles of the face which move the Features. To reach these, the nerve takes a long, winding route from behind after emerging in front of the ear, it is in close proximity to the muscles of the jaws, and actually lies for a considerable part of its course upon two of the largest; but it declines sending a single branch to these muscles: it pursues its way across them, undiminished in size, to the muscles of the features beyond. Again, it is remarkable that these muscles, to which the Portiodura thus goes circuitously, have large branches of the Fifth, not less than three on each side, distributed freely in the midst of them; but they come off from the ganglionic root alone, have no fibrils of the motor root joined to them, and bestow sensation exclusively. Accordingly, the peculiarities of the Portio-dura may be summed up by saying, that it avoids contributing branches, which it could easily have done, to the muscles of the jaws, and exhausts itself altogether on the muscles of the features.
These were the observations which led the author to believe that some important distinctions would be found to exist between the two series of nerves whose peculiarities have been thus shortly set forth. The theory which he advanced to solve the problems it will be my endeavour to lay before the reader, in as brief a space as can be done. And I may premise by stating that it involved an examination of the development of the Nervous System through the whole members of the animal kingdom.
First, the author conceived that a class, formed of the Spinal nerves and Fifth, and to which he applied the term “Original System," ministered to organs, and bestowed nervous endowments, essential for the existence and well-being of creatures of
every grade, high and low, in the animal kingdom. Secondly, that an additional class, of which the Portio-dura was a type, and to which he gave the name "Respiratory System," was needed only by those animals, near or at the summit of the scale, in which the organ of Respiration had become adapted, by successive changes of structure, to be the instrument of Voice, and, in Man, also of Speech and Expression.
Original System of Nerves.
I. It has just been said that the author conceived that the Spinal nerves and Fifth were the representatives, in the higher animals and man, of a class common to them and to all below them-a class which ministered to functions and endowments required equally by animals in general. The mode in which he endeavoured to establish that view was the following. He sought, in the first place, to ascertain the primary objects for which, in the construction of an animal, a nervous system was originally demanded; and in the next place, having satisfied himself concerning that point, to learn whether these objects could be secured, or the necessary functions of an animal be provided for, by a class of nerves furnished with the powers that belong to the Spinal nerves and Fifth.
In order to learn what may be the requirements, in animals generally, which make the introduction of a nervous system into their bodies necessary, he directed his observations to the contrast between members of the Vegetable kingdom, which are destitute of a nervous system, and those of the Animal kingdom, in which it first appears. It is a well-known fact in zoology, that the transition between beings of the two kingdoms is so gradual, that it has always been a source of dispute, in treating of those which reside on the confines, whether particular individuals were the subjects of the one or of the other realm. But looking on the subject differently, the question may be properly asked-What are the generallypervading characters of a Vegetable, viewed in the abstract, as contrasted with those of an Animal, viewed similarly in the abstract?
Now, the cardinal difference between the Vegetable, on the
one hand, and the Animal on the other, is, that the former is a stationary organism, the latter locomotive. In other words, the vegetable obtains its nourishment and lives by means of roots which bind it prisoner to one place; the animal has to shift its locality, seize its food, and, after due preparation in the mouth, convey it into the interior of its body. When the aliment has been swallowed, it is true that the processes of assimilation, and of otherwise dealing with it, are analogous. But the characteristic distinction between them. remains that the one is fixed, the other moveable.
Accordingly, a new question arises :—An organised body, dependent for its subsistence on nourishment which it must procure by voluntarily going in quest of it, being given, what organs and properties must it possess to qualify it for that mode of existence?
The following series of parts appears indispensable :—
First:-Organs of Locomotion-including all varieties of instruments by which animals can change their localities-as Legs, or inferior substitutes for them.
Secondly:-Organs of Prehension-including all varieties of instruments by which animals can seize and secure their prey, or other food-as Arms and Hands, or inferior substitutes for them.
Thirdly:-Organs of Mastication-including all varieties of instruments by which animals can triturate, and reduce the food to a fit condition for being swallowed and conveyed into the stomach-as Jaws armed with Teeth, or their inferior substitutes. . All the above instruments are specially characteristic of Animal, as contrasted with vegetable organisation. Each also may be conceived to exist as an independent structure. But owing to the variety of positions, habits, and instincts of animals, and their obtaining nourishment from infinitely diversified sources, the different organs present themselves in the most multifarious forms. In animals lowest in the scale, slightly removed from vegetables, the instruments are so fused, by mutual interchange of offices, into one another, that it is difficult to recognise the identity of each: the prehensile organ will be found acting in aid of the locomotive, and the manducatory, it may be, in combination with both. But as animals progressively rise in the scale, a gradual departure from that