SECT. II.
Of the Brains and Nerves.
The brains are by all Physiologists esteemed the most mysterious organs of life, being the fountain of the nerves, the organs of sensation, and hence allowed by all to be the seat of the soul, or the sanctum sanctorum of the human understanding.
They are distinguished into the common or great brain called cerebrum; the small part called cerebellum; and the spinal marrow, which projects down the back-bone. The great brain is divided into two great lobes, seated in the upper and fore part of the skull, of which it takes in the greatest cavity. It is so exceedingly sensible, that the least pressure puts an immediate stop to the sensation of the animal; and which is generally termed a apoplexy.[[2]]
[2]. Hence in a fracture of the skull, or any other heavy contusion, where the brain is compressed either by the fractured bone, or the extravasated blood, the patient will remain in a state of stupefaction, till either the fracture is reduced, or the extravasated blood removed by trepannation; after which he immediately comes to himself, as if suddenly revived from a trance.
This part of the brain, though it is so exquisitely sensible, seems to be designed by nature as preparatory to the cerebellum, the immediate root of the nerves.
The cerebellum is the lesser, but the more material part of the brains. It is seated in the hinder cavity of the skull, (which for the sake of protecting this exquisite sensible organ, is remarkably strong and firm) and from thence as the fountain of life itself, the nerves, the instruments of sensation in the whole animal frame, take their commencement: For whereas the other is so sensible, as that the least pressure will cause a stupor; this is so far superior in point of exquisite sensibility, that a pressure on that part, not only occasions an apoplexy, but even stops life itself.
From this cerebellum prolongates the marrow in the back-bone, called medulla spinalis, which is but a continuation of the same, and is equally sensible, and of the same substance with the former.
The whole brain, that is, the great brain, the small brain, spinal marrow, together with all the nerves which spring from thence, are covered with two coats, called the external stronger coat, dura mater; and the thinner and inner coat, pia mater; which continue with each nerve, even to its smallest ramification. The whole in a living subject is in a continual motion peculiar to itself.[[3]]
[3]. This motion in the brain is not unlike the peristaltic motion; this is an observation too seldom taken notice of by anatomists, though evidently plain in living animals, and is of singular service in physiological enquiries.
From the cerebellum, and medulla spinalis, all the nerves take their commencement and origin. Ten pair spring forth from the part in the skull; the principal are those which form the various senses, tasting, smelling, hearing, and seeing, and that common to all, feeling. From the medulla spinalis, or marrow in the back-bone, proceed thirty pair of nerves, which spread themselves variously over the whole system.
All nerves have this in common with each other, namely, exquisite sensibility; but in other respects they differ however widely; particularly the nerves destined for some of the external senses.
It is from that common quality feeling, that they become the subtile springs of life; and give an edge to all the animal functions.
There is nothing so plentifully, and nothing more curiously interwoven with every other substance, as the nerves; and so amazingly quick is their sensation, that the very instant a single nerve, even the smallest and remotest in the system, is affected, the soul, or the common sensorium, is sensible of the identical part so affected.
It has been asserted, that this amazing quick conveyance of the sensation of the nerves, is owing to a subtile nervous fluid; but on a rational reflection, and a just enquiry into the animal œconomy, it must appear, that such conveyance of a fluid is inconsistent with probability and experience: For, neither are the nerves tubulous, nor has there ever been discovered any circulating humour in them. And secondly, is it consistent with reason, that a conveyance of a fluid from the toe to the finger, should be so momentarily quick, as is sometimes experienced in the gout, and other the like ailments? That the quick transposition of sensation of the nerves, is rather effected on a principle of vibration, appears more probable, and will admit of a more convincing experiment; which may be exemplified in the following manner: Suppose (in the language of a seaman) a rope runs from the top-mast-head, or any other part of the rigging, to the deck, and is kept tought and free from any interception; then it will be evident, that the least touch at one end will instantaneously be conveyed to the other end; in like manner the nervous sensation may be communicated, perhaps not unlike electricity; which similarly will in an instant be conveyed to any part, by a direct communication, but equally subject to be intercepted.
Hence, a paralytic limb will lose, together with its irritable sensibility, its circulation of fluids, and its nourishment; and recover again when the pressure or interruption is ceased. Hence also, spasmodic contractions will be affected at the irritation of some principal parts, and from this instantaneous irritability, the motion of every part is thought to obey the will.