tating and ammoniacal liquid, which reacts upon the mucous membrane and aggravates the previous inflammation.

We find, therefore, that the spinal cord, in its character of a nervous centre, exerts a general protective action over the whole body. It presides over the involuntary movements of the limbs and trunk; it regulates the action of the sphincters, the rectum, and the bladder; while at the same time it exerts an indirect influence on the nutritive changes in those parts which it supplies with

nerves.

CHAPTER IV.

THE BRAIN.

By the brain, or encephalon, as it is sometimes called, we mean all that portion of the nervous system which is situated within the cavity of the cranium. It consists, as we have already shown, of a series of different ganglia, connected with each other by transverse and longitudinal commissures.

Since we have found the functions of sensation and motion, or sensibility and excitability, so distinctly separated in the spinal cord, we should expect to find the same distinction in the interior of the brain. These two properties have indeed been found to be distinct from each other, so far as they exist at all, in the encephalic mass; but it is a very remarkable fact that they are both confined to very small portions of the brain, in comparison with its entire bulk. According to the investigations of Longet, neither the olfactory ganglia, the corpora striata, the optic thalami, the tubercula quadrigemina, nor the white or gray substance of the cerebrum or the cerebellum, are in the least degree excitable. Mechanical irritation of these parts does not produce the slightest convulsive movement in the muscles below. The application of caustic liquids and the passage of galvanic currents are equally without effect. The only portions of the brain in which irritation is followed by convulsive movements are the anterior surface of the medulla oblongata, the tuber annulare, and the lower part of the crura cerebri; that is, the lower and central parts of the brain, containing continuations of the anterior columns of the cord. On the other hand, neither the olfactory ganglia, the corpora striata, the tubercula quadrigemina nor the white or gray substance of the cerebrum or cerebellum, give rise, on being irritated, to any painful sensation. The only sensitive parts are the posterior surface of the medulla oblongata, the restiform bodies, the processus e cerebello ad testes, and the upper part of the crura cerebri; that is, those portions of

the base of the brain which contain prolongations of the posterior columns of the cord.

The most central portions of the nervous system, therefore, and particularly the gray matter, are destitute of both excitability and sensibility. It is only those portions which serve to conduct sensations and nervous impulses that can be excited by mechanical irritation; not the ganglionic centres themselves, which receive and originate the nervous impressions.

We shall now study in succession the different ganglia of which the brain is composed.

OLFACTORY GANGLIA.

These ganglia, which in some of the lower animals are very large, corresponding in size with the extent of the Schneiderian mucous membrane and the acuteness of the sense of smell, are very small in the human subject. They are situated on the cribriform plate of the ethmoid bone, on each side of the crista galli, just beneath the anterior lobes of the cerebrum. They send their nerves through the numerous perforations which exist in the ethmoid bone at this part, and are connected with the base of the brain by two longitudinal commissures. The olfactory ganglia with their commissures are sometimes spoken of as the "olfactory nerves." They are not nerves, however, but ganglia, since they are mostly composed of gray matter; and the term "olfactory nerves" can be properly applied only to the filaments which originate from them, and which are afterward spread out in the substance of the Schneiderian mucous membrane.

It has been found difficult to determine the function of these ganglia by direct experiment on the lower animals. They may be destroyed by means of a strong needle introduced through the bones of the cranium; but the signs of the presence or absence of the sense of smell, after such an operation, are too indefinite to allow us to draw from them a decided conclusion. The anatomical distribution of their nerves, however, and the evident correspondence which exists, in different species of animals, between their degree of development and that of the external olfactory organs, leaves no doubt as to their true function. They are the ganglia of the special sense of smell, and are not connected, in any appreciable degree, with

OPTIC THALAMI.—CORPORA STRIATA.—HEMISPHERES. 359

ordinary sensibility, nor with the production of voluntary move

ments.

OPTIC THALAMI.

These bodies are not, as their name would imply, the ganglia of vision. Longet has found that the power of sight and the sensibility of the pupil both remain, in birds, after the optic thalami have been thoroughly disorganized; and that artificial irritation of the same ganglia has no effect in producing either contraction or dilatation of the pupil. The optic thalami, however, according to the same observer, have a peculiar crossed action upon the voluntary movements. If both hemispheres and both optic thalami be removed in the rabbit, the animal is still capable of standing and of using his limbs in progression. But if the right optic thalamus alone be removed, the animal falls at once upon his left side; and if the left thalamus be destroyed, a similar debility is manifest on the right side of the body. In these instances there is no absolute paralysis of the side upon which the animal falls, but rather a simple want of balance between the two opposite sides. The exact mechanism of this peculiar functional disturbance is not well understood; and but little light has yet been thrown, either by direct experiment or by the facts of comparative anatomy, on the real function of the optic thalami.

CORPORA STRIATA.

The function of these ganglia is equally uncertain with that of the preceding. They are traversed, as we have already seen, by fibres coming from the anterior columns of the cord; and they are connected, by the continuation of these fibres, with the gray substance of the hemispheres. They have, therefore, in all probability, like the optic thalami, some connection with sensation and volition; but the precise nature of this connection is at present altogether unknown.

HEMISPHERES.

The hemispheres, or the cerebral ganglia, constitute in the human subject about nine tenths of the whole mass of the brain.

Throughout their whole extent they are entirely destitute, as we have already mentioned, of both sensibility and excitability. Both the white and gray substance may be wounded, burned, lacerated, crushed, or galvanized in the living animal, without exciting any convulsive movement or any apparent sensation. In the human subject a similar insensibility has been observed when the substance of the hemispheres has been exposed by accidental violence, or in the operation of trephining.

Very severe mechanical injuries may also be inflicted upon the hemispheres, even in the human subject, without producing any directly fatal result. One of the most remarkable instances of this fact is a case reported by Dr. William Detmold, of New York,' in which an abscess in the anterior lobe of the brain was opened by an incision passing through the cerebral substance, not only without any immediate bad effect, but with great temporary relief to the patient. This was the case of a laborer who was struck on the left side of the forehead by a piece of falling timber, which produced a compound fracture of the skull at this part. One or two pieces of bone afterward became separated and were removed, and the wound subsequently healed. Nine weeks after the accident, however, headache and drowsiness came on; and the latter symptom, becoming rapidly aggravated, soon terminated in complete stupor. At this time, the existence of an abscess being suspected, the cicatrix, together with the adherent portion of the dura mater, was dissected away, several pieces of fractured bone removed, and the surface of the brain exposed. A knife was then passed into the cerebral substance, making a wound one inch in length and half an inch in depth, when the abscess was reached and about 3iij of pus discharged. The patient immediately aroused from his comatose condition, so that he was able to speak; and in a few days recovered, to a very considerable extent, his cheerfulness, intelligence, and appetite. Subsequently, however, the collection of pus returned, accompanied by a renewal of the previous symptoms; and the patient finally died at the end of seven weeks from the time of opening the abscess.

Another and still more striking instance of recovery from severe injury of the brain is reported by Prof. H. J. Bigelow in the American Journal of Medical Sciences for July, 1850. In this case, a pointed iron bar, three feet and a half in length, and one inch and a

Am. Journ. of Med. Sci., January, 1850.