brane, adhering to the inner table of the scull. The pia mater is a soft and delicate covering, adhering so closely to the folds or convolutions, that it is doubted by some physiologists, whether it should be considered as a distinct membrane from the surface of the brain. Those, who doubt the existence of this membrane, say it is merely the minute cerebral arteries, and those of the corresponding veins, forming at the surface of the brain a minute vascular net work.*

The arachnoid, or spider web membrane, is interposed between the other two, and is the secretory organ of the serum, which moistens the internal surface of the dura mater. It is a shut sack, whose internal surface is in contact with itself, while its external surface adheres to the pia mater and dura mater.

The brain is composed of a cineritious and medullary substance, as is every other nervous apparatus; and both are doubtless necessary to the perfect formation of the instrument, which performs a particular function. The cineritious substance of the brain is principally confined to the surface, and is of a pulpy, gelatinous character. It is usually of a gray or ashy color, but not always; so that pulpiness, and not color, is its distinguishing character. This substance is easily seen also in the ganglions, and in the nervous masses of the head and spine of vertebral animals. It has, distributed through it, an immense number of blood vessels, so that it appears almost as an entire mass of them; but there is besides these a distinct nervous matter interspersed between them. The white, or medullary substance is demonstrated to be fibrous or

* Dunglison's Physiology.

striated. Its fibrosity cannot be seen with equal distinctness in all brains. It should be a fresh brain of a subject, who had been healthy and somewhere near the middle age, in order to show its fibres to the best advantage.

The fibrous nature of the medullary substance was more fully demonstrated by Dr. Spurzheim than by any preceding anatomist. His mode of dissection enabled him to discover several particulars not before known, viz.

1. That the fibres of the medullary substance begin at the apex or medulla oblongata, and extend to near the surface, diverging in every direction.

2. That, as they proceeded to the surface, fibres were added.

3. That there are two great sets of fibres for each half of the brain.

4. That these are folded into convolutions or organs.

5. That the gray, or cineritious matter, is that out of which the medullary matter proceeds; and that the gray matter intervenes among the medullary matter, where the fibres commence.

The fibres are easily discovered by the scalpel; but the convolutions are so exceedingly delicate, that they will not always bear to be separated by the finger, as you would separate the parts of an orange; but by jetting water, with a syringe, upon the brain, or by using a common blow pipe, the parts will separate. The convolutions upon the surface are the mere peripheral expansions of the internal nervous bundles, to which they bear proportion, as the expansion of the optic nerve is in proportion to the nerve itself.

The cerebellum is an apparatus in connextion with, but independent of, the rest of the nervous system, as to its existence and functions. It is composed of the same nervous matter of the brain, presenting cineritious and medullary substances. It is connected with the medulla oblongata in that portion called the corpora restiformia. Circulation in the Brain.

The brain requires much more blood to support it than any other equal part of the system - say eight or ten times as much. And the blood, which goes to the brain, is supposed to be more earated than that, which is distributed to the other parts. It is blood, which has just passed from the lungs, highly charged with oxygen and caloric, and is light red and frothy. Its force is checked both by its specific gravity and the angular curvature, which the artery makes in its course; were it not for this, the cerebral arteries would be likely to break.

The brain requires to be constantly stimulated with properly oxygenated blood. If this be altogether withdrawn, the brain ceases to act, and sensibility and consciousness become extinct. When fixed air is inhaled, the blood passing through the lungs does not become renovated with oxygen, and the mental functions become impaired. If blood be too highly oxygenated, the brain is too much stimulated, and inflammation is liable to ensue. Hence the phenomena of thought and feeling not only depend on certain organs, but those organs require their appropriate stimulus.*

* Combe's principles of Physiology, p. 214.

CHAPTER VI.

SCULL.

THE scull is a strong, but yielding, covering to the brain, intended to protect it from injury. In infants it is in a partially formed state; and in after life the processes of absorption and deposition are so constant, that it furnishes no obstacle to the growth and change of form in the brain. Indeed, where any organ of the brain is very active for a long time, there is so much heat in the region of it, and the absorbent vessels become so active, that the scull becomes very thin against the organ, and a protuberance becomes apparent. Hence the scull enlarges to suit the growth of the brain, either by growth at the sutures of the scull, or by the formation of protuberances. The scull is composed of an outer and inner table, and between these is a diploë. several processes. These serve to fasten the muscles of the scalp to the scull. The mastoid process is directly back of the ear; the zygomatic, just before the ear, and the spinous process, or occipital spine, is at the top of the spine, at the transverse ridge of the occipital bone. The scull is nearly parallel in its two tables, and the scalp is so loose, that the shape of the brain may be determined by the shape of the head, with reasonable certainty. There are cuts of several sculls in a subsequent part of this work, illustrative of them, both physiologically and anatomically.

There are also

CHAPTER. VII.

THE DOCTRINE OF PROPORTION.

THE brain being the instrument of thought and feeling, these, when manifested, are to be practically treated by the same laws, as if they were mere functions of the brain; and the brain being only a part and portion of the whole human frame, we must not estimate its powers of manifestation, independently of certain conditions of the whole system.

1. Then we must regard proportion, as a great indication of both strength and the tendency to well-regulated activity.

We have already observed, that the brain requires a large portion of pure oxygenated blood, and that without this its powers are inert. It will be readily perceived, that, if the brain be too large for a perfect proportion to the body, one of two things must occur; either that the brain will not receive its wonted stimulus, or that it will deprive the rest of the system of a portion of what belongs to it, and enfeeble it; either of which conditions would indicate deranged proportion and consequent weakness. Men of very large brains and feeble bodies may be enabled, under strong excitement, to perform great things occasionally; but this will be followed by subsequent lassitude or disease.

Again, if the blood predominate strongly in the system, and be of a highly stimulating character, and the brain be small, then the brain is too highly stimulated