larities or interruption of the supply, for such is the importance and delicacy of this organ that any great increase in the quantity of blood sent to it, or any interruption of the supply, would result in immediate death. There are four large arterial trunks which serve to convey the blood to the brain-the two internal carotids and the two vertebral. So large are these vessels, and so ample their means of intercommunication, that even if the circulation in one of them be impeded or stopped, the others may supply its place with little diminution of the general flow. The veins do not accompany the arteries here, as in other parts of the body, but are widely dispersed, and ultimately form large, tortuous canals.

From the position of the brain in the interior of a hard, bony case, it is evident that any sudden increase in its volume, either from an over-supply of blood or any other cause, would be productive of very serious consequences to its delicate structure, were not special provision made in order to meet it. This is done by means of a fluid which exists between the layers of the arachnoid membrane in the brain and spinal cord, known as the cerebro-spinal fluid. The amount usually averages about 2 oz., but in cases of atrophy or wasting of the brain as much as 12 oz. may be found, while in cases where the brain has undergone an increase there is either a diminution or a total absence of this fluid. Its presence also doubtless has the effect of preventing an undue expansion of the blood vessels.

The weight of the entire mass of brain differs considerably in different individuals. In the adult male it usually ranges between 40 and 60 oz., the average being about 50, but in some cases it reaches as high as

64 or 65, and in others it is as low as 33 or 34 oz. In the adult female the weight usually ranges from 36 to 50 oz., the average being about 45, but it has been found as high as 56 and as low as 31. In cases of idiocy the amount is sometimes much below this, and as low a weight as 20 oz. has been recorded.1 The average weight of the brain in proportion to that of the body is about 1 to 36. As regards the weight of different parts of the brain, the cerebrum averages in the adult male nearly 44 oz., and in the adult female 38; the cerebellum in the male about 51, and in the female about 4; the pons varolii and medulla oblongata together about 1 oz., being rather larger in the female than in the male.

Physiologists differ as to the time when the brain attains its full size. Sir William Hamilton and others place it as early as the seventh or eighth year. Gall and Spurzheim, and phrenologists generally, maintain that the brain continues to grow till about the fortieth year, and this appears to be nearest the truth. "It appears that in general the weight of the brain increases rapidly up to the seventh year, then more slowly to between sixteen and twenty, and again more slowly to between thirty-one and forty, at which time it reaches its maximum point. Beyond this period there appears

1" When the weight of the brain falls below a certain minimum standard, the possession by its owner of anything like ordinary human intelligence seems to be impossible. Gratiolet, without specifying the sex, supposed this lower limit of weight to be about 312 oz. Broca places it somewhat higher, fixing upon 32 oz. as the limit for the female and 37 oz. as the lower limit of weight for the male brain compatible with ordinary human intelligence. The brain-weight of idiots may, however, and frequently does, fall far below the limits above assigned."-Dr. Bastian. "It would appear that in general throughout the human race no brain under 30 oz. is sufficient for the exercise of its normal functions."-Dr. Allen Thomson.

a slow but progressive diminution in weight of about one ounce during each subsequent decennial period." -Quain's Anatomy. We have here a physiological basis for what is popularly believed to be a fact, that the mind reaches its maximum about the age of forty, and that no great work has been accomplished by any one after that age, the basis of which was not laid before.

The spinal cord forms the means of communication between the brain and most other parts of the body. Through it impressions made on the sensitive spinal nerves are conveyed to the brain, and through it, too, the stimulus sent from the brain is conducted to the motor spinal nerves in order to excite the action of the muscles. It may also be regarded as a series of ganglia or nervous centres united together in one continuous column. In consequence of its continuity and its direct connection with the brain, most of its operations are carried on consciously, and are under the control of the will; but at the same time each of the nervous centres of which it is composed is capable of receiving impressions and giving out stimuli independently of the will or of consciousness. Hence if any part of the cord be destroyed, and communication with the brain be thereby interrupted, sensation and voluntary motion will cease in those parts connected with the cord below the seat of the injury, but the detached portion of the

1 "From forty to fifty years there is a slight diminution in weight and a greater one between fifty and sixty. After sixty years the rate of decrease is still greater; the process of decay becomes more and more rapid, and thus in the eighth decade of existence the average weight of the brain is less by more than 3 oz. than it was in the fourth decade. In the aged, on the average the weight of the brain decreases pari passu with the intelligence."-Dr Thurnam.

2 “Every segment of the spinal cord may act separately as an independent centre; every segment is a little brain."-G. H. Lewes.

cord will still retain the properties of an ordinary nerve centre and respond to stimuli acting on it.1

The spinal cord is composed of both white and grey nervous matter, the former situated externally, and constituting its chief portion, the latter disposed in the interior. When the cord is cut transversely, the grey substance is seen arranged in the form of two crescental masses, one occupying each lateral half, and the two being connected together by a transverse band of the same material.2 The posterior horns are long and narrow, and approach the surface, while the anterior are considerably shorter and thicker. The cord is divided longitudinally into two exactly symmetrical halves by an anterior and a posterior fissure, the two halves being united in the middle by a commissure. The posterior fissure is deeper but less wide and distinct than the anterior. Each half is marked on the sides. by two longitudinal furrows which divide it into three portions or columns,-an anterior, a middle or lateral, and a posterior. The entire cord is, like the brain, enveloped in three membranes,—an outer, the dura mater; an inner, the pia mater; and the arachnoid membrane between the two.

The nerves of the spinal cord consist of thirty-one pairs issuing from the sides of the cord through its whole length, and corresponding in number to the interverte

1 "When the spine is injured or divided there are two centres, and the parts above are not sensitive to impressions on the parts below; nor are the parts below sensitive to impressions on the parts above; but each segment is sensitive to its own affections. The paralytic man does not feel the prick on his leg, but his leg feels it." -G. H. Lewes.

2 "The cells are larger and more numerous in the anterior grey matter of the spinal cord than in the posterior. They are of various sizes, but remarkably multipolar and intimately connected with each other and with the roots of the spinal nerves by means of their numerous branching processes."-Dr. Laycock.

bral foramina through which they pass. Each nerve arises by two roots, an anterior and a posterior, the latter being the larger. The anterior root springs from the groove between the anterior and lateral columns of each half of the cord, and contains only efferent or motor fibres; the posterior rises just in front of the groove between the lateral and posterior columns, and consists entirely of afferent or sensory fibres. "The roots emerge through separate apertures of the sheath of the dura mater surrounding the cord, and directly after their emergence, while the roots lie in the intervertebral foramen, a ganglion is formed on the posterior root. The anterior root lies in contact with the anterior surface of the ganglion, but none of its fibres intermingle with those of the ganglion. But immediately beyond the ganglion the two roots coalesce, and by the mingling of their fibres form a compound or mixed spinal nerve which, after issuing from the intervertebral canal, divides into an anterior and posterior branch, each containing fibres from both roots."Kirkes' Physiology.

Of the root fibres of the nerves, some pass horizontally or obliquely into the substance of the cord, while others proceed upwards or downwards in it. Many of them may be traced into the central grey substance and terminate there; and the experiments of Brown-Sequard make it probable that the grey substance is the channel by which sensitive impressions are conducted to the brain. He has shown that the posterior columns are not, as was previously generally supposed, the course which they follow, but that they pass into the grey substance, through which alone, or aided by the anterior columns, they reach the brain. Further, he has proved that impressions are not con