in layers that are more or less distinguishable from each other. The stratification is best marked in the hippocampal region, which is characterised by a very regular layer of large nervecells; in other parts it is far less definite, so that it is almost impossible to identify each of the five layers of classical description (Meynert). A section of grey matter from a motor' convolution is, however, to some extent distinguishable by the prominence and number of large arrow-head cells (30 to 40 to 100 μ) in the deeper part of the socalled third layer; these are sometimes. characterised as motor,' and are considered to be the analogues of motor cells in the anterior cornua of the cord and of Purkinje's cells in the cerebellar cortex. It nowise follows, however, because a nerve-cell is large, that therefore it is motor, and as regards the various cells above alluded to, those of the anterior cornua are the only ones whose claim to the term motor may be regarded as established upon sufficient data. The motor' cells of the cortex have been so characterised because they are a prominent feature in the experimentally excitable area of the brain; other large cells, whether in the hippocampus or in TION THROUGH THE CORTEX the cerebellum, are not qualified by any OR MOTOR TYPE × 50 diam. such designation. And, as a matter of fact, it happens that the large-cell layers are most definite in the very regions (hippocampus, cerebellum) the functions of which are most indefinite, and in all probability not motor. The supposition that the motor' cells of large muscles, or of 'large movements,' or connected with long nervefibres, are large' as compared with motor cells of small muscles, or of small movements,' or of short nerve-fibres, is a speculation as yet unsupported by any actual proof. The total number of nerve-cells in the human brain has been estimated at 2,000 millions. FIG. 278.-TRANSVERSE SEC (Meynert.) 6 The vascular supply of the brain.-The arterial system of the brain is peculiar in two main particulars: (1) the large arteries by which it is supplied-two internal carotids and two vertebrals -form a free anastomosis at the base of the brain, known as the circle of Willis; (2) the branches that are given off from the circle of Willis, supplying the basal ganglia and the cortex, run their course to their several area of distribution with little or no anastomosis. It follows as a consequence of this disposition of the vessels, that the circulation in the brain suffers little or no change by obstruction on the cardiac side of the circle of Willis, and that, on the other hand, obstruction of a vessel beyond the circle of Willis causes damage which is confined to the area served by the obstructed vessel. The first statement is borne out by the experimental fact that, e.g. on dogs, it is not until all the four vessels leading to the circle of Willis are tied that symptoms of arrested cerebral circulation appear. The second statement is most clearly exemplified by the well-characterised group of clinical symptoms that are caused when the left middle cerebral artery is blocked by an embolus. Considered from the point of view of its vascular supply, the cortex of the brain is divisible into three regions, supplied by the anterior, middle, and posterior cerebral arteries respectively. The cortical territory fed by the middle cerebral, or Sylvian artery, which is the direct continuation of the internal carotid, is physiologically and pathologically the most important; it includes the convolutions in the vicinity of the fissure of Rolando, and those of the island of Reil, i.e. the so-called motor area and speech-centre. If the Sylvian artery is blocked (and in the great majority of cases this happens on the left side), the consequences are motor paralysis of the right side and loss of speech, i.e., in clinical language, right hemiplegia and motor aphasia. In addition to the cortical system formed by branches of the three cerebral arteries, other branches from them are supplied to the basal ganglia, and form the ganglionic system of arteries. Those given off by the middle cerebral are of greatest pathological importance, as being the vessels most apt to give rise to hæmorrhage; one branch in particular, the lenticulo-striate, has been called the artery of cerebral hemorrhage' (Charcot), because it is most liable to give way; the hæmorrhage derived from it may compress or rupture the internal capsule, and so cause hemiplegia. As regards the ultimate distribution of the cerebral capillaries, the most noteworthy point is, that the network is much closer and richer in grey matter than in white matter; the former is physiologically the more active, and requires to be more abundantly nourished with blood; a similar difference holds good in the case of the spinal cord, in well-injected sections of which the closer vascular network of the grey matter defines the pattern of the cornua in the midst of the scantily-supplied white matter. The return of blood from the cortex is chiefly effected by a superficial system of veins opening into the superior longitudinal sinus, and the fact that, whereas the current in the sinus is directed from before backwards, the veins opening into it are directed from below upwards, has been remarked upon as promoting venous stagnation, giving rise, therefore, to a peculiar liability to thrombosis. It was formerly a debated question whether or no the amount of blood can vary within the cranium; taking into account that the cerebral ventricles are in communication with the sub-arachnoid space and filled with a serous fluid, there can be no doubt that more or less blood may enter the cranium, expressing more or less serous fluid into the spinal canal. The sub-arachnoid and intra-ventricular fluid is, indeed, of great mechanical importance, forming as it does a bath within which the cerebrospinal mass is protected from accidental shocks and sudden pressure. The functions of the brain. That the brain is the organ of intelligent sensation and motion is proved by the facts of comparative anatomy already alluded to, and by common experience. The same proposition is established by clinico-pathological facts, and by the study of animals after removal of a hemisphere or of the cortex. Experimentally, we learn that after removal of the cortex an intelligent animal is reduced to the state of a non-intelligent automaton, responding indeed to stimuli, internal as well as external, but failing to interpret the significance of present events in accordance with bygone experience. A brainless dog is stupid; he may see a bone in front of his eyes without showing sign that he knows the meaning of a bone, or the use to which it may be put; he may hear the crack of a whip, but he no longer shows sign of fear, for he does not remember its sting; his former purposeful behaviour has entirely disappeared in short, he has lost memory and judgment. From clinico-pathological observations, as well as from experiment, we learn, moreover, that each hemisphere is the controlling organ of the opposite side of the body. Clinical cases of ordinary hemiplegia are every day found to be due to unilateral disease in the opposite hemisphere. Experimental extirpation or stimulation on one side of the brain is found R Cortex L the corpus callosum, and (3) numerous but scattered association fibres between the various convolutions. These last-named fibres are difficult to trace, least so, perhaps, in the gyrus fornicatus, in which there is a well-marked longitudinal system known as the cingulum. The vertical and horizontal fibres, derived respectively from the internal capsule and the corpus callosum, are also difficult to dissociate, being closely interwoven in their expansion through the corona radiata. Capsule L FIG. 276.-TO ILLUSTRATE THE reptiles 1 to 1,500 birds 1 to 220 mammals 1 to 180 1 to 120 man 1 to 50 ,, ourang But mere weight of brain is not a regular index of degree of intelligence in individual cases; there are many exceptions to the general rule. As regards man very similar considerations hold good, viz., taken in the rough, the brain-weight of distinguished men is above, while that of idiots is below, the average, and the brain-weight of civilised men is above that of savages. Yet there are many individual exceptions to the general rule. The average brainweight is: Of Europeans. ,, negroes 49 oz., or 1,390 grammes As regards sex, the brain-weight cannot be appealed to in evidence of superiority on either side. The average male European brain weighs 49 oz.; the average female European brain weighs 44 oz.; but the average body-weights in the two sexes |