Brain

Brain. The term applied to that portion of the central nervous system which lies within the cavity of the skull. At its upper limit the spinal cord is continuous with the medulla oblongata or bulb which passes upwards through the foramen magnum into the cranial cavity. On the dorsal aspect of the medulla lies the cerebellum, and above the limit of the bulb on the ventral aspect are seen the transversely running fibres of the pons Varolii. Anterior to the pons the two crura cerebri diverge outwards passing into the cerebral hemispheres.

The brain, like the spinal cord, has three enveloping membranes, dura mater, arachnoid, and pia mater; the interval between the two latter is called the subarachnoid space, and is filled by the cerebro-spinal fluid. This fluid serves as a kind of packing material by which the delicate nervous structures are shielded from injury; in particular an accumulation of it at the base of the brain forms a sort of water cushion for its support. Another function of the cerebro-spinal fluid is to adapt the volume of the cranial contents to the unyielding walls of the cavity of the skull. When the amount of blood circulating in the brain is at a maximum, the quantity of cerebro-spinal fluid within the skull is at a minimum: and if on the other hand the supply of blood to the brain diminishes, an increased amount of fluid accumulates in the subarachnoid space, and so compensates for the difference in the bulk of the anaemic as compared with the hyperaemic brain. The fluid of the subarachnoid space is in direct communication with the fluid occupying the central canal of the spinal cord.

The weight of the brain of an adult man averages about 50 oz., that of an adult woman about 45 oz. The human brain is heavier than that of any other animal, the elephant and whale excepted. The proportion of brain-weight to body-weight is also greater in man than in the rest of the animal kingdom, with one or two exceptions among small birds and small monkeys. The relation between brain-weight and intelligence is however not one which can be insisted upon. Probably the extent of infolding of the convolutions of the cerebral hemispheres is a factor of more importance than actual weight in highly developed brains.

The cerebral hemispheres form the main bulk of the human brain; they are divided up by fissures into five lobes on each side, frontal, parietal, occipital and temporo-sphenoidal, with the island of Reel. These lobes are further subdivided into convolutions, by secondary fissures. The most important fissures are the Sylvian, between the parietal lobe, above, and the temporo-sphenoidal below, the fissure of Rolando on the outer aspect, of the parietal lobe, and the parieto-occipital separating the parietal and occipital lobes on the median aspect of the hemisphere. A section of a cerebral hemisphere shows a mass of white matter ensheathed by a thin outer envelope, or cortex, of grey matter. This grey matter follows all the undulations of the convolutions, and thus the more furrowed by fissures a brain is, the larger is the area of grey matter exposed on its surface.

Microscopical examination shows the white matter to be made up of medullated nerve fibres, while in the grey matter numerous ganglion cells are found.

Running across the bottom of the fissure which separates the two hemispheres is the great white commissure, called the corpus callosum. A horizontal section of the brain made just below this structure reveals the so-called basal ganglia, the corpora striata anteriorly, and the optic thalami posteriorly. The two last-named bodies lie on each side of a cavity, called the third ventricle. This cavity communicates in front through the foramen of Munro with the lateral ventricles, which lie one in either hemisphere: behind it is in communication through the aqueduct of Sylvius or iter a tertio ad quartum ventriculum with the fourth ventricle, which lies on the dorsal aspect of the medulla oblongata. There is yet another cavity, that of the fifth ventricle (of different origin to the other ventricles), placed in the septum luciduma partition separating the two lateral ventricles from one another.

Sections made through the basal ganglia reveal certain important structures. The corpus striatum proves to consist of two main masses of grey matter, the nucleus caudatus near the middle line, and the nucleus lenticularis externally. Bounded internally by the nucleus caudatus in front and the optic thalamus behind, and externally by the nucleus lenticularis, is a portion of white matter, called the internal capsule, which presents an anterior limb and a posterior limb, united at an obtuse angle forming a bend, called the genu or knee of the capsule. The posterior limb of the internal capsule is now known to form the route by which motor impulses coming from the cerebral cortex pass downwards on their way to the crura cerebri, pons, medulla and spinal cord. Outside the nucleus lenticularis is another tract of white fibres, the external capsule, bounded externally by a stratum of grey matter, called the claustrum, while outside this, again, is the white matter abutting on the convolutions of the island of Reil.

Immediately posterior to the third ventricle, and beneath the posterior end of the corpus callosum, is the pineal body, and below the third ventricle, visible on the inferior aspect of the brain, is the pituitary body. Just behind and below the pineal body are the corpora quadrigemina, which are concerned with visual sensations and are the homologues of the optic lobes of lower vertebrates.

The Cerebellum consists of an elongated central lobe and two lateral hemispheres. The cerebellum is connected with adjoining structures by means of three pairs of peduncles, the superior peduncles pass upwards and inwards to the cerebrum, the inferior peduncles downwards and inwards to the medulla, and the middle peduncles communicate with the pons. The cerebellum, like the cerebrum, contains white matter internally, with an external grey cortex; in the latter are found peculiar ganglion cells, known as the cells of Purkinje.

The Medulla oblongata connects the brain with the spinal cord; just above the cord on the inferior aspect of the medulla is seen the pyramidal decussation, formed by the crossing over of medullated nerve fibres from the anterior pyramids of the medulla on their road to the lateral columns of the cord, the right and left anterior pyramids going to the left and right lateral columns respectively.

The central grey matter of the medulla is exposed on the upper surface by the opening up of the central canal of the cord into the fourth ventricle. In this grey matter lie important nerve nuclei, constituting the origin of cranial nerves from the fifth to the twelfth. Several outlying portions of grey matter are also found, the largest of which is known as the olivary body.

Functions of the Brain. The Cerebral Hemispheres. The evidence with respect to the functions of these complex structures comes mainly from two sources - experiments upon lower animals and the study of disease in man. With regard to the former it is necessary to refer to the effects of removing the cerebral hemispheres, and to the evidence with respect to localisation of function derived from electrical stimulation of the cerebral cortex.

Removal of the hemispheres in a frog or pigeon reduces the animal to a kind of automaton: it is capable of performing complex movements in response to external stimuli, but if left undisturbed remains motionless and apparently devoid of all power of volition. In animals of higher development the shock produced by an operation of such magnitude is too great to admit of recovery. As regards electrical stimulation, Fritsch and Hitzig showed in 1870 that the application of a galvanic current to certain parts of the cortex of one side was followed by movements of the opposite side of the body. Their results have been extended by Ferrier and others, and the result of recent work has been to map out certain parts of the grey matter into areas, stimulation of which causes definite muscular movements. In the monkey's brain the motor centres of the cortex, as they are called, are situated on each side of the fissure of Rolando, on the convex surface of the hemisphere, the centres for the face lying lowest down, then those for the arm, and uppermost those of the leg; the muscles of the left-hand side of the body being represented in the right hemisphere, and vice versa. Further centres concerned with sight, hearing, taste, and smell, etc., have been described. To turn now to the teaching of disease in the human subject. Aphasia (q.v.) has long been associated with injury of a particular portion of the cerebial cortex, and it was noted from time to time that lesions of certain portions of the cortex were accompanied by palsies of definite muscles, or groups of muscles. Again, Dr. Hughlings Jackson traced certain convulsive phenomena to localised disease of grey matter (Jacksonian epilepsy). The two sets of facts, pathological and experimental, are found to be in the main confirmatory of one another, and by comparing the convolutions of the human brain with those of the monkey, and collecting the evidence obtained from post mortem examinations in man, it has been found possible to acquire a knowledge of cortical topography, which has been put to practical use in the treatment of disease. Of late years, in fact, it has been found possible, in several instances, to form an opinion as to the seat of the lesion from the symptoms of the patient, and the skull has been trephined and the mischief actually remedied by surgical treatment.

It must, of course, be remembered that only a comparatively small part of the cortex has been, so to speak, "used up" in this scheme of localisation. A large portion, for example, of the grey matter of the frontal lobes is apparently insensitive to electrical stimuli, and extensive disease of the frontal lobes has been noted without any ascertained associated defect.

The Cerebellum is probably concerned to a large extent with the co-ordination of muscular movements; thus tumours of the cerebellum are associated with a peculiar staggering gait, and removal of the cerebellum in animals causes marked incoordination.

The Medulla, besides serving as a link between the cord and brain, has most important relations with the respiratory and circulatory mechanisms. The whole brain above the medulla may be removed in animals, and respiration and life still continue, while, on the other hand, injury of a certain limited region in the medulla, wdiich has been called the "noeud vital" (vital knot), produces instant death. Again, most important nerves originate in the medulla.

Diseases of the Brain. (For the results of violence see Head Injuries, for inflammation of the membranes of the brain see Meningitis.) Hydrocephalus, insanity, and certain general and functional cerebral diseases are treated of in separate articles, e.g. Chorea, Tetanus, Epilepsy, Headache, Hydrophobia, Alcoholism, etc., etc. It is necessary here to speak of the general symptoms pointing to disease of the brain, and of certain organic diseases, viz. Haemorrhage, Softening, Abscess, Tumour. Brain-fever is a term used popularly to denote any disease in which delirium and fever are prominent symptoms.

Symptoms suggesting intracranial disease are: - Hemiplegia (q.v.), Convulsions (q.v.), Loss of consciousness and Apoplexy (q.v.), Headache (q.v.), Giddiness (q.v.), Delirium (q.v.), Aphasia (q.v.). Mental symptoms, Vomiting, and affections of cranial nerves, particularly Optic neuritis (q.v.); moreover, fever may be present, and certain characters of pulse and respiration (see Cheyne Stokes On Breathing) suggest cerebral mischief. It is important to note that some symptoms point merely to disease in some part of the brain, while others are of value in localising the actual seat of disease. Thus a diagnosis of cerebral tumour may rest on the presence of the three cardinal symptoms of that disease - headache, vomiting, and optic neuritis, while it may be further possible to indicate where the tumour is, from the associated aphasia, or hemiplegia, or convulsions, or in-coordination of movement, and so on, which may be also present.

Haemorrhage. The most characteristic symptoms of this disease are sudden loss of consciousness with hemiplegia. The most common seat of haemorrhage is the corpus striatum, but the cortex, pons, or other parts may be the site of the lesion. Haemorrhage is much more common after 40 years of age than in younger subjects, and is particularly apt to be associated with granular disease of the kidney, gout, and alcoholism. The longer the initial unconsciousness is prolonged, the less, as a rule, is the chance of recovery, and if there is no sign of improvement after the lapse of twenty-four hours the case usually terminates fatally.

Softening of a portion of brain substance sometimes occurs from occlusion of a blood-vessel (usually an artery) and consequent interference with the circulation. The blocking of the artery may be clue to the lodgment in it of a plug brought by the blood-stream from a distance, or to the formation of a clot in situ. The first condition is spoken of as Embolism (q.v.), the second as Thrombosis (q.v.). The most common cause of embolism is a diseased condition of the valves of the heart, particularly in the affections known as ulcerative endocarditis and mitral stenosis. A thrombus may originate from disease of the arterial wall. The symptoms of softening closely resemble those of haemorrhage; the diagnosis between the two conditions is however often possible from an examination of the condition of the heart and blood-vessels. The term "softening" is popularly applied to almost any species of intracranial disease.

Abscess. A collection of pus in the substance of the brain is occasionally met with as the result of disease of the bones of the skull, particularly in association with ear disease. The possible supervention of this grave condition is one very adequate reason for treating with the greatest care all cases of "discharge from the ear." Aural mischief is too apt to follow after certain "children's diseases," and the importance of not making light of deafness and purulent discharge from the ear in such cases cannot be too forcibly insisted upon.

Tumour. New growths occasionally develop in the brain. The forms of most common occurrence are cheesy tubercular masses, syphilitic gummata and glioma (q.v.). Tubercle usually affects the cerebellum. The symptoms of intracranial tumour have already been briefly alluded to.