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THE STRUCTURE OF THE BRAIN.*
Embryological Sketch. The brain is a sort of pons asinorum in anatomy until one gets a certain general conception of it as a clue. Then it becomes a comparatively simple affair. The clue is given by comparative anatomy and especially by embryology. At a certain moment in the development of all the higher vertebrates the cerebro
spinal axis is formed by a hollow tube containing fluid and terminated in front by an enlargement separated by transverse constrictions into three 'cerebral vesicles,' so called (see Fig. 28). The walls of these vesicles thicken in most
* This chapter will be understood as a mere sketch for beginners. Models will be found of assistance. The best is the 'Cerveau de Texture de Grande Dimension,' made by Auzoux, 56 Rue de Vaugirard, Paris. It is a wonderful work of art, and costs 300 francs. M. Jules Talrich of No. 97 Boulevard Saint-Germain, Paris, makes a series of five large plaster models, which I have found very useful for class-room purposes. They cost 350 francs, and are far better than any German models which I have seen.
places, change in others into a thin vascular tissue, and in others again send out processes which produce an appearance of farther subdivision. The middle vesicle or midbrain (Mb in the figures) is the least affected by change. Its upper walls thicken into the optic lobes, or corpora quadrigemina as they are named in man; its lower walls become the so-called peduncles or crura of the brain; and its cavity dwindles into the aqueduct of Silvius. A section through the adult hu
man mid-brain is shown in Fig. 31.
The anterior and posterior vesi
cles undergo much more consider- PM
(Cb in all the figures) and the pons Varolii below (P. V. in Fig. 33). In its hindmost portions the posterior vesicle thickens below into the medulla oblongata (Mo in all, the figures), whilst on top its walls thin out and melt, so that one can pass a probe into the cavity without breaking through any truly nervous tissue. The cavity which one. thus enters from without is named the fourth ventricle (4 in Figs. 32 and 33). One can run the probe for
ward through it, passing first under the cerebellum and then under a thin sheet of nervous tissue (the valve of Vieussens) just anterior thereto, as far as the aqueduct of Silvius. Passing through this, the probe emerges forward into what was once the cavity of the anterior vesicle. But the covering has melted away at this place, and the cavity now forms a deep compressed pit or groove between the two walls of the vesicle, and is called the third ventricle (3 in Figs. 32 and 33). The aqueduct of Sylvius' is in consequence of this connection often called the iter a tertio ad quartum ventriculum. The walls of the vesicle form the optic thalami (Th in all the figures).
From the anterior vesicle just in front of the thalami there buds out on either side an enlargement, into which the cavity of the vesicle continues, and which becomes the hemisphere of that side. In man its walls thicken enormously and form folds, the so-called convolutions, on their surface. At the same time they grow backwards rather than forwards of their starting-point just in front of the thalamus, arching over the latter; and growing fastest along their top circumference, they end by bending downwards and forwards again when they have passed the rear end of the thalamus. When fully developed in man, they overlay and cover in all the other parts of the brain. Their cavities form the lateral ventricles, easier to understand by a dissection than by a description. A probe can be passed into either of them from the third ventricle at its anterior
end; and like the third ventricle, their wall is melted down along a certain line, forming a long cleft through which they can be entered without rupturing the nervous tissue. This cleft, on account of the growth of the hemisphere outwards, backwards, and then downwards from its starting point, has got rolled in and tucked away beneath the apparent surface.*
At first the two hemispheres are connected only with their respective thalami. But during the fourth and fifth months of embryonic life they become connected with each other above the thalami through the growth between them of a massive system of transverse fibres which crosses the median line like a great bridge and is called the corpus callosum. These fibres radiate in the walls of both hemispheres and form a direct connection between the convolutions of the right and of the left side. Beneath the corpus callosum another system of fibres called the fornix is formed, between which and the corpus callosum there is a peculiar connection. Just in front of the thalami, where the hemispheres begin their growth, a ganglionic mass called the corpus striatum (C.S., Figs. 32 and 33) is formed in their wall. It is complex in structure, consisting of two main parts, called nucleus lenticularis and nucleus candatus respectively. The figures, with their respective explanations, will give a better idea of the farther details of structure than any verbal description; so, after some practical directions for dissecting the organ, I will pass to a brief account of the physiological relations of its different parts to each other.
Dissection of Sheep's Brain.-The way really to understand the brain is to dissect it. The brains of mammals differ only in their proportions, and from the sheep's one can learn all that is essential in man's. The student is therefore strongly urged to dissect a sheep's brain. Full directions of the order of procedure are given in the human dis
* All the places in the brain at which the cavities come through are filled in during life by prolongations of the membrane called pia mater, carrying rich plexuses of blood-vessels in their folds.
secting books, e.g. Holden's Practical Anatomy (Churchill), Morrell's Student's Manual of Comparative Anatomy and Guide to Dissection (Longmans), and Foster and Langley's Practical Physiology (Macmillan). For the use of classes who cannot procure these books I subjoin a few practical notes. The instruments needed are a small saw, a chisel with a shoulder, and a hammer with a hook on its handle, all three of which form part of the regular medical autopsy-kit and can be had of surgical-instrument-makers. In addition a scalpel, a pair of scissors, a pair of dissecting-forceps, and a silver probe are required. The solitary student can find home-made substitutes for all these things but the forceps, which he ought to buy.
The first thing is to get off the skull-cap. Make two saw-cuts, through the prominent portion of each condyle (or articular surface bounding the hole at the back of the skull, where the spinal cord enters) and passing forwards to the temples of the animal. Then make two cuts, one on each side, which cross these and meet in an angle on the frontal bone. By actual trial, one will find the best direction for the saw-cuts. It is hard to saw entirely through the skull-bone without in some places also sawing into the brain. Here is where the chisel comes in-one can break by a smart blow on it with the hammer any parts of the skull not quite sawn through. When the skull-cap is ready to come off one will feel it 'wobble.' Insert then the hook under its forward end and pull firmly. The bony skull-cap alone will come away, leaving the periosteum of the inner surface adhering to that of the base of the skull, enveloping the brain, and forming the so-called dura mater or outer one of its 'meninges.' This dura mater should be slit open round the margins, when the brain will be exposed wrapped in its nearest membrane, the pia mater, full of blood-vessels whose branches penetrate the tissues.
The brain in its pia mater should now be carefully shelled out.' Usually it is best to begin at the forward end, turning it up there and gradually working backwards. The olfactory lobes are liable to be torn; they must be carefully scooped from the pits in the base of the skull to which they adhere by the branches which they send through the bone into the nose-cavity. It is well to have a little blunt curved instrument expressly for this purpose. Next the optic nerves tie the brain down, and must be cut through-close to the chiasma is easiest. After that comes the pituitary body, which has to be left behind. It is attached by a neck, the so-called infundibulum, into the upper part of which the cavity of the third ventricle is prolonged downwards for a short distance. It has no known function and is probably a rudimentary organ.' Other nerves, into the detail of which I shall not go, must be cut successively. Their places in the human brain are shown in Fig. 34. When they