the cerebellum, and the teleology of the whole business would appear to be the maintenance of the upright position. If a man stand with shut eyes and attend to his body, he will find that he is hardly for a moment in equilibrium. Incipient fallings towards every side in succession are incessantly repaired by muscular contractions which restore the balance; and although impressions on the tendons, ligaments, foot-soles, joints, etc., doubtless are among the causes of the compensatory contractions, yet the strongest and most special reflex arc would seem to be that which has the sensation of incipient vertigo for its afferent member. This is experimentally proved to be much more easily excited than the other sensations referred to. When the cerebellum is disorganized the reflex response fails to occur properly and loss of equilibrium is the result. Irritation of the cerebellum produces vertigo, loss of balance, and nausea; and galvanic currents through the head produce various forms of vertigo correlated with their direction. It seems probable that direct excitement of the cerebellar centre is responsible for these feelings. In addition to these corporeal reflexes the sense of rotation causes compensatory rollings of the eyeballs in the opposite direction, to which some of the subjective phenomena of optical vertigo are due. Steady rotation gives no sensation; it is only starting or stopping, or, more generally speaking, acceleration (positive or negative), which impresses the end-organs in the ampullæ. The sensation always has a little duration, however; and the feeling of reversed movement after whirling violently may last for nearly a minute, slowly fading out.

The cause of the sense of translation (movement forwards or backwards) is more open to dispute. The seat of this sensation has been assigned to the semicircular canals when compounding their currents to the brain; and also to the utricle. The latest experimenter, M. Delage, considers that it cannot possibly be in the head, and assigns it rather to the entire body, so far as its parts (blood-ves

sels, viscera, etc.) are movable against each other and suffer friction or pressure from their relative inertia when a movement of translation begins. M. Delage's exclusion of the labyrinth from this form of sensibility cannot, however, yet be considered definitively established, so the matter may rest with this mention.

CHAPTER VII.

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 human mid-brain is shown in Fig. 31.

The anterior and posterior vesicles undergo much more considerable change. The walls of the posterior vesicle thicken enormously in their foremost portion and form the cerebellum on top

(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