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-vessels, 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.[28] [Illustration: FIG. 28. FIG. 29. FIG. 30. (All after Huguenin.)] =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 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 mid-brain (_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. [Illustration: FIG. 31.--The 'nates' are the anterior corpora quadrigemina, the spot above _aq_ is a section of the sylvian aqueduct, and the tegmentum and two 'feet' together make the Crura. These are marked _C.C._, and a cross (+) marks the aqueduct, in Fig. 32. ] [Illustration: FIG. 32 (after Huxley).]