The cerebellum is situated at the basis of the cerebrum towards its posterior part. (Fig. ii. 6, G.) Its form is elliptical, its largest diameter extending transversely from one aide to the other. (Fig. ii. 6.) Like the cerebrum it is divided into two lateral halves or hemispheres ()ig. ii. 6), which are separated by the falx cerebelli. In the centre of its upper surface there is a distinct prominence, termed the vermiform process (fig. ii. 7), which may be considered as the fundamental part of the organ, because, in the lower animals, whatever other parts of the cerebellum are absent, this is invariably present, affording thus the nucleus or rudiment of the organ, from which, by the addition of other parts, as the hemispheres or lateral lobes, kes, the more perfect organ of the higher animal is built up.
The external surface of the cerebellum is divided iuto flattened strata or layers (fig. ii. 6), separated by fissures which correspond to the clefts or sulci between the convolutions. The pin mater, bearing the nutrient arteries of the cerebellum, passes between every one of these fissures ; while the arachnoid membrane in simply extended over them. If a vertical section be made through either hemisphere of the cerebellum, a thick mass of white substance is seen in the centre, which, as it divides into the several strata, presents an arbor escent appearance commonly denominated the arbor vitae. (Fig, iii. 3.) These strata diverge towards the circumference of the cerebellum, and are covered externally by gray substance. (Fig. iii. 3.) In front of the cerebellum is placed a large mass of nervous matter, forming a very considerable eminence, commonly termed the tuber annulare, or the pone varolii. (Fig. ii. 10.) The external surface of this body is convex, and it is divided into two lateral halves by a middle groove. (Fig. ii. 10.) It is joined to the cerebrum by two flick white cords named tho crime, cerebri, and to the cerebellum by two similar cords named the crura cerebelli. The erura cerebri aro continued (from the tuber) outwards and forwards to the under and middle part of each hemisphere of the cerebrum, in which they are lost. In like manner the erura cerebelli are continued outwards and backwards into the hemispheres of the cerebellum, in which they tenuinate.
The 3Iedulla Oblongata is that portion of the cerebral mass which intervenes between the tuber annulare and the foramen (fig. H. 7): beyond the foramen magnum it takes the name of spinal
cord. On the anterior surface of the medulla oblongata there are four eminences contiguous to each other. (Fig. ii. 7.) The two internal are named corpora pyramidalia, or the pyramids (fig. ii. 8) • and the two external the corpora olivaria (fig. ii. 9), or the olivary If the membranes which invest the medulla oblongata are carefully removed, and its middle groove be gently drawn asunder, there will be discovered four or five bands of white substance ascending obliquely from one side of the medulla to the other. (Fig. ii. 11.) These bands on each side decussate, some of them passing above and others below those of the other side, so that they are interwoven like plaited straw. (Fig. ii. 11.) These bands are named the decussating bands of the corpora pyramidalia, and their decumation is conceived to explain the phenomenon familiar to the physician and surgeon, that when injury is done to one side of the brain a consequent disturbance of function is manifested on the opposite aide of the body.
Taken as a whole, the nervous mass constituting the brain is strictly symmetrical, that is, the different parts of which it is composed are so arranged, that, if the organ be supposed to be divided into two lateral halves by a plane passing perpendicularly through its centre, the parts placed on each side of this plane have a perfect correspond ence with each other, and form in fact reduplications of each other. (Fig. E.) The principal parts of the cerebral malts are thus double, but they are all united on the median line with their fellows of the opposite side. This union is effected by medullary halide of various sizes and figures which pass from ono to another, called cominissurca Thus the double parts of the cerebellum are united by means of the large mass of cerebral matter already spoken of under the name of tuber annulare, or pons varolii. (Fig. ii. 10.) The hemispheres of the cerebrum are united chiefly by a broad expansion of medullary matter, which extends transversely across from the bottom of one hemisphere to that of the opposite side, called the corpus callosum, or the great coininissure of the brain. (Fig. iii. 6, 6.) There are other connecting bands of smaller size by which minor portions of the cerebral mass are placed in communication, into a description of which it is not necessary to enter here.