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convolutions, viz. : the posterior central, the parietal superior, the inferior, and superior parietal arches. The temporal lobe has three quite definite lines or fissures, and four convolutions : the superior and inferior, the middle, and vertical, The occipital lobe is nearly triangular in form, and also has four fissures and four convolutions, viz. : the arcus occipitalis, and the three convolutions of Ecker.

Proceeding on a radical and developmental basis, the brain has been otherwise divided into the prosencephalon-the fore-brain; the thalamencephalon-the between-brain; the mesencephalon—the mid-brain; the epencephalonthe hind-brain; the metencephalon—the after-brain, or the oblongata. The first of these, or the great brain, will be referred to hereafter. The 'tween brain includes, as its most important parts, the optic thalami, the optic tracts, the corpora mammillaria, the two corpora geniculata, and the third ventricle. The mid-brain contains the great cerebral peduncle, and the corpora quadrigemina. The hind-brain,

, or the cerebellum, comprises a large body of crossing fibres and is covered, like the fore-brain, with grey matter. Numerous superficial fissures cover its external surface; they are irregularly arranged and of unequal depth, and serve to divide it into several more or less distinct lobes. These fissures impart to the organ its characteristic appearance. The after-brain, or the medulla oblongata, is the intermediate body uniting the spinal cord and the brain, and measures about three cm. Its dorsal surface is nearly covered by three sulci which extend up from the cord, and serve to bring into view more or less distinctly certain prominences, the most prominent of which is the inferior olive.

There are two kinds of matter which enter largely into the constitution of the brain, and which are quite distinct from each other in appearance and character—the grey and

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a. Gyrus fornicatus.
b. Corpus callosum.
6. Septum lucidum,
d. Columnar fornicis.
e. Corpus striatum.
f. Stria terminalis.
g. Thalamus opticus.
h. Pulvinar.
i. Brachium conjunctivum anti-

cum.
j. Peduncular cerebri.

k, l, m. Crus cerebelli, n. Origin of the abducens. 0. Ala cinerea. p. Obex. 9. Funiculus gracilis. r. Funiculus cuneatus. s. Clava. t. Calcar avis. u. Hippocampus. V. Caudate nucleus. w. Corpus geniculatum mediale.

-r. Thalamus opticus. y. Capsula interna. 2. Claustrum. 1. Nucleus lentiformis. 2. Island of Reil. 3. Capsula externa.

interna. 5. Caput nuclei caudati. 6. Cornu anticum,

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white. The characteristic element of the former is the nerve cell, of the latter medullated fibres; while the function of the former is the origination or rather the transformation, and that of the latter the transmission of nerve impulses. Grey matter covers the whole exterior and upper convex surface, and is termed the cortex cerebri. It is somewhat unevenly arranged; is enclosed in two or three membranes on the external surface, and passes into the different sulci and the median division, until it reaches the commissural fibres.

This cortex, composed so largely of grey matter, is of the highest significance to the student of mental diseases, as it constitutes the great centre to which all influences or im: pressions are radiated from the periphery, and from which they are again reflected outward. It comprises those combinations of cells, fibres, and blood vessels, whose physiological function constitutes so important an element in the evolution of mental activities. It, with other agglomerations of ganglia composed of grey matter, constitutes the sub-structure, the physiological derangements and changes of which produce the several genera of mental disorders which are grouped under the term insanity. Its depth varies, as does also the depth of the sulci or fissures which intersect it, in different orders of animals and in different individuals of the same order.

The numerous and intricate sulci into which the grey matter dips, serve to very largely increase its extent of surface, the material being, as it were, doubled up on itself; whereas, if the grey matter was simply extended in a smooth form along the concave cavity of the cranium, a greatly less extended surface of it could be enclosed within. In the monkey and the higher orders of the mammalia, below the genus homo, these sulci are much more rudimentary, and only in the human species do they become complicated and fully pronounced. This fact appears to confirm the theory that in some manner the higher degree of intelligence depends upon the superficial area, as also upon the depth of the grey matter. This view is further strengthened by the fact that in imbeciles, idiots, and some classes of criminals, these sulci are much more superficial than in the higher and more intellectual classes of persons.

In passing downward from the superior surface of the grey substance comprising the cortex, we perceive that it varies to some extent in depth in the different regions of the brain, and is arranged in more or less distinctly-marked concentric layers. (Meynert. In the convolutions are found no less than five of these layers, the first of which is made up largely of connective tissue, with a small number of quite irregularly shaped cells scattered through it, but which are found more often near the external surface of the layer, and with extensions downward. In it are also found fine medullated and non-medullated nerve fibres and bloodvessels.

The second layer is much more sharply bounded, and is filled with large numbers of a peculiar triangular or pyramidal shaped cell, which is termed the nerve cell. These cells vary considerably in size, and have small processes which radiate from their sides and angular points. In form and general appearance they differ from the cells of any other portion of the system, and all have their apices turned toward the surface of the convolutions, and are arranged in layers more or less parallel with each other. They are smaller than those in the layer next below, and have been called the “small pyramids."

In the third layer these cells become four or five times larger than those of the second layer, with a diameter from

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SECTIONS THROUGH THE CORTEX CEREBRI, SHOWING THE CELLS OF THE

DIFFERENT LAYERS.

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