THE EXTERNAL BA SAL GANGLION :—The gray matter external to the internal capsule, and enclosed by its funnel shaped concavity, is the lenticular ganglion, so called because its parts are disposed in lenticular layers. Its layers are super imposed and separated from each other by lamime of white fibres, called capsules. A transverse section of the entire mass of the lenticular body, including the parietal and tem poral portions of the internal capsule, is triangular. The apex of the triangle is directed downward and inward, and its base is opposed to that of the insula. It will be seen that the lenticular nucleus is bounded above and below by the internal capsule and is in three layers or parts. The outer sur face of each layer is convex and composed mostly of gray matter, while the inner portion of each section is traversed by numerous white tracts for some distance from the septum or capsule. The three bodies which comprise the lenticular nucleus increase in size and thickness from within outwards. The innermost is a conical wedge spreading the fibres of the internal capsule at the bottom of its funnel shaped concavity. The inner and middle portions are together called the globus pallidus, on account of being much paler in color than the outermost, which is called the putamen. Externally the outer body or portion of the lenticular nucleus is covered by a thick layer of radiating white fibres which diverge from below upwards.
The layer of diverging fibres external to and overlying the lenticular body, forming the external capsule, is thicker above than below, and is united with the outer surface of the internal capsule above and along the upper margin of the lenticular nucleus The external capsule is covered upon its outer surface by a thin layer of gray matter which is thicker below than above, and is called the claustrum, (a wall,) external to which is the subinsular layer of white matter, and the insula. These parts therefore enclosed by the funnel
shaped whorl of the fibres of the internal capsule are, from within outward, the three divisions of the lenticular nucleus, the external capsule, claustrum and the white and gray matter of the insula, the latter being enclosed in the Y shaped expansion of the bottom of the fissure of Sylvius.
The internal capsule will be studied in detail subsequent to the description of those parts below which enter into its formation. It is sufficient at this time to remember its shape, its relations to the basal ganglia and that fibres from these ganglia contribute to swell its volume.
Because of the funnel shape of the internal capsule, a horizontal section of the brain, as generally represented in diagram, shows an anterior and a posterior arm, and locates the motor portion of the capsule behind the knee in the anterior part of the posterior arm.
The hemispherical ganglion and lateral dissections of it will be described after its internal anatomy has been traced in a longitudinal direction. The crura cerebri or peduncles of the cerebrum, as before stated, consist of two portions, the crusta and the tegmentum. The crusta is a broad band of fibres upon the outer and anterior part of either emus cere bri. The tegmentum is all that portion of the peduncle behind and internal to the crusta. The crusta originates entirely from the pous Varolii, but the tegmentum is con nected with the cerebellum and with the posterior layer of the Pons which includes the fillet and fasciculi teretes, and through these with the spinal cord. The connections aud the relations of the various tracts which compose the peduncle of the cerebrum will be more clearly understood after the struc ture of the cerebellum and the pons Varolii have been studied. We will again return to the anatomy of the cerebrum in order to trace the parts of the peduncle to their connections with those of the hemisphere.