THE BRAIN AND ITS MEMBRANES. 785 



between them and the inner margins of the crustae or peduncles of the cerehrum 

 (mira cerebri), is a lozenge-shaped interval, the interpeduncular space, in which 

 are found the following parts : the tuber cinereum, infundibulum, pituitary body, 

 corpora albicantia, and the posterior perforated space. 



The remaining structures above enumerated have all been previously described, 

 each in its own region. 



Structure of the Hemispheres. 



Each hemisphere is made up of gray and white matter. The latter constitutes 

 nearly the whole of the deeper portion (medullary centre), and enters into the 

 .structure of the convolutions. The gray matter covers in the convolutions, form- 

 ing the cortex of the hemisphere, and also is collected into three masses or nuclei 

 situated in the hemisphere the corpus striatum, the claustrum, and the nucleus 

 amygdalce. These last might be regarded as subdivisions of one large nucleus, 

 since they are more or less connected in the anterior perforated space. 



The WHITE MATTER OF THE HEMISPHERES consists of medullated fibres, vary- 

 ing in size and arranged in bundles, separated by neuroglia. They may be 

 divided into three distinct systems, according to the course which they take : 

 1. Projection fibres, which connect the hemispheres with the medulla oblongata 

 and cord. 2. Transverse or commissural fibres, which connect together the two 

 hemispheres. 3. Association-fibres (Meynert), which connect different structures 

 in the same hemisphere. 



1. The projection or peduncular fibres consist of a main body, which originates 

 in the cord and medulla oblongata, forms the longitudinal fibres of the pons, which 

 last are then continued up into the mid-brain, where, as has been before described, 

 the fibres are arranged in two strata, which are separated by the locus niger, the 

 ventral or superficial stratum forming the crusta, and the dorsal or deeper stratum 

 the bulk of the tegmentum. The fibres derived from these two sources take a dif- 

 ferent course, and will have to be separately considered. 



The fibres of the crusta are derived from the pyramid of the medulla, which 

 fibres are continued upward through the pons to form the crusta ; they are rein- 

 forced in their passage through the crus by accessory fibres derived from the cen- 

 tral gray matter around the Sylvian aqueduct, from the nuclei pontis, and from 

 the locus niger (see page 742). Most of the fibres of the crusta (except the 

 mesial fillet, p. 742) pass into the hemisphere as part of the internal capsule, 

 which last, passing upward, diverges into fibres which radiate forward, upward, 

 and backward, thus constituting the corona radiata, Each fibre of this last- 

 named structure proceeds to the corresponding portion of the cortex, where it 

 becomes the direct prolongation of an axis-cylinder process of a pyramidal cell 

 (see below). Some, if not all, of the fibres of the internal capsule give off col- 

 laterals to the optic thalamus and the nucleus caudatus and lenticularis of the 

 corpus striatum. 



From these ganglia (see pages 747, 760) there are also fibres which proceed 

 into the internal capsule and the corona radiata, thus forming parts of each in 

 addition to the fibres from the crusta. The fibres which arise from the ganglia 

 are more numerous than those which terminate in the ganglia, so that more fibres 

 pass out of the ganglia than pass into them. 



The fibres of the tegmentum are continuous with those longitudinal fibres of 

 the pons which are derived from the formatio reticularis of the medulla (which 

 see), including also fillet (per corpora quadrigemina) and posterior longitudinal 

 bundle. They are reinforced by fibres from the corpora quadrigemina and cor- 

 pora geniculata, and from the superior peduncle of the cerebellum. Superiorly, 

 some are lost in the subthalamic region, while others enter the optic thalamus and 

 terminate in its gray matter, whence they are continued into the internal capsule 

 as the various bundles of fibres which have been already referred to both in the 

 description of the optic thalamus and just above. Thus, the tegmental fibres 



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