BASAL GANGLIA 573 



are convolutions on the surface^ythe base of the brain and in the 

 gray matter by the sides of tl^^reat longitudinal fissure. In the fissure 

 of Sylvius, near its ascending branch, between the anterior and the 

 posterior lobes of the brain and beneath the third frontal convolution, is 

 a group of convolutions constituting the island of Reil, or insula. 



Fig. 140 shows the most important parts observed on the inner sur- 

 face of the left hemisphere. These parts do not demand any explana- 

 tion beyond that given in the diagram in outline. 



Basal Ganglia. The ganglia at the base of the brain are the 

 olfactory ganglia, the corpora striata, optic thalami, tubercula quadri- 

 gemina and the gray matter of the pons Varolii. The olfactory ganglia 

 will be described in connection with the physiology of the sense of 

 smell. The corpora striata and the optic thalami are important in their 

 relations to the internal capsule and the paths of motor and sensory 

 conduction. 



Corpora Striata, Optic Thalami and Internal Capstde. The corpora 

 striata are pear-shaped bodies, situated at the base of the brain, with 

 their rounded bases directed forward, and the narrower ends, backward 

 and outward. Their external surface is gray, and they present, on 

 section, alternate striae of white and gray matter. They present what 

 is called an intraventricular portion, projecting into the anterior part of 

 the lateral ventricles, and an extraventricular portion, which is embedded 

 in the white substance at the base of the brain. The corpora striata are 

 to be regarded as subsidiary centres connected with motion. 



The optic thalami are oblong bodies situated between the posterior 

 extremities of the corpora striata and resting upon the crura cerebri on 

 the two sides. These are white externally, and in their interior they 

 present a mixture of white and gray matter. They are subsidiary nerve- 

 centres connected with sensation. 



In a horizontal section made through the brain, involving the corpora 

 striata and the optic thalami, the corpora striata present a division into 

 two nuclei. These are the caudate nucleus, which is internal, and the 

 lenticular nucleus, which is external to and behind the caudate nucleus. 

 External to the lenticular nucleus, is a layer of white substance, the 

 external capsule, in which there is a band of gray matter, called the 

 claustrum. External to the external capsule, at its anterior portion, is 

 the insula, or island of Reil. 



Between the caudate nucleus and the lenticular nucleus in front, is 

 a broad band of white fibres, continuous with a band of white fibres 

 lying posteriorly, between the lenticular nucleus and the optic thalamus 

 on either side. This band is the internal capsule. The portion of the 

 internal capsule that lies between the caudate nucleus and the lenticu- 



