NERVOUS SYSTEM 849 



developed. In these animals the corpus striatum seems to be a reflex 

 center of a higher order than the thalamus. 



There is doubt as to the exact function of the corpus striatum. 

 Ramon y Cajal thinks that in mammals, at least, this body functions as 

 a reinforcement center of the descending motor impulses coming from 

 the cortex, as these fibers give off collateral branches when passing 

 through the corpus striatum, while the striatum itself sends important 

 descending tracts into the thalamus and cerebral peduncle. 



The white matter consists of fibers that pass between the cortex 

 and deep parts of the brain-stem which have no functional connection 

 with the striatum itself. These are called projection-fibers, and are partly 

 ascending and descending fibers which pass between the thalamus and 

 the cortex, and partly descending motor projection-fibers of the cortico- 

 spinal or pyramidal tract, cortico-bulbar tract, and cortico-pontine tracts 



The gray matter of the corpus striatum forms the two nuclei named 

 after their shape the caudate and the lentiform nucleus (Fig. 475). Most 

 of the projection-fibers pass between these two nuclei in a wide band 

 of white matter which is called the internal capsule. These same fibers 

 radiating from the internal capsule toward the capsule are called the 

 corona radiata. 



The external capsule is formed of a thinner sheet of fibers external 

 to the lentiform nucleus. 



As many cases of apoplexies and other cerebral diseases cause 

 hemorrhage and other injuries in the internal capsule, there destroying 

 some of the fibers, the study of the exact arrangement of sensory and 

 motor projection fibers within the internal capsule is of great clinical 

 importance. 



Claustrum is the name given to the thin band of gray matter lying 

 between the external capsule and the cortex of the insula (Fig. 475, B). 



Nucleus amygdalae is a small mass of sub-cortical gray matter under 

 the tip of the temporal lobe. It forms part of the nucleus olfactorius 

 lateralis. 



Thalamus (Fig. 475). The middle and larger subdivision of the 

 diencephalon, sometimes even applied to the entire diencephaloii and 

 called the optic thalamus. 



As all nervous impulses which reach the brain cortex, except those 

 that come from the olfactory organs, pass through the thalamus, this 

 organ serves as a sort of vestibule for the cortex and probably also as a 

 great relay station for the incoming and outgoing nerves. 



It is to be remembered that the optic fibers which occupy the thala- 

 mus take up much of that organ, but it should not be called the optic 

 thalamus because all fibers to and from the cortex, regardless of whether 

 coming from the eye or not, pass through the thalamus. 



Two parts of the thalamus are to be noted. The ventral portion, 

 contains chiefly motor coordination centers. In man this position is not 



