546 THE NERVOUS SYSTEM 



anterior limb of the capsule; b, the pyramidal fibers in the genu and the anterior 

 part of the posterior limb ; c, the temporo-pontine fibers in the posterior part 

 of the posterior limb. Fibers connecting the optic thalami and corpora 

 striata with the cerebral cortex run in the capsule. The pes and the corona 

 radiata form the great sensory and motor highway to and from the cerebral 

 cortex. 



The tegmentum is the continuation anteriorly of the reticular formation of 

 the medulla. It ends for the most part in the neighborhood of the optic thala- 

 mus and in the parts beneath. The tegmentum of either side is supposed to 

 be concerned chiefly with afferent impulses. It is made up to a very consid- 

 erable extent of collections of gray matter, the most important of which are 

 the substantia nigra, separating the pes and tegmentum, and the nucleus ruber, 

 which is a rounded mass situated more toward the aqueduct of Sylvius; it 

 serves as a way-station in the cerebello-cerebral conduction paths and also 

 has important connections with the spinal cord. The locus niger extends 

 back as far as the posterior corpus quadrigeminum. Posteriorly, the teg- 

 mentum is chiefly reticular in structure. 



Corpora Quadrigemina. There are two corpora quadrigemina on 

 each side, the anterior and posterior. They form prominences on the dorsal 

 surface of the mid-brain, dorsal to the aqueduct of Sylvius. The posterior 

 corpora quadrigemina receive through the lateral fillet fibers from the coch- 

 lear division of the eighth nerve. They are closely associated with the median 

 corpora geniculata, and, like these, give origin to fibers which continue the 

 auditory conduction path upward to the auditory center. The anterior 

 corpora quadrigemina receive fibers from the optic nerve, the mesial fillet, 

 and also from the occipital cortex, as will be more fully described later. They 

 are closely associated with the external corpora geniculata. They also form 

 reflex centers for eye muscles in the ocular adjustments. 



Corpora Geniculata. These are two on each side of the brain 

 stem, the external or outer and the median or inner. The external corpus 

 geniculatum is at the side of the crus and appears to be a swelling on the lateral 

 division of the optic tract, and actually receives terminations of the optic fibers, 

 thus constituting a way-station in the optic conduction paths. Similarly 

 the median appears to be the termination of the median division of the optic 

 tract, from which it receives some fibers, figure 416, but it is more intimately 

 connected with the auditory tracts, forming a way-station between the lateral 

 fillet and the auditory cortical center, figure 389. 



The Optic Thalami. The optic thalami are oval in shape, and rest 

 upon the crura cerebri. They form part of the floor of the lateral ventricles and 

 their inner sides bound the third ventricle. They are connected by a trans- 

 verse tract, the middle commissure. 



Each thalamus has several collections of gray matter, forming somewhat 

 indistinctly defined masses separated by white fibers. These masses of gray 



