586 THE NERVOUS SYSTEM 



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

 also has important connections with the spinal cord. The locus niger ex- 

 tends back as far as the posterior corpus quadrigeminum. Posteriorly, the 

 tegmentum is chiefly reticular in structure. 



Corpora Quadrigemina. There are two corpora quadrigemina on each 

 side, the superior and inferior. They form prominences on the dorsal 

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

 corpora quadrigemina receive through the lateral fillet fibers from the 

 cochlear division of the eighth nerve, figure 414. 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 superior corpus quadrigeminum receives fibers from the optic nerve, 

 the mesial fillet, and also from the occipital cortex, as will be more fully de- 

 scribed later. It is closely associated with the external corpus geniculatum. 

 The corpora geniculata also form reflex centers for the eye muscles in the 

 reflexes that result in the adjustments of the eye to vision at different dis- 

 tances. The nervous connections of these nuclei are discussed in presenting 

 the optic tract and the auditory tracts. 



V. THE FORE -BRAIN. 



The fore-brain or prosencephalon consists of two divisons, the thalamen- 

 cephalon or thalami, and the telencephalon, or corpora geniculata, corpora 

 striata, and the cerebral hemispheres. These subordinate divisions will be 

 discussed in order. 



The 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 transverse 

 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 

 matter are known as the nuclei of the thalamus, which are six in number. 

 They are called the anterior nucleus, the median nucleus, the lateral nucleus, 

 the ventral nucleus, the pulvinar, and the posterior nucleus. The corpora 

 geniculata are also closely associated with the optic thalamus. The anterior 

 nucleus is composed of large nerve cells which receive the terminations of 

 axones of cells of the corpora mammillaria at the base of the brain (bundle 

 of Vicq d'Azyr). There they meet the fibers of the fornix, which establish 

 a relation between this tubercle of the thalamus and the hippocampal con- 

 volutions. The median nucleus is connected by its axones with the cortex 

 of the island of Reil and the second and third frontal convolutions. The 

 lateral nucleus is quite large and lies against the internal capsule, into which 

 it sends fibers. It is connected with the central convolutions of the cortex. 



