THE CENTRAL NERVOUS SYSTEM. 79 



The upward prolongation of the nucleus of the fifth nerve lies in 

 the grey matter lateral to the cerebral aqueduct. The nuclei of the 

 third and fourth nerves are found in the grey matter of the floor 

 of the aqueduct. The posterior longitudinal bundle has a position 

 immediately ventral to the latter nuclei. Some fibres of this bundle 

 arise in a nucleus, the nucleus of the posterior longitudinal bundle, which 

 lies at the upper part of the mid-brain, immediately under the thalamus. 

 The bundle also receives a number of fibres from the superior colliculus. 

 Its other connections have already been described. 



The mid-brain, pons, and medulla oblongata together form the brain- 

 stem, which conveys the conducting paths between the fore brain and 

 the spinal cord. In addition to its conducting function, the mid-brain 

 forms a cell-station on the optic and auditory paths, and it also serves 

 as a reflex centre for contraction of the pupil through the oculo-motor 

 nucleus. It has also been suggested that the red nucleus may form a 

 cell-station on an indirect motor path from the cerebral hemisphere to 

 the spinal cord, the route being by way of the cerebro-cerebellar path 

 through the pons, then by the superior peduncle to the red nucleus, 

 and by the rubro-spinal tract from the latter to the spinal cord. 



SECTION VIII. 

 THE FORE-BRAIN. 



The fore-brain consists of the two cerebral hemispheres, together 

 with certain masses of grey matter and other structures situated around 

 the third ventricle, and comprising the thalami, the corpora geniculata, 

 the hypophysis or pituitary body, and the pineal gland. The thalami 

 arid geniculate bodies are composed of grey matter. Each thalarnus 

 forms the lateral boundary of the third ventricle on its own side. 

 The internal (medial) and external (lateral) geniculate bodies lie 

 ventral to the thalamus on each side, and are in close relation with 

 the superior colliculi. 



The pineal gland lies immediately above the superior colliculi. It 

 contains no nerve structure, being composed of alveoli with earthy 

 phosphates in their interior, and it has not, so far as is known, any 

 function. It is supposed to be the homologue of the pineal eye of 

 the lizards. 



THE CEREBRAL HEMISPHERES. 



The cerebral hemispheres constitute the largest part of the brain. 

 Each consists of an external layer of g'rey matter, thrown into folds 

 or convolutions, with white matter internally. A mass of grey matter, 



