THE NERVOUS SYSTEM. 633 



in the corpus subthalamicum, thence to the optic thalamus and the cere r 

 bral cortex. 



(b.) Posterior longitudinal bundles. A bundle of fibres which appear 

 to begin the bulb as certain fibres of the anterior column of the cord, 

 which are the short longitudinal commissures between segments of the 

 cord. It is traceable upward as far as the nucleus of the third nerve. 

 It is supposed to connect the nuclei of the fourth and sixth nerves with 

 the third, and with the anterior corpus quadrigeminum. 



(c.) Superior peduncle of the cerebellum. This arises on either side 

 from the superficial gray matter, but chiefly from the corpus dentatum, 

 and passes forward outward beneath the posterior corpus quadrigeminum, 

 and beneath it and the anterior corpus quadrigeminum decussates with 

 its fellow; the fibres then pass forward in the anterior district of the 

 tegmentum and end in the red nucleus. 



(d.) Fibres from the corpora quadrigemina. From each corpus quad- 

 rigeminum passes forward and downward a tract called the brachium. 

 The anterior brachium goes to the lateral corpus geniculatum, and then 

 to the optic tract, other fibres pass into the tegmentum, and thence 

 directly to the occipital cortex. The posterior brachium goes to the 

 median corpus geniculatum, thence to the tegmentum, and through it 

 possibly to the temporal region of the cerebral cortex. 



Commissural fibres. In addition to the fibres of the corpus callosum, 

 which connect all parts of the hemispheres, and fornix, there are three 

 other commissures, the anterior white commissure, and the posterior 

 white commissure in the third ventricle connect by white fibres the two 

 sides of the brain. The fibres in the anterior come from the temporo- 

 sphenoidal convolution chiefly, but a few are part of the olfactory tract. 

 The posterior connects the optic thalami and tegmenta. The middle 

 is chiefly composed of gray matter, but also contains some transverse 

 fibres. x 



Functions of the Cerebrum. 



Speaking in the most general way, and for the present omitting 

 the accumulating evidence in favor of the direct representation of the 

 various co-ordinated movements of the muscles of the body in ganglia 

 situated in different parts of the cerebral cortex, it may be said that : 

 (1.) The cerebral hemispheres are the organs by which are perceived 

 those clear and more impressive sensations which can be retained, and 

 regarding which we can judge. (2.) The cerebrum is the organ of the 

 will, in so far at least as each act of the will requires a deliberate, how- 

 ever quick determination. (3.) It is the means of retaining impressions 

 of sensible things, and reproducing them in subjective sensations and 

 ideas. (4.) It is the medium of all the higher emotions and feelings, and 



