260 THE NERVOUS SYSTEM 



nences on the dorsal surface of the pons and crura above the 

 aqueduct of Sylvius. They contain white and gray matter. 

 The posterior tubercles are connected with the eighth nerve, 

 the sensory tract, the temporal region of the brain, and the 

 lateral corpora geniculata. The anterior tubercles are con- 

 nected with the optic nerve, with the occipital region, and 

 with the median corpora geniculata. 



The function of the anterior of these bodies is mainly 

 connected with the eye; the posterior are associated with 

 the sense of hearing. 



The Cerebrum, 



The great size of the cerebral hemispheres in man ob- 

 scures the fact that the different parts of the brain are dis- 

 posed in a linear series ; these, from before backward, are, 

 the olfactory lobes, cerebral hemispheres, optic thalami, 

 corpora quadrigemina, cerebellum, medulla oblongata. This 

 arrangement exists in theliuman fetus, and persists through- 

 out life in some of the lower animals. 



Anatomy. The substance of each hemisphere is divided 

 by fissures into five lobes (a) frontal, (b) parietal, (c) 

 occipital, (d) temporo-sphenoidal and (e) central. The 

 main fissures are four in number (i) The fissures of Syl- 

 vius running from the front and under part of the brain 

 backward, outward and upward; (2) the fissures of Rolando 

 running from the median line near the center of the longi- 

 tudinal fissure forward, outward and downward; (3) the 

 parie to -occipital fissure, little of which is evident upon the 

 surface of the brain, but which appears on longitudinal sec- 

 tion separating the occipital and parietal lobes; (4) the 

 calloso-marginal fissure, also evident only on the internal 

 aspect of the hemisphere, parallel with and above the cor- 

 pus callosum. (Figs. 81, 82.) 



(a) The frontal lobe is bounded internally by the longitu- 

 dinal fissure, posteriorly by the fissure of Rolando and be- 



