210 MANUAL OF ANIMAL BIOLOGY 



and on which the infundibulum lies is known as the crura cerebri. 

 The pair of large optic nerves, which innervate the retina of the eye, 

 arise in the diencephalon and then continue ventrally and ante- 

 riorly a short distance. The two nerves meet in the mid-ventral 

 line where they cross each other, thus forming the optic chiasma 

 underneath which the infundibulum lies. The crossing of the 

 optic nerves, so that the fibers from the right side of the dien- 

 cephalon supply the left eye and vice versa, is a noteworthy feature 

 of the optic nerves in the Vertebrates. (W. f. 142.) 



One of the characteristic features of the vertebrate central 

 nervous system is the fact that it is hollow. The central cavity 

 when first formed is quite large, but gradually the walls of the 

 central nervous system thicken and most of the original cavity is 

 thus obliterated, leaving only a small central canal which per- 

 sists in the spinal cord throughout life. In the brain the anterior 

 end of the central canal is connected with a series of cavities 

 (ventricles) which in turn communicate with each other through 

 definite openings (foramina). 1 



The ventricles and the foramina arise as modifications of the 

 embryonic central cavity of this region of the central nervous 

 system. The ventricles may be seen to good advantage in longi- 

 tudinal sections through the brain in either a vertical (sagittal) 

 or a horizontal (frontal) plane. If, for example, a frontal section 

 through the brain is studied, the ventricles will be seen as paired 

 structures. The first pair of ventricles (lateral ventricles) 

 form the cavities of the cerebral hemispheres and extend anteriorly 

 into the olfactory lobes. The lateral ventricles are connected 

 near the posterior end of the cerebral hemispheres by the trans- 

 verse foramen of monro which communicates with the unpaired 

 third ventricle lying in the diencephalon. An optic ventricle 

 is present in each of the optic lobes. These communicate with 

 each other, also anteriorly with the third ventricle, and posteriorly 

 with the single large fourth ventricle which lies in the medulla. 

 The fourth ventricle decreases in size posteriorly and merges into 

 the central canal of the spinal cord. (W. f. 140, A, B, C.) 



The brain of the higher Vertebrates is characterized structurally 

 by a great increase in the size of the cerebral hemispheres so that 

 they overshadow the other parts. This increasing dominance of 

 the cerebral hemispheres reaches its culmination in the Mammals, 



1 Singular, foramen. 



