294 STRUCTURE OF THE VERTEBRATES 



The commissure is an accurate indication of the number of cere- 

 bral cells, for each sends a fiber posteriorly and most of these 

 cross in the cerebral commissure. The posterior commissure, 

 crossing at the junction of the diencephalon and the midbrain, 

 is also very primitive. The pons, connecting the two sides of the 

 cerebellum, has been mentioned. 



Exceptions must be made to the impression that all impulses 

 eventually cross from one side of the central nervous system to 

 the other. The incomplete crossing of optic fibers in the higher 

 animals is discussed on page 308. Also, it will be understood 

 that simple motor reflexes do not decussate, for a sensory stim- 

 ulus reaching the cord is transferred to a motor cell on the 

 same side; but the sensory fiber which carries the impulse to 

 the brain when it enters the cord, crosses to the opposite side 

 before reaching the higher centers. In the same way motor 

 impulses from, the brain cross before reaching the ventral motor 

 cells of the cord, and then pass out to the body. Therefore, 

 destruction of fibers in the cord would be more liable to affect 

 the same side of the body, whereas a destruction of brain 

 tissue would affect the opposite side. 



F. Cranial Nerves 



The cranial nerves are those which pass out to the somatic 

 regions of the body through foramina of the skull. There is 

 evidence of two cranial nerves in Amphioxus, and it is thought 

 that these are homologous with the first two (the olfactory and 

 optic) nerves of the higher vertebrates. Synchronously with the 

 development of a chondrocranium in the cyclostomes the num- 

 ber of cranial nerves increases to eight; and, as cephalization 

 proceeds and the cranial cavity increases in size, the number of 

 cranial nerves is increased to ten in the fishes and amphibia, 

 and to twelve in the amniotes. 



The above evidence, and the structure of the nerves them- 

 selves, have given rise to the theory that the cranial nerves pos- 

 terior to the first two are modified spinal nerves, and that the 

 brain is metameric in nature. According to this theory there 

 are at least eight neuromeres, the first three corresponding to 

 the three primary vesicles described above. The third brain 



