STRUCTURE OF THE VERTEBRATES 295 



pouch would then form the cerebellum, and the posterior five 

 neuromeres would form the medulla. 



Two other factors give evidence to the validity of the theory. 

 (1) The appearance of muscle segments in the eye and ear 

 regions indicates the former extension of metamerism in the 

 higher vertebrates. (2) The structure of the cranial nerves is 

 further evidence. Although most of the nerves are highly spe- 

 cialized, some being entirely sensory and others entirely motor, 

 most of them are mixed like the spinal nerves; and some arise 

 from the brain as several roots. 



The cranial nerves are known by names which describe their 

 function or location, and are also designated by numbers. 

 These numbers are usually written Nerve I, Nerve II, etc. As 

 the cranial nerves are very important, and vary widely in func- 

 tion, each will be treated separately. 



Nerve I, Olfactory. The olfactory nerve is sensory and con- 

 nects the olfactory lobe of the brain with the epithelium of the 

 nasal sacs. It is usually considered that the epithelium of the 

 sac gives rise to the receptors and their axones which extend 

 posteriorly toward the brain. In many vertebrates, including the 

 fish, reptiles and mammals, these axones are very short and 

 make connection with the long dendrites extending outward 

 from the olfactory lobe. In this case the nerve is really an ol- 

 factory tract, as is found in the elasmobranchs. In other verte- 

 brates the sensory axones are long and pass toward the brain, 

 forming an olfactory nerve. These axones meet the short den- 

 drites of the brain nerves within the olfactory lobe. 



Terminal Nerve. The terminal nerves lie mediad to the 

 olfactory, and should be designated as Nerve I, but they were 

 discovered after the more prominent nerves were numbered. 

 Their function is unknown. They are found in most elasmo- 

 branchs and dipnoans, and in the embrj'os of many higher 

 vertebrates, including man and other mammals. Apparently they 

 are sensory nerves of the rostrum, whose function has been 

 replaced in the higher groups by Nerves V and VII. 



Nerve II, Optic. The optic nerve is formed by the axones of 

 association neurones which connect the receptors of the retina 

 (sensory layer of the eye) with the brain. The retina arises as 

 an evagination from the primitive forebrain, and the nerves 



