CRANIAL NERVES. 155 



olfactory lobe or from a part of the brain developed from it, and 

 does not at first consist, like the ordinary cranial and spinal 

 nerves, of a series of non-nucleated axis fibres, but has the form of 

 nucleated bundles, which arise from a network of star-shaped 

 cells of the olfactory lobe. These nuclei are therefore identical 

 with the nuclei of nerve-cells. 1 



These observations were made on the human subject, and it is very desirable 

 that similar researches should be extended to the lower Vertebrates." It might 

 then be possible to explain the fact that the roots of the olfactory nerve are 

 usually double. The roots either eventually unite on either side to form a 

 common trunk, or else (less commonly) remain entirely separate, so that two 

 olfactory nerves perforate the ethmoid on each side (Gymnophiona). 



A definite cribriform plate is riot always present ; far more 

 commonly the whole undivided trunk of the olfactory nerve ex- 

 tends into the nasal cavity, and only then begins to break up. 

 This holds good for by far the greater number of Vertebrates below 

 Mammals, as well as for Monotremes. 



It appears very probable that the olfactory nerve, in spite of its 

 peculiarities in the human subject, arises primitively, that is, 

 phylogenetically, in a similar manner to the ordinary cranial nerves. 

 The important circumstance that in Fishes it arises from the primi- 

 tive ridge of the medullary tube, from which all the other nerves 

 take their origin, is in favour of this supposition. 



Optic Nerve. As already mentioned, the optic nerve arises 

 from the stalk of that outgrowth of the primary fore-brain which is 

 spoken of as the primary optic vesicle. Inasmuch, therefore, 

 as it represents a part of the brain, it cannot be compared with 

 any of the other nerves. It remains for further researches to show 

 whether it is possible, on phylogenetic grounds, to consider it as 

 originally a segrnental nerve. 



Three more or less sharply differentiated portions may in most 

 cases be distinguished in the optic nerve ; these are spoken of from 

 the proximal to the distal end respectively as the optic tract) 

 chiasma, and nerve. 



The chiasma, that is, the crossing of the two optic nerves, is 

 always present, though not always freely exposed, for it may re- 

 tain a primitive position deeply embedded in the base of the brain, 

 as in Cyclostomes. 



While in most Teleosteans the optic nerves simply overlie one 

 another (Fig. 128, A), in some of these Fishes (Harengus, Engraulis, 



1 The olfactory lobes, which are closely united with the hemispheres, are originally 

 directly applied to the olfactory mucous membrane, which is perforated by numerous 

 fine fibres from them. This condition persists throughout life in. Cyclostomes : in 

 other Fishes, in correspondence with the separation of the olfactory organs and brain 

 by the secondary growth of the head, each olfactory lobe becomes drawn out into an 

 olfactory tract, the main mass of the lobe remaining in connection with the 

 olfactory organ, and thus being widely separated from the brain. In Elasmobranehs 

 the olfactory lobes are usually short and thick, while in Teleosteans they are long and 

 slender. 



