156 ORGANIZATION OF THE HEAD v. 9- 



and the reason in the case of the optic nerve is quite clear. It is not 

 morphologically a peripheral nerve at all. The eye is formed as a 

 vesicle attached to the brain ; the optic 'nerve' therefore develops as a 

 bundle of fibres joining two portions of the central nervous system; in 

 fact it is now usually called the optic tract, not the optic nerve. 



This reasoning will not apply to that very peculiar and interesting 

 structure the olfactory nerve. This is unique among all craniate 

 nerves in consisting of bundles of fibres whose cell bodies lie at the 

 periphery. The cells of the olfactory epithelium, like the sensory cells 

 in invertebrates and some of those of amphioxus, are neurosensory 

 cells, that is to say, their inner ends are prolonged to make the actual 

 nerve-fibres that pass into the brain. This fact does not by itself solve 

 the problem of fitting the nerve into the series of dorsal and ventral 

 roots, but it reminds us that the nerve is very ancient, and suggests 

 that it does not fall into the rhythm of the rest of the series because it 

 precedes the other cranial nerves either in time or space, or perhaps 

 even both. The olfactory nerve may have existed before any seg- 

 mental structure appeared, possibly as the nerve of sense-organs on 

 the front end of the ciliated larva which we suppose gave rise to our 

 stock (p. 76). Alternatively we can say that the olfactory nerve is as 

 it is because it lies in front of the region over which the segmenting 

 process operates; it is, as it were, 'prostomial'. If we wish we can hold 

 both these views together. 



There are one or two other exceptions to the rhythmic arrangement 

 of nerves, perhaps more difficult to account for than the first and 

 second cranial nerves. If all connexions between centre and periphery 

 are made by dorsal and ventral roots what is the status of the fibres 

 that run down the infundibular stalk to reach the cells of the pituitary 

 body? This glandular tissue, derived from the epithelium of the 

 hypophysial folding of the roof of the mouth, is undoubtedly a peri- 

 pheral organ. Does it receive its nerve-fibres direct from the brain ? 

 If so presumably we must say that the pituitary, like' the nose, 

 is prostomial, lying in front of the segmental region, and this is 

 reasonable enough from its position. There is good reason to believe 

 that it is an extremely ancient organ, already present in the earliest 

 chordates. 



A still more puzzling exception is the nervus terminalis. This is 

 a small bundle leaving the brain ventrally behind and below the olfac- 

 tory nerves and running to the olfactory mucosa or to the accessory 

 olfactory organ of Jacobson, where this is present (p. 350). In some 

 vertebrates it carries a small ganglion. The fibres are probably 



