412 BIOLOGY. [Book vi, 



tlie organ of smell seems to exist spite of the absence of brain. 

 It is represented by one or two depressions with vibratile 

 epithelium. 1 



In every class of vertebrates, man not excepted, the nervous 

 olfactory threads are composed of pale fibres, finely granulous, 

 without envelopment of myeline : in short of grey fibres, 

 which all, in the craniote vei-tebrates, emanate from two special 

 and intracranian nervous expansions called olfactory bulhs. 

 These masses of nervous substance, very little developed in man, 

 are much more so in the other vertebrates. In many mammifersand 

 vertebrates, moreover, the sense of smell is much more developed 

 than in man. It controls the sense of sight, often comes to 

 its aid, and sometimes takes its place. The olfactory bulbs of 

 many mammifers are very voluminous, situated in advance of 

 the brain : often they are hollow, and communicate with the 

 lateral ventricles. The mole, which is almost blind,^ has 

 enormous olfactory bulbs. In many birds and reptiles also the 

 olfactory bulbs form an important cerebral expansion. The 

 olfactory bulb of fishes is often as large as the cerebral lobe, 

 properly so called. Sometimes even it is much larger, and this 

 conformation is no doubt in relation with the difficulty of 

 olfaction in an aquatic medium. 



The olfactory nerves are nervous threads, proceeding in great 

 number from those bulbs, and ramifying into the superior portion 

 of the nasal cavities. The mucous membrane of those cavities, 

 habitually clothed with vibratile cilia, is destitute thereof 

 precisely in the portion, somewhat limited, where it receives the 

 olfactory threads. There it is furnished with a cylindrical epithe- 

 lium, whose cells terminate in fine filaments, which go to lose 

 themselves in the dermis of the mucous membrane. Beneath 

 these epithelial cells are other special cells, sui-mounted each 



^ Huxley, Anatomia Compa/rata dei Vertebrati. Italian translation of E. 

 Giglioli. Gegenbaur, Anatomic Comparie, p. 709. 



" On tKe contrary, the mole is said to have very sharp sight, though its eyes 

 are peculiarly placed. — Translator. 



