OLFACTORY ORGAN 



197 



The primary origin of the olfactory organs is by no means understood : 

 possibly it may have arisen by a modification of primitive integumentary 

 sense-organs. It is doubtful whether the organ can be said to have a true 

 olfactory function in Fishes and perennibranchiate Amphibians. 



In its simplest form, the olfactory organ consists of a ventral, 

 paired, pifc-like depression of tlie integument of the snout opening 

 on to the surface by an external nostril. It is lined by an epithelium 

 which is connected with the brain by the olfactory nerves. The 

 olfactory mucous membrane contains sensory cells, or olfactory cells 

 proper — usually provided with sensory 

 hairs, separated by isolating or supporting 

 cells, both kinds having a smilar origin 

 (Fig. 156). 



These olfactory cells are said to constitute the 

 only true neiifo-epithelinm in Vertebrates, as the 

 nerve arises in connection with the cell itself, 

 witli whicli it remains continuous, as in many 

 Invertebrates {primanj sensory cell, Retzius). Tlie 

 cell is therefore not merely surrounded by a 

 nerve-network as in other secondfit-;/ nerve-cells, 

 and the olfactory organ thus probably represents 

 a very ancient structure phylogenetically. It is 

 possible, however, tliat the central cells of the in- 

 tegumentary sense-organs of Anamnia (e.g. end- 

 buds) may be directly continuous with their 

 nerves, although surrounded by a nerve-network. 



From the Amphibia onwards glandular 

 elements are present, the secretion of which 

 serves to keep the nasal cavity moist. 



The olfactory organs of all the true 

 Fishes exhibit the above-described simple 

 sac-like form, but from the Dipnoi onwards 

 they come to communicate with the cavity 

 of the mouth as well as with the exterior. 

 In consequence of this, anterior or external, 



and posterior or internal nostrils (choance) can be distinguished,. 

 and as a free passage is thus formed through which air can pass, 

 the olfactory organ takes on an important relation to the respira- 

 tory apparatus. 



In Amphioxus, the ciliated pit situated above the anterior end of the 

 central ner\'ous system probably represents an unpaired olfactory organ. 

 Traces of a structure possibly homologous with this are said to occur in the 

 embryos of the Lamprey and Sturgeon. 



Cyclostomes.— In these forms (Fig. 54) the olfactory organ 

 consists of a sac, containing numerous radial folds of the mucous 

 membrane, and unpaired externally. It lies close in front of the 

 cranial cavity, and opens on the dorsal surface of the anterior 

 part of the head by a longer or shorter chimney-like tube. In 



Fig. 



OF 



156. — Epithelium 



THE OlFAOTORY 



Mucous Membkane. 

 A, of Petromyzon plan- 

 eri; B, of Salamanclra 

 atra. 



R, olfactory 

 interstitial 

 cells. 



cells ; E, 

 epithelial 



