206 PHILOSOPHY OF ZOOLOGY. 



trance, is placed on the anterior part of the head. It is di- 

 vided by a partition which varies in its breadth and posi- 

 tion, and gives to the nostrils a great variety of character. 



1. Structure of the organs of Smell. In the vertebral 

 animals which respire by lungs, the partition which is ob- 

 served externally, is continued to the posterior opening into 

 the throat, forming what is termed the septum narium. 

 This division is formed by the vertical plate of the ethmoidal 

 bone and the vomer. The cavities on each side are, in 

 part, filled by the turbinated processes of the ethmoidal 

 bone which occupy the vault, and the inferior turbinated 

 bones, which adhere to the maxillary bones, and project 

 into the middle of the cavity. 



The olfactory membrane lines these cavities of the nose, 

 covering its septum, walls, and projecting laminae. It is 

 merely a continuation of the external skin, which is attach- 

 ed by cellular substance to the periosteum of the bones, and 

 which unites posteriorly with the integuments of the throat 

 and gullet. Near the external openings it resembles the 

 skin, but towards the interior it is of a red colour, which is 

 derived from the numerous bloodvessels with which it is 

 supplied. Its surface presents numerous little eminences, 

 which have been regarded, by some, as nervous papillae, 

 and, by others, as the termination of excretory canals. It 

 is kept constantly moist by a watery fluid, termed nasal 

 mucus, in general, a secretion from the whole membrane, in 

 other cases produced in particular cells. 



This membrane is supplied with nerves from the first pair 

 or olfactory nerves. These take their rise in the anterior lobes 

 of the brain. Each nerve proceeds to its corresponding nos- 

 tril, and after dividing into numerous filaments, is lost in the 

 substance of the olfactory membrane, where it covers the 



