Bawden, Nose and Jacobsoris Organ. 121 



mouth at the front end of a groove running back to the pos- 

 terior nares. 



In birds the nasal sac is extended to form a groove. The 

 hinder part becomes enclosed in the mouth and closes except at 

 the posterior where it remains open, forming the posterior (or 

 internal) nares. 



3. An interesting field for further investigation is the evo- 

 lution of the respiratory regions of the nose. The nose is 

 one of the organs which serve a double purpose. It is 

 not only the seat of the sense of smell, but was evidently 

 intended to be also the principal organ through which an 

 animal breathes. Its more or less circuitous passages, warm 

 and moist, protect the lungs by taking the chill from the inspir- 

 ed air and arresting irritating dust. The cavity of the nostril 

 in many types is extended by means of certain so-called turbinated 

 bones, covered by the extremely sensitive olfactory membrane. 

 This cavity is further separated into two parts, the respiratory 

 passages of the nose, and the true olfactory centers. The latter 

 are usually situated farther entad, so that in general it is not true 

 that the entire olfactory epithelium is concerned in the act of 

 smelling. In man, for example, the terminal nerve cells which 

 alone take cognizance of odors are situated in the upper third 

 of the nasal chambers out of the line of the ordinary inhalation. 

 For this reason we do not usually notice odors except when we 

 sniff — draw the air into the upper part of the nostrils and hold 

 it there a moment — when we may become aware of even the 

 faintest scent. 



The fact that certain Amphibia during their aquatic life do 

 not possess ciliated epithelium but when entering upon their 

 terrestrial life acquire it, seems to indicate that the possession of 

 such epithelium bears some relation to the necessary circulation 

 of the air through the nasal passages during the land-life of these 

 types. There certainly is some significance in the fact that as a 

 class aquatic animals are not supplied, while as a class land 

 animals are supplied with this ciliated olfactory epithelium. 

 Some have suggested that the cilia serve to renew the medium 

 bearing the odoriferous particles, but this perhaps would conflict 



