484 HISTORY OF THE HUMAN BODY 



into the naso-palatine canal, thus retaining its early relation- 

 ship to the primitive choanse, but in anthropoids and some 

 other mammals, it is quite vestigial and appears only in em- 

 bryonic life. In the monotremes it reaches the highest de- 

 velopment attained among mammals, and is here entirely en- 

 cased in its cartilage, through which passes a small branch of 

 the olfactory nerve to supply the organ. From the lateral 

 wall of the. cartilage a turbinal process develops, similar to 

 the turbinalia of the main cavity, but very simple in form and 

 covered with indifferent non-olfactory epithelium. Remains 

 of this process are seen in marsupials and even in rodents, 

 forms in which the entire organ is well developed. 



If we except such special adaptations as the tubular pro- 

 longations of the nostrils which exist in certain fishes and a 

 few aquatic birds, an external nose as a separate organ is 

 a mammalian characteristic. It possesses a cartilaginous frame- 

 work derived from the primordial skeleton and thus, in part, 

 homologous with the cartilaginous capsule of amphibians, and 

 it is supplied with superficial muscles from the mimetic group. 

 It shows great power of adaptation in the various mammals, 

 sometimes forming a flexible snout or trunk, as in swine, ele- 

 phants, and moles, and sometimes developing a moist, sensitive 

 surface, as in carnivores and ruminants. In the anthropoids 

 it is reduced in size, corresponding to the lessened importance 

 of the olfactory sense, although its muscles are very mobile 

 and assist greatly in the expression of emotions. These latter 

 powers seem to be undergoing degeneration in Man in spite 

 of the fact that the external nose is more prominent than in 

 most primates. 



The ultimate organ of smell is the olfactory membrane, 

 which in fishes and amphibians is distributed over the entire 

 nasal cavity, but which, with the establishment of air-breathing 

 and the setting apart of a respiratory portion, tends to confine 

 itself to the more dorsal region. It consists of a highly differ- 

 entiated form of epithelium in which occur the terminal olfac- 

 tory cells surrounded by supporting cells, some or all of which 

 may be ciliated. In certain fishes and in the lower urodeles 



