THE ORGANS OF THE OUTER GERM -LAYER. 515 



into the oral cavity, which has been effected in all Vertebrates that 

 breathe by means of lungs, a second function has been assumed. It 

 is now not exclusively a sensory organ for the perception of odors, 

 but serves at the same time to conduct currents of air both to and 

 from the oral and pharyngeal cavities and the lungs. It has become 

 a kind of respiratory atrium for the apparatus of respiration. The 

 assumption of this accessory function gives a special stamp to the 

 later stages of the development of the organ, and is to be taken into 

 account in a proper estimate of it. For the course of the further 

 development is most of all determined by the tendency to an exten- 

 sive enlargement of the surface of the olfactory chamber. The 

 increase of surface, however, does not affect the real olfactory 

 mucous membrane or sensory epithelium, to which the olfactory 

 nerve is distributed, but rather the ordinary ciliate mucous membrane. 

 It is therefore less connected with an improvement of the sense of 

 smell than with an accessory function in the process of respiration. 

 By an increase of the surface of the soft, vascular mucous membrane 

 the air that is swept over it becomes warmed and freed from particles 

 of dust, which are caught by the moist surface. From this time 

 forward therefore one must distinguish a regio olfactoria and a regio 

 respiratoria. The former, which is derived from the sensory 

 epithelium of the original olfactory pit, remains relatively small, 

 receives the terminations of the olfactory nerve, and is limited in the 

 case of Man to the region of the upper turbinal process and a part 

 of the septum nasi. It is the respiratory function that causes the 

 vast dimensions which the organ of smell attains in the higher 

 Vertebrates. 



The increase in the surface of the nasal cavity is produced by three 

 different events : (1) by the formation of the hard and soft palate, 

 (2) by the development of the turbinal bones, (3) by the appearance 

 of the accessory cavities of the nose. 



The first event begins in Man toward the end of the second month. 

 There is then formed on the inner surface of the maxillary process 

 (fig. 289) a ridge, which projects into the wide primitive oral cavity 

 and grows out horizontally into a plate. The right and left palatal 

 plates at first embrace between them a broad fissure, through which 

 may be seen the original roof of the oral cavity and on this the inner 

 nasal orifices, which become more and more slit-like and are separated 

 by a bridge of substance which has arisen from the median frontal 

 process and can now be designated as the nasal septum. In the 

 third month the embryonic palatal fissure becomes gradually narrower. 



