Mammalia: Nervous System 675 



turbinate, secondary and tertiary, are restricted to the more lateral or 

 outer region and are accordingly called ectoturbinals. 



The turbinal surfaces are all covered by a thin periosteum (or 

 perichondrium over unossified parts) overlaid by a continuation of the 

 epithelium which lines the walls of the nasal cavity. 



In contrast to the 1 concha of reptiles, most mammals have from 

 4 to 6 primary turbinate in each nasal cavity and there may be as 

 many as 10 or 11 (e.g., in the African anteater, Orycteropas) . Even 

 among individuals of the same species the number may vary. The 

 nasoturbinal is usually less extensive than the others, or may be 

 rudimentary. 



In mammals having the more numerous turbinate, there are no 

 large open spaces in the nasal cavity. It is a labyrinth of narrow 

 passages. The relation of these passages to the nostrils is such that the 

 respiratory current of air passes mainly through the ventral region of 

 the nasal cavity. In this region olfactory cells are few or usually entirely 

 lacking. Accordingly, a ventral respiratory part and a dorsal ol- 

 factory part are distinguished, but with no sharp demarcation be- 

 tween them. There are, in mammals, no olfactory cells on the maxillo- 

 turbinals, but they are more or less abundantly present on the other 

 turbinate, as well as on the main wall of the nasal cavity, especially the 

 median nasal septum. 



It is important to distinguish between the olfactory epithelium 

 which contains olfactory sensory cells, each of which produces a nerve- 

 fiber passing, via the olfactory nerve, into the olfactory lobe of the 

 brain, and the nonol factory nasal epithelium. Nerve-terminations 

 whose stimulation produces sensations of pressure (touch), heat, cold, 

 or pain are present in both the olfactory and nonolfactory regions. The 

 receptors for these senses are related to branches of the trigeminal 

 nerve (V). 



The entire nasal surface is more or less richly glandular. The glands 

 range from unicellular to complex multicellular, and the secretions are 

 both serous and mucous. 



Keenness of sense of smell is correlated with the area of olfactory 

 surface but not with that of the nasal surface, for much of the latter is 

 not olfactory and a region which is olfactory in one animal may not be 

 so in another. Most important for the olfactory sense are the ethmo- 

 turbinals, but their surfaces may not be completely olfactory. The 

 human olfactory organ (Fig. 506) is an extreme case of reduction of 

 olfactory surface. In the human embryo as many as seven turbinate in 

 each nasal cavity have been recognized — a maxilloturbinal which per- 

 sists in the adult, a vestigial and temporary nasoturbinal, and five 

 ethmoturbinals, of which usually only the lower two persist. The adult 



