286 THE SENSORIAL FUNCTIONS. 



shown at t in Fig. 385. Eight or more principal branches 

 arise from the main trunk; and each of these is afterwards 

 divided- and subdivided to an extreme degree of minuteness, 

 so as to form, in all, many luindred plates. The olfactory- 

 membrane, with all its nerves, is closely applied to every 

 plate in this vast assemblage, as well as to the main trunk, 

 and to the internal surface of the surrounding cavity: so that 

 its extent cannot be less than 120 square inches in each nos- 

 tril. An organ of such exquisite sensibility requires an ex- 

 traordinary provision for securing it against injury, by the 

 power of voluntarily excluding noxious vapours; and nature 

 has supplied a mechanism for this express purpose, enabling 

 the animal to close, at pleasure, the orifice of the nostril. 

 The boo;, which, in its natural state, subsists wholly on ve- 

 getable food, resembles herbivorous tribes in the external 

 form and relative magnitude of the turbinated bones; but 

 they are more simple in their structure, being formed of sin- 

 gle, and slightly convoluted plates, without partitions or per- 

 forations. In this respect, they approach to the human 

 structure, which is even less complicated, and indicates a 

 greater affinity with vegetable than with animal feeders. 

 Man, indeed, distinguishes more accurately vegetable odours 

 than those proceeding from animal substances; while the 

 reverse is observed with regard to quadrupeds whose habits 

 are decidedly carnivorous. A dog, for instance, is regard- 

 less of the odour of a rose or violet; and, probably, as he 

 derives from them no pleasure, is unable to discriminate the 

 one from the other. Predacious animals, as Sir B. Harwood 

 observes, require both larger olfactory nerves, and a more 

 extensive surface for their distribution, than the vegetable 

 eaters. The food of the latter is generally near at hand; 

 and as they have occasion only to select the wholesome from 

 the noxious plants, their olfactory organs are constructed for 

 the purpose of arresting the effluvia of odorous substances 

 immediately as they arise. The former are often under the 

 necessity of discovering the lurking places of their prey at 

 a considerable distance, and are, therefore, more sensible to 



