196 FOUNDATIONS OF BIOLOGY 



3. Sense of Smell 



The special sense organs of smell, or OLFACTORY BUDS, 

 reside in the membrane which lines a pair of invaginations of 

 the anterior end of the head, termed OLFACTORY POUCHES. 



The buds are in communication with the brain by the olfac- 

 tory, or first pair of cranial nerves. The pouches constitute 

 relatively simple sacs in the lower Vertebrates, but in the air- 

 breathing forms, and especially in the Mammals, the walls of 

 the pouches are thrown into folds, ridges, and secondary 

 pouches. This is necessitated by the concentration of the 

 olfactory surface to the air passages of the nose which lead 

 to the lungs. On the other hand, in Man the olfactory appa- 

 ratus has fallen somewhat from the complexity which it attains 

 in the lower Mammals, as is attested not only by its structure 

 in the adult but also by transient remnants in the human em- 

 bryo. 



4. The Ear 



The ears, or organs of hearing and equilibration, arise as 

 paired depressions of the ectoderm of the head, which, in 

 all Vertebrates above the lower Fishes, lose their connection 

 with the exterior and form the so-called INNER EAR, or LABY- 

 RINTH. This becomes divided into two chief parts, the SAC- 

 CULUS and the UTRICULUS from which are developed three 

 SEMICIRCULAR CANALS, one in each plane of space. The sac- 

 culus is largely devoted to the reception of vibrations of the 

 surrounding medium, that is to hearing in the usual sense of 

 the word. Accordingly the sacculus becomes progressively 

 differentiated as we ascend the Vertebrate scale a complex 

 derivative in the mammalian ear being the COCHLEA. On the 

 other hand, the utriculus and the semicircular canals provide 

 for sensations of loss of equilibrium, or orientation of the 

 body in space, and show far less change. It is probable 



