SENSATION AND THE SENSIFEROUS ORGANS. 277 



orbits in adult human beings. But, if we trace back 

 the nasal chambers to their origin in the embryo, we 

 find, that, to begin with, they are mere depressions of 

 the skin of the fore part of the head, lined by a con- 

 tinuation of the general epidermis. These depressions 

 become pits, and the pits, by the growth of the adjacent 

 parts, gradually acquire the position which they finally 

 occupy. The olfactory organ, therefore, is a specially 

 modified part of the general integument. 



The human ear would seem to present greater dif- 

 ficulties. For the essential part of the sense organ, in 

 this case, is the membranous labyrinth, a bag of com- 

 plicated form, which lies buried in the depths of the 

 floor of the skull, and is surrounded by dense and solid 

 bone. Here, however, recourse to the study of devel- 

 opment readily unravels the mystery. Shortly after the 

 time when the olfactory organ appears, as a depression 

 of the skin on the side of the fore part of the head, 

 the auditory organ appears as a similar depression on 

 the side of its back part. The depression, rapidly deep- 

 ening, becomes a small pouch; and then, the commu- 

 nication with the exterior becoming shut off, the pouch 

 is converted into a closed bag, the epithelial lining of 

 which is a part of the general epidermis segregated from 

 the rest. The adjacent tissues, changing first into car- 

 tilage and then into bone, enclose the auditory sac in a 

 strong case, in which it undergoes its further metamor- 

 phoses ; while the drum, the ear bones, and the exter- 

 nal ear, are superadded by no less extraordinary modi- 



