270 SENSATION AND THE SENSIFEROUS ORGANS. [LECT. 



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

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

 continuation of the general epidermis. These depres- 

 sions 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 

 difficulties. For the essential part of the sense organ, 

 in this case, is the membranous labyrinth, a bag of 

 complicated 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 

 development 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 de- 

 pression on the side of its back part. The depression, 

 rapidly deepening, becomes a small pouch ; and then, 

 the communication 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 cartilage and then into 

 bone, enclose the auditory sac in a strong case, in 

 which it undergoes its further metamorphoses ; while 

 the drum, the ear bones, and the external ear, are 

 superadded by no less extraordinary modifications of 

 the adjacent parts. Still more marvellous is the 

 history of the development of the organ of vision. 

 In the place of the eye, as in that of the nose and 



