THE OUTER TISSUES OF ANIMALS. 319 



I entertain no doubt whatever of the fact. The outer ends 

 of the olfactory sacs remain open, but those of the ocular 

 auditory sacs rapidly close up, and shut off their contents 

 from all direct communication with the exterior." That is 

 to say, the eye considered as an optical apparatus is pro- 

 duced by metamorphoses of the skin: the only parts of it 

 not thus produced, being the membranes lying between the 

 sclerotic and the vitreous humour, including those retinal 

 structures formed in them. All is tegumentary save that 

 which has to appreciate the impressions which the modified 

 integument concentrates upon it. 



Thus, as Prof. Huxley has somewhere pointed out, there 

 is a substantial parallelism between all the sensory organs in 

 their modes of development; as there is, too, between their 

 modes of action. A vibrissa may be taken as their common 

 type. Increased impressibility by an external stimulus, 

 requires an increased peripheral expansion of the nervous 

 system on which the stimulus may fall; and this is secured 

 by an introversion of the integument, forming a sac on the 

 walls of which a nerve may ramify. That the more extended 

 sensory area thus constituted may be acted upon, there 

 requires some apparatus conveying to it from without the 

 appropriate stimulus; and in the case of the vibrissa, this 

 apparatus is the epidermic growth which, under the form of 

 a hair, protrudes from the sac. And that the greatest 

 sensitiveness may be obtained, the external action must be 

 exaggerated or multiplied by the apparatus which conveys it 

 to the recipient nerve ; as, in the case of the vibrissa, it is by 

 the development of a hair into an elastic lever, that trans- 

 forms the slight force acting through considerable space on 

 its exposed end, into a greater force acting through a smaller 

 space at its rooted end. Similarly with the organs of the 

 higher senses. In a rudimentary eye, the slightly modified 

 sense cell has but a rudimentary nerve to take cognizance of 

 the impression; and to concentrate the impression upon it, 

 there is nothing beyond a thickening of the epidermis into a 



