236 ELEMENTAR Y ANA TOM Y. [LESS. 



LESSON VII. 



THE EXTERNAL SKELETON, THE SKIN AND ITS 

 APPENDAGES. 



1. THE second category of skeletal parts remains to be 

 considered, namely, those which together constitute the 



EXTERNAL SKELETON. 



As has been said when the description of the skeleton was 

 begun, fibrous tissue pervades the whole of the human frame, 

 bounding and supporting all its organs. 



With certain exceptions shortly to be noticed, it is the 

 skin (taking this word in its wide sense) alone of all the fibrous 

 structures which, in other animals than man, develops tissue 

 of a dense and solid nature horny or bony. 



2. Under the head of " external skeleton," or exo-skeleton, 

 will be considered the skin and its appendages, that is to 

 say, the skin, whether external or internal, together with 

 structures developed from or in it. 



Such structures in man are the hair, nails, and teeth, but 

 we shall find that many and diverse exo-skeletal parts may 

 exist which have no direct counterparts in the human frame. 



3. The SKIN of man invests his body pretty closely. 

 Nevertheless the roots of the fingers and toes (especially of 

 the three middle toes) are held together by folds of skin, 

 and in some cases these folds extend far along, binding the 

 digits together, and causing the person so affected to be 

 what is called " web-fingered " or " web-toed." Such a con- 

 dition is constant with regard to two of the toes in an animal 

 so nearly resembling man in structure as the Siamang 

 Gibbon, and is familiar to us in Ducks and Geese. 



Extensions of the skin far greater than this may, how- 

 ever, take place. Thus, in the so-called " Flying Squirrels" 

 the skin of the sides, between the arms and the legs, is 

 much expanded, serving for a parachute. A skin parachute, 



