238 ELEMENTARY ANATOMY. [LESS. 



The minute structure and more important characters of 

 the dermis, epidermis, and epithelium have been described 

 in the First and Twelfth Lessons of " Elementary Physiology." 



5. As the skin (or integument) is thus divisible into two 

 parts, so also the parts annexed to it are similarly divisible 

 according as they have their place of origin in the substance 

 of the ecteron or in that of the enderon. 



In the first case they are termed ecteronic, and may be 

 called epidermal or epithelial structures according as they 

 arise in the external surface of the body or in the internal 

 passages. In the second case they are called enderonic or 

 dermal structures. 



Thus our hair and nails are epidermal parts of the exo- 

 skeleton, because they are modifications of the external layer 

 of the skin. 



On the contrary, our teeth are dermal structures, because 

 they are developed from the deeper layer or enderon. 



We will consider now more carefully the skin itself before 

 proceeding to treat of its APPENDAGES. 



6. In the soft nature of his skin man agrees with the great 

 majority of warm-blooded Vertebrates (i.e. of Beasts and 

 Birds), though exceptions are to be met with even in his own 

 class. 



On the other hand, though the vast majority of cold- 

 blooded Vertebrates (e.g. Reptiles and Fishes) have in one or 

 ^both layers of the skin solid and dense structures, yet almost 

 all Batrachians (Frogs, Toads, and Efts) agree with man in 

 having an entirely soft and flexible integument. 



In order to estimate the condition which man presents, 

 it will be well to consider what the examination of other 

 creatures shows us to be amongst the possibilities of skin- 

 structure. 



7. First with regard to the EPIDERMIS. This, as we know, 

 is shed in man in minute fragments constantly removed by 

 friction and ablution, and constantly replaced. Only under 

 abnormal conditions and after certain diseases does it come 

 away in large and continuous patches. 



The case is very different in some of the lower animals. 

 Thus the Snakes cast off the entire epidermal investment of 

 the body even that of the eyes at once, and this process is 

 repeated at intervals. 



The little Efts, so common in our pools, also shed the 

 entire epidermis at once. Separating first at the lips, the skin 

 is pushed back over the head by the action of the fore-limbs ; 



