LEPUS CUNICULUS 271 



however, they are more marked and become large after the birth of 

 the young. They are marked externally by papilla-like projections 

 known as the teats or mammae. The ducts of the gland open on to 

 the apex of this projection, and so form an outlet through which the 

 young animal can suck the milk that is its only food for the first 

 weeks of its life. 



Integument and Muscular System. 



The skin of the rabbit fits far more closely to the under- 

 lying muscular tissue than in the frog, for there are no subcutaneous 

 lymph sinuses. A certain amount of independent movement is 

 provided for, however, by the development of a layer of sub- 

 cutaneous or areolar connective tissue, which connects the skin to 

 the body wall. The structure of this tissue has already been dealt 

 with. The detailed structure of the skin and its glands also differ 

 in Rana and Lepus, although both are composed of the same two 

 fundamental layers, the epidermis and the dermis. 



The epidermis is a stratified epithelium divisible into two layers ; 

 it is derived from the ectoderm of the embryo, and to be regarded as 

 forming a protective layer for the underlying tissues. Its basal 

 portion is composed of a number of layers of polygonal cells, which 

 are composed of protoplasm and contain nuclei. These constitute 

 the rete mucosum or Malpighian Layer, and the cells at its outer side 

 become filled with secretory granules. The outer layer of the 

 epidermis, known as the stratum corneum, is many layers thick, and 

 the cells become more and more flattened as they pass outwards, at 

 the same time their nuclei disappear and the protoplasm becomes 

 replaced by a substance allied to keratin, a process termed cornifica- 

 tion. The outermost cells form small dead horny scales that are 

 constantly being shed. As noted above, this is a protective layer, 

 and in certain parts, for example the heel of man, becomes extremely 

 thick. 



The dermis or corium is composed of dense connective tissue, 

 which on its outer side is thrown up into a series of folds into which 

 pass the blood vessels nourishing the skin, and in which are situated 

 the various touch corpuscles. As it passes inwards the texture of the 

 dermis becomes looser, and it incloses islets of fatty tissue, and finally 

 it merges into the subcutaneous tissue. 



Hairs, the characteristic covering of the mammalian skin, are 

 the products of the epidermis, and are borne in deep tubular pit-like 

 downgrowths of that layer termed the hair follicles. Each hair is a 

 long rod-like structure with an enlarged root, and contains an axis 

 or medulla of large angular cells containing granules and sometimes 

 air globules. Surrounding this is a fibrous layer composed of long 



