CHAPTER XIL 



THE SKIN AND ITS APPENDAGES. 



The cutaneous system comprises the skin, hair and 

 glands. The teeth are sometimes considered as be- 

 longing to this system. The skin is composed of two 

 distinct structures, viz. : the cuticle and cutis. The 

 cuticle is derived from the ectoderm and the cutis is 

 derived from the mesoderm. The cuticle consists of 

 four layers ; the stratum corneum, the stratum lucidum, 

 the stratum granulosum and the stratum Malpighii. 

 The first layer is composed of cells that have lost the 

 appearance of ever having been nucleated, called the 

 cornified layer. The second layer is composed of cells, 

 wilier! are polyhedral in shape and have almost lost 

 their nuclei. The third layer is composed of cells ir- 

 regularly polyhedral in shape and are joined together 

 by little spines. These cells are called prickle cells, 

 and outside of these are seen other cells, which have a 

 granular protoplasm. These two layers constitute the 

 stratum granulosum. The fourth layer is composed 

 of cells, which are columnar in shape and are in a 

 stage of active growth, as shown by the property they 



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