8 ANATOMY FOR NURSES. [Chap. II. 



body not in themselves distinguished by the preponderance of 

 any tissue. Such are : — 



Blood-vessels. Synovial membranes. 



Lymphatic vessels. Mucous membranes. 



Lymphatic glands and bodies Secreting glands. 



of like structure. Integument or skin. 

 Serous membranes. 



Thus, though we may say the greater bulk of the body is made 

 up of a combination of four distinct tissues, — the epithelial, 

 connective, muscular, and nervous, — there are parts in which 

 these tissues are so intimately mixed that we cannot distinguish 

 any distinct variety, and we are therefore obliged to class them 

 by themselves. 



As the structure of an organ depends upon the properties of 

 the tissues composing it, so the characteristics of each tissue 

 depend upon their ultimate structural units — the cells and the 

 products of the cells. ^ 



The early embryo is an agglomeration of cells, and the whole 

 of the body is developed out of one cell, called the ovum, 

 which measures 2^0 ^^ 120 ^^ ^^ ^'^^^ (0.106 to 0.211 mm.) 

 in diameter. In the beginning of the formation of the body, 

 the protoplasm of the ovum divides and subdivides, and the 

 daughter cells thus formed eventually arrange themselves in 

 three la3'ers. These layers are known respectively as the epi- 

 blast, or upper layer ; the mesoblast, or middle layer ; the hypo- 

 blast^ or under layer. The epiblast is supposed to give rise to the 

 nervous tissue and most of the epithelial tissue ; the mesoblast 

 to the connective and muscular tissues, and also to a portion of the 

 epithelial tissue; the hypoblast to the rest of the epithelial tissue. 

 Of these tissues, the epithelial is the simplest and most nearly 

 allied to the primitive tissue, and will first engage our attention. 



Epithelial tissue. — Epithelial tissue is composed entirely of 

 cells united together by adhesive matter. The cells are gener- 

 ally so arranged as to form a skin or membrane, covering the 

 external surfaces, and lining the internal parts of the body. 

 This membrane is seen when the skin is blistered, the thin and 

 nearly transparent membrane raised from the surface being 



^ By the products of the cells is meant, for example, the fibres of connective 

 tissue, or the intercellular substance of cai-tilage and bone. 



