1 8 ELEMENTS OF MAMMALIAN ANATOMY 



complex and is separated from the rest of the cell by a nu- 

 clear membrane which performs an important function in 

 general cell activity. The nucleus contains a highly organized 

 substance, the cJiromatin, which is important in reproduction 

 and metabolism and is supposed to be the bearer of hereditary 

 characters. During cell-division the chromatin material may 

 be collected in definite masses or rods, chromosomes, the num- 

 ber and appearance of which are constant for the species. The 

 animal cell has no definite cell wall but the peripheral layer of 



protoplasm is so modified as to act like a 

 semipermeable membrane which plays, an 

 important role in cellular activity. 



Cells vary in size, shape, structure and 



arrangement according to function or 



pressure. Cells differentiated to perform 



a certain definite function are grouped 



Fig. 8.— In\'oluntary together w^ith a certain amount of in- 



MuscLE-cELLs. X 250. tcrccllular substance to form a tissue. 



n. Nucleus of a cell. 



There are four primary tissues: epithelial, 

 connective, rnusctdar, and nervous. 



Epithelial tissues line all the inner and outer surfaces of 

 the body. They are distinguished by the importance of the 

 cells and the small amount of intercellular substance, cement. 

 They serve for protection, secretion, excretion, and reaction 

 to stimuli. They may be simple, that is made of a single row 

 of cells as in the intestine, or complex or stratified, made up of 

 several layers as in the skin. Epithelial cells form the paren- 

 chyma of all secreting glands. 



Connective tissues are distinguished by the large amount of 

 intercellular substance. They form the supporting elements 

 of the body. Ordinary connective tissues are recognized by 

 the character of their fibers or their cells. Those recognized by 

 their fibers are white fibrous or tendon, yellow elastic, reticu- 

 lar, and areolar. The areolar is the commonest kind and is 

 found everywhere, as just beneath the skin, between muscle 



