GENERAL ANATOMY U 



GENERAL ANATOMY 



Although in every respect a continuous structure and forming 

 a single organism, the body is differentiated into a large number of 

 parts, or organs, which are more or less individual in form, com- 

 position, or function. Organs are arranged for the most part in 

 systems, each of which is concerned with some general or funda- 

 mental function, to which several organs may contribute. 



In a more general way the body may be considered as an 

 assemblage of tissues, since these are the materials of which the 

 organs are composed. Tissues may be defined as layers or aggre- 

 gations of similarly differentiated cells. They are of several 

 different kinds and are variously associated in the formation of 

 organs. Being structures of intermediate rank, they may be 

 considered either as organ components or as combinations of 

 specialized cells. 



As a body-unit a cell consists of a small mass of living proto- 

 plasm, containing a central body, the nucleus imbedded in a mass 

 of cytoplasm. The latter is surrounded or enclosed on its free 

 border by a cell-membrane. The nucleus is a highly organized 

 body, having an important function in the reproduction of the 

 cell and also in its general activity or metabolism. It contains a 

 characteristic formed material, chromatin, and frequently also a 

 minute spherical body, the nucleolus. The chief features of 

 a typical cell are illustrated in the accompanying figure (1) of the 

 developing ovum, the latter being a single cell, noteworthy for 

 its large size, and also one in which the external form is not greatly 

 modified, as it is in the majority of the cells of the body. Its 

 enclosing membrane, the zona pellucida, by which in its natural 

 position in the ovary it is separated from the surrounding follicular 

 cells, is considered to belong in part to the latter. 



As fundamental living matter, protoplasm possesses certain 

 properties on which the functions of the body ultimately depend. 

 Considered collectively, these functions are not so well illustrated 

 in the higher or multicellular organisms, in which particular 

 functions are assigned to particular cells, as in the lower unicellular 

 organisms, in which all functions are discharged by a single cell. 

 In simple or protozoan animals the protoplasm is seen to be capable 



