CHAPTER III. 



THE STRUCTURE OF THE ELEMENTARY TISSUES. 



THE careful examination of the minute anatomy of the body has 

 shown that there are certain elementary structures, of which, alone or 

 when combined in varying proportions, the whole of the organs and 

 tissues of the body are made up. These Elementary Tissues are four 

 in number, called : (1.) The Epithelial / (2.) The Connective ; (3.) The 

 Muscular, and (4.) The Nervous. To these four, some would add a fifth, 

 looking upon the Blood and Lymph, containing, as they do, formed 

 elements in a fluid menstruum, as a distinct tissue. 



All of these elementary tissues consist of cells and of their altered 

 equivalents. It will be as well therefore to indicate some of the differ- 

 ences between the cells of the body. They are named in various ways, 

 according to their shape, situation, contents, origin, and functions. 



(a.) From their shape, cells are called spherical or spheroidal, which 

 is the typical shape of the free cell; this maybe altered to polyhedral 

 when the pressure on the cells in all directions is nearly the same; of 

 this the primitive segmentation cells afford an example. The discoid 

 form is seen in blood-corpuscles, and the scale-like form in superficial 

 epithelial cells. Some cells have a jagged outline and are then called 

 prickle cells. Cells of cylindrical, conical, or prismatic form occur in 

 various places in the body. Such cells may taper off at one or both 

 ends into fine processes, in the former case being caudate, in the latter 

 fusiform. They may be greatly elongated so as to become fibres. Cells 

 with hair-like processes, or cilia, projecting from their free surfaces, are 

 a special variety. The cilia vary greatly in size, and may even exceed 

 in length the cell itself. Finally, cells may be branched or stellate with 

 long outstanding processes. 



(b.) From their situation cells may be called free, as in the blood, 

 or combined, when connected together or with other elements to form 

 organs and tissues. 



(c.) From their contents cells are called, when containing fat for 

 example, fat cells ; when containing pigment, pigment cells, etc. 



(d.) From their function cells are called secreting, protective, sensi- 

 tive, contractile, and the like. 



(e.) From their origin cells are called epiblastic and mesoblastic and 

 hypoblastic. 



26 



