CHAPTER II. 



THE STRUCTURE OF THE ELEMENTARY TISSUES. 



THE cells of the body are described in various ways ; for example, ac- 

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



(a.) Their shape varies: Starting f rom the spherical or spheroidal 

 (Fig. 10, a) as the typical form assumed by a free cell, we find this altered 

 to a polyhedral shape when the pressure on the cells in all directions is 

 nearly the same (Fig. 10, b). 



Of this, the primitive segmentation-cells may afford an example. 



The discoid shape is seen in blood-cells (Fig. 10, c), and the scale-like 

 form in superficial epithelial cells (Fig. 10, d). Some cells have a jagged 

 outline (prickle-cells) (Fig. 27). 



Cylindrical, conical, or prismatic cells occur in the deeper layers of 

 laminated epithelium, and the simple cylindrical epithelium of the in- 

 testine and many gland ducts. Such cells may taper off at one or both 

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

 fusiform (Fig. 11). They may be greatly elongated so as to become 

 fibres. Ciliated cells (Fig. 10, d) must be noticed as a distinct variety : 



FIG. 10. Various forms of cells, a. Spheroidal, showing nucleus and nucleolus ; 6. Polyhedral ; 

 c. Discoidal (blood cells) ; d. Scaly or squamous (epithelial cells). 



they possess, but only on their free surfaces, hair-like processes (cilia). 

 These vary immensely in size, and may even exceed in length the cell 

 itself. Finally, we have the branched or stellate cells, of which the large 

 nerve-cells of the spinal cord, and the connective tissue corpuscle are 

 typical examples (Fig. 11, e). In these cells the primitive branches by 

 secondary branching may give rise to an intricate network of processes. 



(b.) According to their situation in the tissues cells are known as 

 epithelial, connective tissue cells, blood cells, glandular, and the like. 



(c.) According to their contents, they are called fat cells when their 

 protoplasm contains an excess of fat, pigment cells when it contains pig- 

 ment ; colored, when their protoplasm is infiltrated with a coloring 

 matter, as haemoglobin. 



