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HAND-BOOK OF PHYSIOLOGY. 



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

 fusiform (Fig. 10). They may be greatly elongated so as to become 

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

 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 



\ 



FIG. 10. Various forms of cells, a. Cylindrical or columnar, b. Caudate, c. Fusiform, d. Cilia- 

 ted (from trachea), e. Branched, stellate. 



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

 typical examples (Fig. 10, e). In these cells the primitive branches by 

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



Classification of Cells. Cells may be classified in many ways. 

 According to: 



(a.) Form : They may be classified into spheroidal or polyhedral, dis- 

 coidal, flat or scaly, cylindrical, caudate, fusiform, ciliated and stellate.. 



(b. ) Situation : we may divide them into blood cells, gland cells, 

 connective tissue cells, etc. 



(c.) Contents : fat and pigment cells and the like. 



(d.) Function: secreting, protective, contractile, etc. 



(e.) Origin: hypoblastic, mesoblastic, and epiblastic cells. (See 

 chapter on Generation.) 



It remains only to consider the various ways in which cells are con- 

 nected together to form tissues, and the transformations by which inter- 

 cellular substance, fibres and tubules are produced. 



Modes of connection. Cells are connected: 



(1) By a cementing intercellular substance. This is probably always 

 present as a transparent, colorless, viscid, albuminous substance, even 

 between the closely apposed cells of cylindrical epithelium, while in the 

 case of cartilage it forms the main bulk of the tissue, and the cells only 

 appear as imbedded in, not as cemented by, the intercellular substance. 



This intercellular substance may be either homogeneous or fibrillated. 



In many cases (e.g. the cornea) it can be shown to contain a number 



