30 HANDBOOK OF PHYSIOLOGY. 



B. Special Varieties. 



a. Gelatinous. 



b. Adenoid or Eetiform. 



c. Neuroglia. 



d. Adipose. 



II. Cartilage. 

 III. Bone. 



Structure of Connective Tissues. 



All of the varieties of connective tissue are made up of two elements, 

 namely, cells and intercellular substance. 

 (A.) Cells. The cells are of two kinds. 



(a.) Fixed. These are cells of a flattened shape, with branched pro- 

 cesses, which are often united together to form a network : they can be 

 most readily observed in the cornea, in which they are arranged, layer 



above layer, parallel to the free surface. 

 They lie in spaces, in the intercellular or 

 ground substance, which are of the same 

 shape as the cells they contain, but rather 

 larger, and which form by anastomosis a sys- 

 tem of branching canals freely communicat- 

 ing (Fig. 28). 



To this class of cells belong the flattened 

 tendon corpuscles which are arranged in 

 long lines or rows parallel to the fibres (Fig. 

 34) 



FIQ. 29. Ramified pigment- 

 cells, from the tissue of the choroid These branched cells, in certain situa- 

 coat of the eye. x 350. a, cell with 



pigment; 6, colorless fusiform tions, contain a number of pigment-granules, 



cells. (Kolliker.) . . ' 



giving them a dark appearance : they form 



one variety of pigment-cell. Branched pigment-cells of this kind are found 

 in the outer layers of the choroid (Fig. 29) . In many of the lower animals, 

 such as the frog, they are found widely distributed, not only in the skin, 

 but also in internal parts, e. g., the mesentery and sheaths of blood-ves- 

 sels. In the web of the frog's foot such cells may be seen, with pigment- 

 granules evenly distributed throughout the body of the cell, and its 

 processes ; but under the action of light, electricity, and other stimuli, 

 the pigment-granules become massed in the body of the cell, leaving the 

 processes quite hyaline ; if the stimulus be removed, they will gradually 

 be distributed again throughout the processes. Thus the skin in the 

 frog is sometimes uniformly dusky, and sometimes quite light-colored, 

 with isolated dark spots. In the choroid anb retina the pigment-cells 

 absorb light. 



(b.) Amoeboid cells, of an approximately spherical shape: they have a 

 great general resemblance to colorless blood-corpuscles (Fig. 2), with 



