STRUCTURE OF THE ELEMENTARY TISSUES. 



-'8, 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 inter- 

 cellular or ground substance, which are of the same shape as the cells 

 tlu-v contain but rather larger, and which form by anastomosis a system 

 of branching canals freely communicating (Fig. 2-4). 



Fig. 24. Horizontal preparation of cornea of frog, stained in gold chloride; showing the network 

 of branched cornea corpuscles. The ground-substance is completely colorless. X 400. (Klein.) 



To this class of cells belong the flattened tendon corpuscles which are 

 arranged in long lines or rows parallel to the fibres (Fig. 29). 



These branched cells, in certain situations, contain a number of pig- 

 ment-granules, giving them a dark appearance: they form one variety of 

 pigment-cells. Branched pigment-cells of this kind are found in the 

 outer layers of the choroid (Fig. 25). In many 

 lower animals, such as the frog, they are found 

 widely distributed, not only in the skin, but also in 

 many internal parts, e.g., the mesentery and sheaths 

 of blood-vessels. In the web of the frog's foot such 

 pigment-cells may be seen, with pigment evenly 

 distributed through 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 all over the pro- 

 cesses. Thus the skin in the frog is sometimes 



FIG. 25. Ramified pig- 

 ment-cells from the tissue 

 of the choroid coat of the 

 eye. X 350. a, cell with 

 pigment; 6, colorless 

 form cells. (Kolliker.) 



In the choroid and retina the pigment- 



*y - 



pigment; 6, colorless fusi- 



uniformly dusky, and sometimes quite light-col- 

 ored, with isolated dark spots, 

 cells absorb light. 



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

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



