KETIFORM OR RETICULAE TISSUE. 239 



only when they are heaped together that their dark colour distinctly appears. The 

 nucleus of the pigment-cell is not coloured, but is very often hidden from view by 

 the black particles. 



In the lower animals remarkable movements are often observed in the ramified 

 pigment-cells, e.g., those of the frog's skin. In these the dark particles of pigment 



Fig. 279. A SMALL PORTION OF THE CHOKOID COAT OF THE EYE I HIGHLY MAGNIFIED. (E. A. S.) 



p, pigment-cells ; /, elastic fibres ; n, nuclei of epithelioid cells (the outlines of the cells are not 

 indicated) ; I, lymphoid cells. 



are at one time dispersed through the whole cell and its branches, but at another 

 time they gather into a heap in the central part, leaving the rest of the branched 

 cell vacant, but without alteration of its figure. In the former case the skin is of a 

 dusky hue ; in the latter, pale. The aggregation of the pigment-molecules can be 

 excited through the nerves, both directly and also in a reflex manner, as by the 

 stimulus of light upon the retina. 



RETIFORM OR RETICULAR TISSUE. 



This is a distinct variety of connective tissue which is met with widely dis- 

 tributed in various parts of the body, constituting the whole framework of some 

 organs and entering largely into the constitution of many mucous membranes. It 

 is composed of a very fine network or reticulum of connective-tissue fibrils, which 

 in their behaviour to staining reagents and in their general microscopic appearance 

 closely resemble the white fibres of areolar tissue, with which, in the lymphatic glands, 

 they are undoubtedly continuous. In their chemical character they are, according 

 to Mall, more nearly allied to elastic fibres, or rather with the membranes of the elastic 

 fibres. However this may be, it is nevertheless clear from its anatomical continuity 

 with the white fibrils of connective tissue that the retiform tissue is merely a variety 

 of areolar tissue, being formed of very fine anastomosing bundles, with the meshes 

 of the network occupied by fluid ; the ground substance having disappeared. In 

 most situations the fixed cells of the tissue are applied to and are wrapped round 

 the strands of the network, which may thus be in great measure concealed by the 

 cells. The tissue then appears formed of a network of branching and anastomosing 

 cells, and was for a long time so described, but if the cells are brushed away or 



