56 CONNECTIVE TISSUES. 



surface and covered in glycerin. Sections in this direction 

 are preferable, because the branching of the cells and their 

 mode of communication cannot be so well seen in others. 

 We shall return to these subsequently. In the membrana 

 nictitans of the frog there occur networks of large, coarsely 

 granulated cells, containing flattened oblong nuclei, and with 

 branches which run for the most part parallel with the surface. 

 This structure must be prepared with chloride of gold in 

 exactly the same way as the cornea. 



Pigment Cells. These are closely related to the fixed 

 cells now under consideration. They are more or less 

 branched corpuscles, which are sometimes isolated, sometimes 

 foriiL.a. network. The}- are, in general, larger than the ordi- 

 nary connective tissue corpuscles. Each contains an oblong 

 44ear nucleus, while both their bodies and processes are beset 

 with pigment granules. In mammalia they are 4i****l. as is 

 well known, especially in the skin, and in the sclerotic, iris, 

 and chor.oid. In the lower vertebrates, c.ij.. in the frog, they 

 are very numerous, not only in the skin, but in the peritoneum, 

 and in several mucous membranes. Pigment cells can be 

 made to retract their pigmented processes when stimulated 

 either mechanically, chemicall}', or electrically, as well as 

 under the influence of light. Let us examine them (a) in the 

 we*b, (b) in the mesentery of the frog, (c) in the tail of the 

 tadpole, and (d) in the choroid of a mammal, (a) A common 

 frog (jR. temporaria) is secured on a plate similar to that 

 shown in Fig. 11, and the toes are extended by ligatures at- 

 tached to their tips. With this view, the hole O is surrounded 

 by five or six small perforations into which wooden pins can 

 be stuck ; the ends of the ligatures are drawn through the 

 holes and fastened with the pins. In those parts of the web, 

 which appear to the naked eye dark, it is seen, even with a 

 linear magnification of 100, that the pigment cells are con- 

 nected by an extraordinary number of fine dark processes 

 which are either penicilliated or dendritic. Often the distinc- 

 tion between body and process is not marked ; it looks rather 

 as if the whole network were made up of processes. In other 

 parts, which are not so dark to the naked eye, groups of 

 pigment cells are found in which the bodies are round or 

 oblong, and the processes broader and less numerous the 

 latter being either in continuity with those of neighboring 

 corpuscles, or broken off abruptly by a gnawed edge. The 

 pigment granules do not extend to the end of these broad 

 processes ; so that it is possible to see that the substance in 

 which they are embedded is hyaline. 



If the dark parts are touched once or twice with a camel-hair 

 pencil (especially if it has been dipped in oil of turpentine), 

 the processes are gradually retracted, while, pari passu, the 



