238 PIGMENT. 



being usually found in great abundance in those situations in which fat is becoming 

 developed (fig. 278). Sometimes, however, the deposition takes place in the flattened 

 connective tissue cells (fig. 277, /), or in cells which are apparently intermediate 

 between these and the granular cells, being rounded like the latter, but consisting 

 of a much clearer protoplasm (fig. 277, /') When deposited in ramified or flattened 

 cells these acquire a spherical shape as they enlarge, in consequence of the distension 

 produced by the accumulating fat. 



The fat in some parts, and especially in the serous membranes, is formed at the 

 expense of pre-existing lymphatic tissue, the lymphoid cells probably becoming 

 enlarged and transformed into fat-cells, whilst a considerable development of blood- 

 vessels accompanies the change. A similar transformation is also witnessed in the 

 thymus gland, which, in the foetus and infant, is chiefly composed of lymphoid 

 tissue, but as growth proceeds becomes wholly converted into a mass of fat. 



The superficial resemblance which adipose tissue often bears to many glands, in its 

 lobulated structure and the arrangement of its blood-vessels, has led some histologists to 

 look upon the fat-cell as a corpuscle of specific nature, and totally distinct from any other 

 kind of cell met with in the connective tissue. But against this view it may be urged that 

 the situations in which fat is deposited are, previously to its appearance, in no way dis- 

 tinguishable from the rest of the areolar tissue ; that the cells in which it is produced are, so- 

 far as can be seen, the same as those which are met with in almost all parts of the areolar 

 tissue ; and further, that when from any natural cause the fat is entirely removed from the 

 cells of a part and not again deposited in them, the part eventually acquires all the ordinary 

 characteristics of the areolar tissue. The great development of blood-vessels in adipose tissue 

 is obviously related to the function which it subserves in storing up the fatty materials 

 derived from the food in such a form and situation as to be readily re-absorbed into the- 

 circulation when needed. Fat may be deposited in other cells besides those^of connective tissue. 



PIGMENT. 



An accumulation of coloured pigment-granules is met with in many cells of the 

 animal body, but most frequently in epithelium-cells and in cells belonging to the 

 connective tissue. A well-marked example of pigmented epithelium-cells in the 

 human body is afforded by the black coating which forms the external layer of the 

 retina of the eye, and covers the posterior surface of the iris. Pigment is also met 

 with in the deeper layers of the cuticle, especially in the coloured races, in the 

 cortical substance of the hairs, in certain epithelial cells of the membranous 

 labyrinth of the ear, and the olfactory region of the nose. 



In the connective tissue the pigment is met with in enlarged and irregularly 

 branched corpuscles which are termed pigment-cells. Such ramified cells are 

 very common in many animals. In the human body cells of this description are 

 found in the dark tissue on the outer surface of the choroid coat (fig. 279), in the 

 iris, where they are often variously coloured, and on the pia mater covering the 

 upper part of the spinal cord. Pigment is also found in some of the ramified cells 

 which form part of the retiform tissue of the medullary substance of lymphatic 

 glands, and occasionally in some of the similar cells of the spleen. It may also be 

 seen in migratory cells, and these are believed to carry the pigment to the epithelial 

 structures in which it is deposited. 



The pigment (melanin) which is contained within the cells, consists of black or 

 brown granules or molecules of a round or oblong shape, and almost too small 

 for exact measurement. These molecules are densely packed together in some 

 cells ; in others they are more scattered, and then it may be seen that there is a 

 certain amount of colourless matter included along with them. When they escape 

 from the ruptured cells, they exhibit very strikingly the " Brownian " molecular 

 movement ; and in consequence of this movement the apparent figure of the 

 particles is subject to change. It is worthy of remark, that when viewed singly with 

 a very high magnifying power they look transparent and almost colourless, and it is 



