50 



GENERAL ANATOMY. 



in addition to its occurrence in adipose tissue is also widely present in the body, 

 as in the fat of the brain and liver and in the blood and chyle, etc. 



Fat-cells (Fig. 19) consist of a number of vesicles, varying in size, but of about 

 the average diameter of -^-^ of an inch. They are formed of an exceedingly 

 delicate protoplasmic membrane, filled with fatty matter, which is liquid during 

 life, but becomes solidified after death. They are round or spherical where they 



have not been subjected to pressure; 

 otherwise they assume a more or less 

 angular outline. A nucleus is always 

 present, and can be easily demonstrat- 

 ed by staining with hgematoxylin; in 

 the natural condition it is so com- 

 pressed by the contained oily matter 

 as to be scarcely recognizable. These 

 fat-cells are contained in clusters in 

 the areolse of fine connective tissue, 

 and are held together mainly by a 

 network of capillary blood-vessels, 

 which are distributed to them. 



Fat is an inorganized substance, 

 consisting of a liquid material (gly- 

 cerin) in combination with certain 

 fatty acids, stearic, palmitic, and 

 oleic. Sometimes the acids separate 

 spontaneously before the fat is exam- 

 ined, and are seen under the micro- 

 scope in a crystalline form, as in Fig. 



19, a. By boiling the tissue in ether or strong alcohol the fat may be extracted 

 from the vesicle, which is then seen empty and shrunken. 



Fat is said to be first detected in the human embryo about the fourteenth 

 week. The fat-cells are formed by the transformation of the protoplasmic con- 

 nective-tissue corpuscles, into which small globules of fat find their way, and 

 increase until they distend the corpuscle into the thin mantle of protoplasm which 

 forms the cell-wall, and in which its nucleus is still to be seen (Fig. 20). 



FIG. 20. Development of fat. (Klein an.l Noble Smith.) 

 a. Minute artery, v. Minute vein. c. Capillary blood- 

 vessels in the course of formation; they are not yet com- 

 pletely hollowed out, there being still left in them proto- 

 plasmic septa, d. The ground-substance, containing 

 numerous nucleated cells, some of which are more dis- 

 tinctly branched and flattened than others, and appear 

 therefore more spindle-shaped. 



PIGMENT. 



In various parts of the body pigment is found ; most frequently in epithelial 

 cells and in the cells of connective tissue. Pigmented epithelial cells are found 

 forming the external layer of the retina (Fig. 21) and on the posterior surface of 

 the iris. Pigment is also found in the epithelial cells of the 

 deeper layers of the cuticle in some parts of the body such as 

 the areola of the. nipple and in colored patches of skin, and 

 especially in the skin of the colored races, and also in hair. It 

 is also found in the epithelial cells of the olfactory region and 

 of the membranous labyrinth of the ear. 



In the connective-tissue cells pigment is frequently met with 

 in the lower vertebrates. In man it is found in the choroid coat 

 of the eye, and in the iris of all but the light-blue eyes and the albino. It is 

 also occasionally met with in the cells of retiform tissue and in the pia mater of 

 the upper part of the spinal cord. These cells are characterized by their larger 

 size and branched processes, which, as well as the body of the cells, are filled 

 with granules. The pigment consists of dark-brown or black granules of very 

 small size, closely packed together within the cells, but not invading the nucleus. 

 Occasionally the pigment is yellow, and when occurring in the cells of the cuticle 

 constitutes "freckles." 



FIG. 21. Pigment- 

 cells of retina. 



