1092 GENERAL ANATOMY OR HISTOLOGY. 



diameter of -3-^3- of an inch. They are formed of an exceedingly delicate proto- 

 plasmic 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 demonstrated by staining 

 with haematoxylin ; in the natural condition it is so compressed by the contained 

 oily matter as to be scarcely recognizable. These fat-cells are contained in clus- 

 ters in the areolae of fine connective tissue, and are held together mainly by a 

 network of capillary blood-vessels, which are distributed to them. 



Chemically the oily material in the cells is composed of the fats, olein, 

 palmitin, and stearin, which are glycerin compounds with fatty acids. Some- 

 times fat-crystals form in the cells after death (Fig. 617, 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 connective-tissue 

 corpuscles, in which small droplets of oil are formed ; these coalesce to produce a 

 larger drop, and this increases until it distends the corpuscle, the remaining 

 protoplasm and the nucleus being crowded to the periphery of the cell (Fig. 618). 



THE 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 

 in the external layer of the retina 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 fre- 

 quently met with in 'the lower vertebrates. In man 

 it is found in the choroid coat of the eye (Fig. 619) 

 and in the iris of all but the light-blue eyes and the 

 albino. It is also occasionally met with in the cells 



FIG. 619. Pigment-cells from r.-r , i ~ < i 



the choroid coat of the eyeball. oi retitorm 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." 



THE CARTILAGE. 



Cartilage is a non-vascular structure which is found in various parts of the 

 body in adult life chiefly in the joints, in the parietes of the thorax, and in 

 various tubes, such as the air-passages, nostrils, and ears, which are to be kept 

 permanently open. In the foetus at an early period the greater part of the skele- 

 ton is cartilaginous. As this cartilage is afterward replaced by bone, it .is called 

 temporary, in contradistinction to that which remains unossified during the whole 

 of life, and which is called permanent. 



Cartilage is divided, according to its minute structure, into hyaline cartilage, 

 fibro-cartilage, and yellow or elastic white fibro-cartilage. Besides these varieties 

 met with in the adult human subject, there is a variety called cellular cartilage, 

 which consists entirely, or almost entirely, of cells, united in some cases by a 



