ADIPOSE TISSUE 75 



tissues, it is not the only material which may have filled them, and 

 therefore to demonstrate the presence of fat, special methods must be 

 employed. Fresh tissue may be preserved in osmic acid, which blackens 

 not only fat but some related substances; or frozen sections of tissue may 

 be stained with Sudan III or Scharlach R, which color fat droplets red 

 and demonstrate them even when minute. These stains may also be 

 used after preservation of the tissue in formalin. It may be noted that 

 Sudan III has been fed to animals, thus imparting a pink color to the 

 living adipose tissue. If the animal is lactating, the fat globules in the 

 milk also become pink. 



Fat vacuoles occur in many sorts of cells which do not belong to adi- 

 pose tissue, such as the cells of the liver, cartilage, and striated muscle. 

 These cells are not called fat cells, even if their protoplasm contains many 

 vacuoles, and they do not resemble the cells of adipose tissue. 



Since fat cells occur in lobular masses in definite places, as under the skin, around 

 the kidney, in the bone marrow, etc., and since they supply the body with nutriment, 

 it has been proposed to regard them as constituting glandular organs. They receive 

 fat from the adjacent vessels and store it, or quite possibly they absorb carbohydrates 

 and convert them into fats. The formation of fat has been said to begin in or near 

 the nucleus with the production of granules, but the part which the nucleus plays 

 is uncertain. The small vacuoles often seen within it apparently arise after the cell 

 is full of fat. Mast cells have often been found associated with fat cells and it has 

 been supposed that they contained secretory granules which were concerned with 

 fat production. Like an internal secretion, fat is taken from the cells into the vessels 

 and distributed over the body. 



TENDON. 



Tendons consist essentially of very dense connective tissue. They 

 are composed almost wholly of parallel white or collagenous fibrils, com- 



FIG. 62. LONGITUDINAL SECTION OF THE FIG. 63. TENDON CELLS FROM THE TAIL 



TENDON OF THE FLEXOR LONGUS DIGITORUM. OF A RAT. STAINED WITH METHYLENE BLUE. 



X 160. INTRA VITAM. (Huber.) 



pactly bound together in bundles. The cementing matrix contains tendo- 

 mucoid. Closely applied to the bundles are the tendon cells which pro- 

 duced them. In ordinary longitudinal sections of tendon, the protoplasm 

 of the cells is indistinct or imperceptible, but the nuclei appear in rows 

 as seen in Fig. 62. In special preparations, particularly in those of the 



