5i6 THE CELL AND MAMMALIAN HISTOLOGY 



or its ground substance, or of more than one of these. In adipose 

 tissue large numbers of mechanocytes become so distended with 

 fat that each appears as a large droplet of fat with thin cyto- 

 plasmic walls, and at one point a nucleus squeezed between the 

 fat and the cell wall. The fat cells are probably specifically 

 different from the fibroblasts, which are also present, and make 

 white fibres which form a skeleton for the tissue. It is these 

 fibres, together with the small amount of cytoplasm, which have 

 to be removed when beef or mutton fat is clarified in the kitchen. 

 Adipose tissue is chiefly found beneath the skin (where it makes 

 the layer called panniculus adiposus), in the mesenteries, and 

 round the kidneys. It is a food reserve, the fat being removed for 

 oxidation or conversion into other substances when the food supply 

 is low, but it also in places acts as a cushion, as in the eye-socket, 

 and in naked mammals, such as man and the whale, the sub- 

 cutaneous fat is an important heat insulator. The camel's hump 

 is largely fat, which provides water when oxidized. 



FIBROUS TISSUE 



If there is in connective tissue little but collagenous fibres 

 and the cells which form them we have white fibrous tissue. This 

 is tough, strong, and non-elastic, and is found in parts of the body 

 where these properties are important. The fibres may be arranged 

 parallel to one another to form a cord, as in the tendons by which 

 the pull of a muscle is transmitted to a bone or the ligaments 

 which join two bones, or they may run criss-cross in one plane 

 to form a sheet, as in the pericardium and the dura mater w^hich 

 covers the brain. The fibres of tendons are continuous with those 

 which run in the general connective tissue of the muscle and bone, 

 so that there is a very intimate connection between the two. 

 Because of the high tensile strength of white fibres — of the order 

 of 10,000 lbs. per square inch, which is similar to that of oak or 

 cast iron — tendons seldom break, which is a good thing, as they 

 regenerate only slowly. In elastic tissue little is present but yellow 

 fibres, and it is accordingly found where elasticity is important. It 

 may form cords, as in the ligamentum nuchae by which the skull 

 is attached to the cervical vertebrae, or sheets as in the walls of 

 the trachea, lungs and arteries. Elastic fibres in the skin keep 

 it taut and smooth and cause the edges of wounds to gape. 

 Their deficiency in the aged allows the formation of wrinkles. 



