CONNECTIVE TISSUES 43 



prevents the loss of heat from the body, being, in fact, an 

 extra garment. 



2nd. Chemical. — Fat, on account of its great quantity of 

 unoxidised carbon and hydrogen, is the great storehouse of 

 energy in the body. 



(C) Pigment Cells. — In various parts of the eye the 

 connective tissue and other cells contain a black pigment — 

 Melanin. The precise mode of origin of this pigment is not 

 known. It contains carbon, hydrogen, nitrogen, oxygen, 

 and it may also contain iron. Melanin is closely related to 

 a series of dark pigments which are produced by heating 

 protein with mineral acids — the melanoidins — and like them, 

 when heated with potash, it yields indol and skatol (see p. 

 330). It is therefore probably connected with tyrosin 

 or Avith tryptophan (see p. 17). It has nothing to do 

 with the blood-pigments. Melanin-like pigments are widely 

 distributed in nature, occurring not only in the connective 

 tissue pigment-cells of animals, but also in epithelial cells of 

 the epidermis, hair, and eye, and in the tissues of some 

 plants. Its function in the eye is to prevent the passage of 

 light through the tissues in which it is contained. 



The fibrous-tissue cells containing the pigment are 

 branched, and in many cases they possess the power of move- 

 ment. This is specially well seen in such cells in the skin 

 of the frog, where contraction and expansion may be studied 

 under the microscope. By these movements the skin is 

 made lighter or darker in colour. The movements of these 

 cells are under the control of the central nervous system. 



3. Cartilage. — While fibrous tissue is the great binding 

 medium of the body, support is afforded in fcetal life and in 

 certain situations in adult life by cartilage. 



Where cartilage is to be formed, the embryonic cells become 

 more or less oval, and secrete around them a clear pellucid 

 capsule. This may become hard, and persist through life, as 

 in the so-called iiarenchymatous cartilage of the mouse's ear. 



(1) Hyaline Cartilage. — Development, liowever, usually 

 goes further, and before the capsule has hardened, the 

 cartilage cells again divide, and each half forms a new 



