174 THE CIVIL ENGINEERS OF THE BODY 



(3) Pigment cells. Fibrous tissue cells (and other cells) in 

 certain parts of the body (e.g. eye) may contain a pigment 

 melanin. How this pigment is formed and what exactly are its 

 functions remain matters of conjecture. Chemically, melanin 

 is closely related with the melanoidins dark pigments resulting 

 from digestion of proteins with hot mineral acids. They serve 

 (a) as light niters preventing the passage of light through the 

 pigment cell. That is, the pigment absorbs energy. These 

 pigmented areas are nearly always found in places exposed to 

 light, and one may suppose that the incidence of strong light on 

 fibrous tissue may cause the formation of melanin from the cell 

 protein. Inorganic examples of the formation of light-absorbing 

 chemical compounds by the absorption of light will occur to the 

 student (cf. Silver Salts), (b) Their function is not only to 

 protect the underlying tissue from the harmful action of radiant 

 energy, but in many cases the pigment cells act as a transmitting 

 station receiving the light stimulus and transmitting it to the 

 effectors. This dermatoptic function has been studied and 

 described by R. Dubois. Light falling on the pigment cells of the 

 epithelium of the siphon of Pholas, a mollusc, causes a reflex 

 retraction of the siphon. Observation under the microscope 

 has shown that the pigment cells in the skin of the frog contract 

 when light falls on them. The pigments of the eye and their 

 action in transmitting light stimuli will be dealt with in another 

 chapter. 



Two modifications of fibrous tissue warrant separate treatment, 

 i.e. cartilage and bone. In lower animals and during the foetal 

 life of higher animals (as well as in certain situations in adult life) 

 rigidity is given to the body by cartilage. The function of 

 cartilage cells will be dealt with later under bone and lubrication. 

 Here it is sufficient to note that the peculiarity of this tissue is the 

 secretion of a homogeneous translucent gel which is tough and 

 elastic. This chondro-mucoid material is a mixture of at least 

 three colloids : ordinary protein, collagen and chondroitin. On 

 decomposition this latter substance yields substances of a carbo- 

 hydrate nature, glucosamine and glycuronic acid (cf. emulsions). 



(4) The great supporting tissue of the body is calcified fibrous 

 tissue or bone. 



(i) Development. Bone is formed by a deposition of calcium 

 salts in white fibrous tissue. Some bones which are more or 

 less flat, e.g. vault of the skull and the scapula, are formed 

 directly in fibrous tissue. This is the so-called intra-membranous 



