CHAP. VIII.] EPITHELIAL TISSUES, &C. 303 



Prepara- The cartilaginous investment of Ascidians, e.g. the 



tion. "mantle" of Phallusia and Cynthia and the external coat of 



Salpa consist mainly of Tunicin and may be employed in its preparation. 



These structures are digested in hot water, then for a short time in 

 dilute acids and alkalies, lastly in alcohol and ether ; the residue, which 

 preserves the original form of the structures, consists of tuiiicin. 



Tunicin is by the action of acids converted entirely into a reducing 

 sugar susceptible of the alcoholic fermentation (dextrose?). It is maintained 

 by Berthelot 1 that tunicin presents certain differences from ordinary 

 cellulose, as, for instance, that it is coloured yellow by iodine and is less 

 affected by certain reagents. 



SECT. 3. ON CERTAIN COLOURING MATTERS OF THE EPITHELIAL 

 TISSUES OF VERTEBRATES. 



Brown and black Pigments. Melanin. 



The cells of the rete Malpighii of the human skin often contain 

 granules of a black pigment; this is especially the case in the skin 

 of the negro, which owes its colour to these pigment-bearing cells. 

 A similar pigment is found in the hexagonal epithelial cells which 

 constitutes the most external layer of the retina, and which used 

 formerly to be considered as belonging to the choroid; also in the 

 connective tissue cells of the outer layer of the choroid. In the 

 bronchial lymphatic glands, of adults and aged persons, in the lung 

 tissue and in melanotic tumours, similar brown or black pigments are 

 discovered, which are all included under the name of Melanin, though 

 it is certain that the substance obtained from these various sources 

 does not present an uniform composition; in all probability, however, 

 all these colouring matters derive from the decomposition of haemo- 

 globin. The formation of such a black pigment has actually been 

 traced in the interior of the red blood-corpuscles, in cases of per- 

 nicious intermittent fevers (see p. 163). 



Characters Melanin occurs in the form of minute amorphous 



and reaction granules which when suspended in water exhibit 

 of Melanin. Brownian movements. It is soluble in ether, alcohol, 

 water and acids. When boiled with solution of caustic potash the 

 black colouring matter is slowly and imperfectly dissolved, a brown 

 liquid being formed, which is discolourized by chlorine. 



In the lung tissue, particles of carbon sometimes occur ; these are some- 

 times in a finely granular condition, though occasionally they present the 

 appearance of minute fragments of coal. The latter are distinguished from 

 melanin by their complete insolubility in boiling caustic potash, in 

 boiling sulphuric acid, and when boiled in strong hydrochloric acid and 

 potassium chlorate. 



1 Berthelot, Ann. de Chim. et de Phys., Vol. LVI. p. 149. 



