ON THE ORIGIN OF COLOUR 183 



consistent with physiological necessities, and, therefore, 

 not necessarily a pathological condition or development. 



The alteration in the colour of the hair, as the process 

 of ageing advances, affords a good example of the physio- 

 logical modification of the function of pigmentation, while 

 the sudden blanching of the hair observed under the 

 influence of great mental shock or emotion might be 

 recognised as an example of the pathological variety of 

 the alteration, although on strictly physiological lines. 



Pigment, as observed physiologically in the human 

 body, is detected in a large number of textures, both 

 superficial and deep, and histologically it may be said to 

 be almost consistently related to nerve textures mainly 

 terminal, both proximal and distal as, for instance, in the 

 skin, retina, Schneiderian mucosa, the otic muscular tex- 

 tures, and the smaller and larger pigmentary deposits to 

 be found scattered up and down it. It may, therefore, be 

 inferred that the nervous system, as has already been 

 remarked, is mainly instrumental in its production and 

 deposition, and that the play of nerve force on the material 

 constituting the pigment under normal as well as abnormal 

 biological conditions eventuates in a molecular transmuta- 

 tion or re-arrangement whereby the impression of colour 

 is given and the pigmentation secured the existence in 

 some nerve cells of colouring matter and the occurrence 

 of pigmentation in connection with the terminal ends of 

 some nerve fibrils being thus accounted for by histological 

 continuity. 



As it is in the physiological state so it is in the patho- 

 logical state, in which latter a morbid excitation or dis- 

 turbance of the nerve terminal textures or a solution of 

 continuity of nerve fibril investments takes place, whereby 

 a leakage or escape of nerve substance and force is per- 

 mitted into the surrounding structures, and there occurs 

 the production of more or less pigmentation "negative 

 or positive," fainter or more marked, according to the 

 site, the structures involved in the lesion, and the con- 

 tinuation and intensity of the morbid processes involved. 



Associated with the subject of pigmentation of the skin 

 is that of the life-long modification or changing textural 

 proportions due to evolution and involution of its external 



