Malpighi, on the discovery of this membrane, offered a conjecture respecting the cause of the colour of nc grocs : he supposed that this membrane contained a juice or fluid of a black colour, from which their black ness arose : the actual existence of a black pigment has been since ascertained, but has never been procured in sufficient quantity to admit of minute and analytical ex amination.
The rete mucosum is of very different colours in dif ferent nations ; and the difference of its colour so com pletely agrees with the difference of their complexions, that there can be no doubt that it is the sole, or at least the principal scat of the colour of the human complexion. Its thickness varies in different parts of the body ; and the depth of its colour, for the most part, is in propor tion to its thickness. Where the cuticle is thickest, there it is also found of the firmest and thickest texture; and it is most easily detected in the palms of the hands, and in the inner parts of the fingers, and on the soles of the feet. There appears, however, one exception to this general rule. According to Kaaic, " in pudendis vero tenue, sed fuscum ita, ut 'legal-es de Europiti cute detractum cssc." Perspiratio dicta Hippocran, /:rr Uni versum corpus ?natomice illustrata, .1uctore Abrahamo Kaaic, M. D. Lug. Bat. 1733, p. 21.
Although this circumstance of the different colours of the rete mucosum, corresponding exactly with the different complexions of the human race, seems satis factorily to fix the seat of colour in that membrane, yet some authors arc of opinion, that the black complexion of the negroes arises, partly at least, from the cuticle. This opinion is particularly maintained by Dr Mitchell, in his " Essay on the Causes of the different Colours of persons in different Climates." Phil. Trans. abridged by Martyn, vol x. p. 926. " The cuticle of negroes," hu says, " is much thicker than it is in people of a fair com plexion ; and hence it cannot so easily or perfectly trans mit the colour of the cutis through it." But it is evident, that even allowing the fact on which this reasoning is founded to be true, namely, that the cuticle of negroes is much thicker than that of fair complexioned people, it will rather militate against, than support the inference which he draws from it ; since, if the thickness of the cuticle weakens its power of transmitting colour, the black colour of the rete mucosum can have little or no ffect in producing the black complexion of the ne groes.
But though the cuticle cannot, in this manner, give rise to the colour of the human complexion, there is reason to suppose, that in some degree, and in some cases, it contributes to this effect. Kaaic maintains, that
this membrane, even when separated from the rete muco sum, is found to possess different colours in different nations ; and that it also varies in men living under the same climate. That this variation does not arise entirely from different degrees of exposure to the sun and air, clearly appears from an experiment which this anatomist made. He macerated the cuticles of twins, and separat ed the rete mucosum ; the colour of the cuticles he found to differ in a considerable degree.
The black colour of the negroes is destroyed by several causes ; indeed whatever destroys the rete mucosum destroys it, as wounds, burns, &c. and as this membrane is never reproduced, the scar remains white ever after wards. Hawkins, in- his Travels into the Interior of -Africa, (p. 120.) mentions, that the land-cloud of Africa, called by the Portuguese ferrino, changes the black colour of the negroes into a dusky grey ; according to some other authors, the change is into a red copper colour. At Darfur, a species of leprosy prevails among the natives, which they call borras, and which gives them the appearance of being pie-bald, changing to a white colour parts both of their skin and their hair.* There are also several instances of the colour of ne grocs being either entirely or partially changed, from the operation of causes which cannot be detected or explained. A boy who was born in Virginia, of black parents, continued of his native colour till he was three years old : at that period a change of colour began to take place, though the health of the boy continued good, and there was no assignable cause for the altera tion in his food, or mode of life. At first white specks made their appearance on his neck and breast, which soon increased in number and size ; from the upper part of his neck down to his knees, he was completely dappled ; his hair was also changed, but not to the same degree, since, though some parts of it were white, in general it retained the black colour and crispature of the negro. The colour of those parts of his body which had undergone the change, was of a more livid white than is found among the fairest Europeans ; nor did the flesh and blood appear through these parts of his skin 30 clear and lively as through the skin of white people. Ile was not liable to be tanned. Philosophical Transac 'ions, vol. xix. p. 781.