Such cases are interesting as representing the last step in the graded series through which the condition of complete pigmentation passes into that of complete albinism.
The individuals in which this diminished pigmentation is found are for the most part those living in caves, and it is probable that their condition is not truly albinotic, but only temporary and due to the absence of the stimulus of light.
In extraneous pigmentation we have coloured substances either in a solid or fluid state, gaining entrance into the organism and accumulating in certain tissues.
A moment's consideration, however, will show that, while an albino may be an individual in which one or more of the complementary bodies of pigmentation are absent, a pigmented animal is something more than an individual which carries all the factors necessary for the development of colour.
In heredity, complete albinism among animals is always recessive; and partial albinism (piebald) is always recessive to complete pigmentation (self-coloured).
Such cases suggest that we should be more correct in regarding, not albinism as correlated with constitutional defects, but rather pigmentation as correlated with powers of immunity or increased resistance against certain injurious processes.
In hepatogenous pigmentation (icterus or jaundice) we have the iron-free pigment modified and transformed by the action of the liver cells into bile pigment (bilirubin).
These are the atrial coelomic funnels or brown funnels, so called on account of the characteristic pigmentation of their walls.
A tendency to pigmentation also develops in certain tissues of the body, such as the nerve and muscle cells.