. Comparative anatomy. Anatomy, Comparative. THE INTEGUMENTARY SYSTEM 179 Italians are more liable than lighter stocks to rickets, which is a sunlight deficiency disease. So it may be that in high latitudes, in a wooded coun- try or one that has much cloud and fog, a fair skin that is still able to tan may have a selective value and be accounted for on Darwinian principles. Skin color plays queer tricks. Any two parents, even two Negroes, may have an albino child; two dark-haired parents may somehow miss with the brown pigment and have red-haired offspring. The most that anyone can say is that
. Comparative anatomy. Anatomy, Comparative. THE INTEGUMENTARY SYSTEM 179 Italians are more liable than lighter stocks to rickets, which is a sunlight deficiency disease. So it may be that in high latitudes, in a wooded coun- try or one that has much cloud and fog, a fair skin that is still able to tan may have a selective value and be accounted for on Darwinian principles. Skin color plays queer tricks. Any two parents, even two Negroes, may have an albino child; two dark-haired parents may somehow miss with the brown pigment and have red-haired offspring. The most that anyone can say is that "Nordic" man probably began as a mutant from a dark stock. Possibly, after the mutation appeared, it was admired and selective mating kept it to the fore. ASSOC I ATI OM NEURONE. EFFERENT NERVE • SWEAT GLAND iCAPILI-ARIES Fig. 136.—A diagram illustrating the nervous mechanism of temperature regulation in man. The quantity of secretion of tubular glands (and consequently the amount of sweat which may evaporate to cool the body) depends upon the quantity of blood in the capillaries associated with the glands and dermal papillae. Through a reflex arc the circulation is regulated by the temperature of the skin. (Redrawn after Hough and Sedgwick.) CUTANEOUS GLANDS Since among invertebrates most glands are unicellular, it has generally been assumed that the multicellular glands of vertebrates have evolved from such beginnings, an increase in the size of the secreting cells tending to carry them into the underlying corium, where together with other epidermal cells, they become multicellular organs. Be this as it may, cutaneous glands develop, much as hairs do, from solid cords, which are proliferated from the stratum germinativum and grow downward into the underlying corium. The lumen of the gland forms later, to connect with the exterior, and the gland anlage differen-. Please note that these images are extracted from scanned page images that may have been digitally en
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