OF THE ANIMAL FRAME. 



123 



layer in warm-blooded animals ; and, thirdly, the cuticle, which covers the 

 whole, and is furnished in the different classes with peculiar organs for the 

 formation and excretion of a variety of ornamental or defensive materials — as 

 hairs, feathers, wool, and silk. 



The CUTIS, or true skin, is seldom uniformly thick, even in the same ani- 

 mal : thus, in man, and other mammals, it is much thicker on the back than 

 in the front of the body; but in the different classes or g^enera of animals it 

 offers us every possible variety. Generally speaking-, it is thinnest in birds, 

 excepting in the duck tribe and in birds of prey. Its consistency and elasti- 

 city in horses, oxen, sheep, and other cattle, render it an object of high value, 

 and lay a foundation for a variety of our most important trades and manufac- 

 tures. In many animals it is so thick and tough, as to be proof against a 

 musket-ball. It is sometimes found so in the elk, but usually so in the ele- 

 phant, which, at the same time, possesses the singularity of being sensible 

 to the sting of flies. The skin of the rhinoceros despises equally the assault 

 of swords, musket-balls, and arrows. 



I have observed already, that in many animals the skin performs the office 

 of a muscle, though it is seldom that any thing like a fibrous structure can be 

 traced in it. The skin of man offers a few partial instances of this power, as 

 in the forehead and about the neck. In most quadrupeds we trace the power 

 extending over the whole body, and enabling them to throw off at their option 

 insects and other small animals that irritate them. The skin of the horse 

 shudders through every point of it at the sound of a whip, and is said to be 

 generally convulsed on the appearance of a lion or tiger. Birds, and espe- 

 cially the cockatoo and heron tribes, derive hence a power of moving at plea- 

 sure the feathers of the crest, neck, and tail ; and the hedgehog, of rolling 

 himself into a ball, and erecting his bristles by way of defence. 



The colour of the skin is derived from the rete mugosum, or mucous web, 

 which, as I have already remarked, is disposed between the true skin and the 

 cuticle. The name of rete, or web, however, does not properly apply to this 

 substance, for it has no vascularity, and is a mere butter-like material, which, 

 when black, has a near resemblance in colour, as well as consistency, to the 

 grease introduced between the nave of a wheel and its axletree. It is to this 

 we owe the beautiful red or violet that tinges the nose and hind-quarters of 

 some baboons, and the exquisite silver that whitens the belly of the dolphin 

 and other cetaceous fishes. In the toes and tarsal membrane of ravens and 

 turkeys it is frequently black ; in hares and peacocks, gray ; blue in the tit- 

 mouse; green in thewaterhen; yellow in the eagle; orange in the stork; 

 and red in some species of scolopax or woodcock. It gives that intermixture 

 of colours which besprinkles the skin of the frog and salamander ; but it is 

 for the gay and glittering scales of fishes, the splendid metallic shells of 

 beetles, and the gaudy eye-spots that bedrop the wings of the butterfly, that 

 nature reserves the utmost force of this wonderful pigment, and sports with 

 it in her happiest caprices. 



The different colours, and shades of colours, of the human skin, are attri- 

 butable to the same material. Most of these, however, are intimately con- 

 nected with a very full access of solar light and heat ; for a deep sun-burned 

 skin has a near approach to a mulatto.* And hence the darkness or blacks 

 ness of the complexion has been generally supposed to proceed from the 

 effect produced upon the mucous pigment by the solar rays, and especially 

 those of the calorific kind, in consequence of their attracting and detaching 

 the oxygen of the pigment in proportion to the abundance with which it 

 impinges against the animal surface, and in the same proportion setting at 

 liberty the carbon, which is thus converted into a more or less perfect char- 

 coal. As this, however, is a subject which I shall have occasion to revert to 

 in a distinct study upon the varieties of the human race,t it is unnecessary 

 to pursue it any farther at present. 



It is a most curious circumstance, that the children of negroes are uniformly 



Humboldt, Essai Polit. sur la Nouvelle Eapagne, &c. 



t Series n. Lecture iii. 



