ON THE DIGESTIVE FUNCTION, &c. 



125 



Silk, however, is chiefly secreted by insects, as some species of spider, 

 whose threads, like the hair of the Angora goat, assume a silky gloss and 

 lubricity, and the phalaena mori, or silk-worm, which yields it in great abun- 

 dance. Yet there are a few shell-fishes which generate the same, and espe- 

 cially the genus pinna, or nacre, in all its species ; whence Reaumur calls 

 this kind the sea silk-worm. It is produced in the form of an ornamental 

 byssus or beard : the animal is found gregariously in the Mediterranean and 

 Indian seas ; and the weavers of Palermo manufacture its soft threads into 

 glossy stuffs or other silky textures. And I may here observe, that there are 

 various trees that possess a like material in the fibres of their bark, as the 

 morus papyrifcra, and several other species of the mulberry : in consequence 

 of which it has been doubted by some naturalists whether the silk-worm 

 actually generates its cocoon, or merely eliminates it from the supply 

 received as its food ; but as the silk-worm forms it from whatever plants it 

 foeds on, it is obviously an original secretion. 



From the integument of the skin originates also that beautiful plumage 

 which peculiarly characterizes the class of birds, and the colours of M^hich 

 are probably a result of the same delicate pigment that produces, as we have 

 already remarked, the varying colours of the skin itself; though, from the 

 minuteness with which it is emplo5'^ed, the hand of chemistry has not been 

 able to separate it from the exquisitely fine membrane in which it is involved. 

 But it is impossible to follow up this ornamental attire through all its won- 

 derful Matures of graceful curve and irridescent colouring, — of downy deli- 

 cacy and majestic strength, — from the tiny rainbow that plays on the neck of 

 the humming-bird, to the beds of azure, emerald, and hyacinth, that tesselate 

 the wings of the parrot tribe, or the ever-shifting eyes that dazzle in the tail 

 of the peacock ; — from the splendour and taper elegance of the feathers of 

 the bird of paradise, to the giant quills of the crested eagle or the condur — 

 that crested eagle, which in size is as large as a sheep, and is said to be able 

 to cleave a man's scull at a stroke ; and that condur which, extending its 

 enormous wings to a range of sixteen feet in length, has been known to fly 

 off with children of ten or twelve years of age. 



Why have not these monsters of the sky been appropriated to the use of 

 man'? How comes it that he who has subdued the ocean and cultivated the 

 earth ; who has harnessed elephants, and even lions, to his chariot wheels, 

 should never have availed himself of the wings of the eagle, the vulture, or 

 the frigate pelican ? That, having conquered the difficulty of ascending into 

 the atmosphere, and ascertained the possibility of travelling at the rate of 

 eighty miles an hour through its void regions, he should yet allow himself to 

 be the mere sport of the whirlwind, and not tame to his use, and harness to 

 his car, the winged strength of these aerial racers, and thus stamp with reality 

 some of the boldest fictions of the heathen poets ] The hint has, indeed, 

 long been thrown out; and the perfection to which the art of falconry was 

 carried in former times sufficiently secures it against the charge of absurdity 

 or extravagance. 



LECTURE XII. 



ON THE DIGESTIVE FUNCTION AND THE ORGANS CONTRIBUTORY TO IT : THE DIF- 

 FERENT KINDS OF FOOD EMPLOYED BY DIFFERENT ANIMALS I CONTINUANCE OF LIFE 

 THROUGH LONG PERIODS OF FASTING. 



Under every visible form and modification matter is perpetually changing: 

 not necessarily so, or from its intrinsic nature ; for the best schools of 

 ancient times concur with the best schools of modern times, in holding its 

 elementary principles, as I have already observed, to be solid and unchange- 

 able ; and we have still farther seen, that even in some of its compound, but 



