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born white, or nearly so; and that the black pigment which colours them is
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 abundance. Yet there are a few shell-fishes which generate the same, and especially 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 inorus o 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 feeds 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 which 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 employed, 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 wonderful features of graceful curve and irridescent colouring, of downy delicacy 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.
ON THE DIGESTIVE FUNCTION AND THE ORGANS CONTRIBUTORY. To IT : THE DIF. FERENT KINDS OF FOOD EMPLOYED BY DIFFERENT ANIMALS : 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 unchangeable; and we have still farther seen, that even in some of its compound, but
gaseous, etherealized, and invisible forms, it is probably alike exempted from
* Study of Med. ii. 2.
digestive canal, the food on which they feed being already of their own nature, and containing a larger quantity of nourishment in a less bulk; and hence demanding a smaller proportion both of time and space to become fit for use. In this respect man holds a medium between the two: his digestive canal is less complex than that of most animals that feed on grass alone, and more extensive than that of most animals that are confined to a diet of their own kind. Man is hence omnivorous, and is capable of subsisting on an aliment of either sort; and from his digestive organs, as well as from various others, is better qualified for every variety of soil and climate than any other animal.
Man, however, is by no means the only omnivorous animal in the world; for the great Author of nature is perpetually showing us that, though he operates by general laws, he is in every instance the lord and not the slave of them. Hence, among quadrupeds, the swine, and among insects the ant, possesses as omnivorous a power as man himself, and feeds equally on the fleshy parts of animals, and on grain, and the sweet juices of vegetables. In consequence of this omnivorous power in the ant, we may often make use of him as a skilful anatomist; for, by putting a dead frog, mouse, or other small animal in a box perforated with holes, and placing it near an ant-hill, we shall find it in a few days reduced to a perfect and exquisite skeleton, every atom of the soft parts being separated and devoured.
The solid materials of the food are first masticated and moistened in the mouth, excepting in a few cases, in which it is swallowed whole. It is then introduced into the stomach, and converted into an homogeneous pulp or paste, which is called chyme; and shortly afterward, by an additional process, into a fluid for the most part of a milky appearance, denominated chyle; in which state it is absorbed or drunk up voraciously by thousands, and tens of thousands of little mouths of very minute vessels, which are not often found in the stomach, but line the whole of the interior coating of that part of the intestinal tube into which the stomach immediately empties itself, and which are perpetually waiting to imbibe its liquid contents. These vessels constitute a distinct part of the lymphatic system; they are called lacteals from the usual milky appearance of the liquid they absorb and contain. They progressively anastomose or unite together, and at length terminate in one common trunk, named the thoracic duct, which conveys the different streams thus collected and aggregated to the sanguineous system, to be still farther operated upon, and elaborated by the action of the heart and the lungs.
The means by which the food is broken down and rendered pultaceous after being received into the stomach are various and complicated. . In the first place, the muscular tunic of the stomach acts upon it by a slight contraction of its fibres, and so far produces a mechanical resolution: secondly, the high temperature maintained in the stomach by the quantity of blood contained in the neighbouring viscera and sanguiserous vessels, gives it the benefit of accumulated heat, and so far produces a concoctive resolution: and, thirdly, the stomach itself secretes and pours forth from the mouths of its minute arteries a very powerful solvent, which is by far the chief agent in the process, and thus produces a chemical resolution. In this manner the moistened and manducated food becomes converted into the pasty mass we have .." called chyme: and, fourthly, there are a variety of juices separated from the mass of the blood by distinct glands situated for this purpose in its vicinity, which are thrown into the duodenum, or that part of the canal into which the stomach immediately opens, by particular conduits, and in some way or other appear to contribute to the common result, and to transform the chyme into chyle, but concerning the immediate powers or modes of action of which we are in a considerable degree of darkness. Of these glands the most remarkable and the most general are the liver and the pancreas or sweet-bread; the first of which secretes the bile, and is always of a considerable size, and appears "Poo a very striking effect on the blood itself, by a removal of several of its principles independently of its office as a digestive organ.
From this brief survey of the process of digestion it is obvious that the stomach itself performs by far the principal part ; in some animals, indeed, it appears to perform the whole; and it is hence necessary that we examine the general structure and powers of this organ with a little more minuteness.
In man the stomach is situated on the left side of the midriff; in its figure it resembles the pouch of a bag-pipe; its left end is most capacious ; its upper side is concave, its lower convex; and the two orifices for receiving and discharging the food are both situated in the upper part. In its substance it consists of three distinct coats or layers, the external and internal of which are membranous, and the middle muscular. The internal coat, moreover, is lined with a villous or downy apparatus, and is extremely convoluted or wrinkled ; the wrinkles increasing in size as the diameter of the stomach contracts.
From what I have already observed, it must appear that the process of digestion in man consists of three distinct acts: mastication, which is the office of the mouth, and by which the food is first broken down; chymification, or its reduction into pulp, which is the office of the stomach; and chylification, or its dilution into a fluid state, which is the office of that part of the intestinal canal which immediately communicates with the stomach. The whole of this process is completed in about three hours, and under certain states of the stomach, to which I shall advert presently, almost as quickly as the food is swallowed. The most important of these three actions is that of chymification; and, while it takes place, both orifices of the stomach are closed, and a degree of chilliness is often produced in the system generally, from the demand which the stomach makes upon it for an auxiliary supply of heat, without an augmentation of which it appears incapable of performing this important function.
Considering the comparatively slender texture of the chief digesting organ, and the toughness and the solidity of the substances it digests, it cannot appear surprising that mankind should have run into a variety of mistaken theories in accounting for its mode of action. Empedocles and Hippocrates supposed the food to be softened by a kind of putrefaction. Galen, whose doctrine descended to recent times, and was zealously supported by Grew and Santarelli, ascribed the effect to concoction, produced, like the ripening and softening of fruits beneath a summer sun, by the high temperature of the stomach from causes just pointed out. Pringle and Macbride advocated the doctrine of fermentation, thus uniting the two causes of heat and putrefaction assigned by the Greek writers; while Borelli, Keil, and Pitcairn resolved the entire process into mechanical action, or trituration; thus making the muscular coating of the stomach an enormous mill-stone, which Dr. Pitcairn was extravagant enough to conceive ground down the food with a pressure equal to a weight of not less than a hundred and seventeen thousand and eighty pounds, assisted, at the same time, in its gigantic labour, by an equal pressure derived from the surrounding muscles."
Each of these hypotheses, however, was encumbered with insuperable objections; and it is difficult to say which of them was most incompetent to explain the fact for which they were invented.
Boerhaave endeavoured to give them force by interunion, and hence combined the mechanical theory of pressure with the chemical theory of concoction; while Haller contended for the process of maceration. But still a something else was found wanting, and continued to be so till Cheselden in lucky hour threw out the hint, for at first it was nothing more than a hint, of a menstruum secreted into some part of the digestive system; a hint which was soon eagerly laid hold of, and successfully followed up by Haller, Reaumur, Spallanzani, and other celebrated physiologists. And though Cheselden was mistaken in the peculiar fluid to which he ascribed the solvent energy, namely, the saliva, still he led forward to the important fact, and the *Astric Juice was soon afterward clearly detected, and its power incontrovertibly established.
* See Series I. Lecture x