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During the action of chewing, the food is mixed with the saliva or spittle. The food is then carried backwards into the pharynx, which is a sort of pouch at the back part of the mouth, from which it immediately descends, into the œsophagus, or gullet, at the extremity of which is the stomach, into which the food is. deposited.
We may here mark a wonderful contrivance. The passage from the mouth, to the windpipe, lies immediately before the passage to the stomach: we might suppose that the food would pass into the first opening, viz. the passage to the windpipe, before it reached its own proper passage. And this would be the case, were it not that there is a little valve standing erect before the passage to the windpipe, which the food in its way to the gullet, presses down, and thus closes the anterior opening of the gullet. Were this passage left open, we would be in danger of being choked by every morsel we endeavoured to swallow.
The stomach is a kind of membraneous bag, not very unlike the bag of a bag-pipe, lying across the body, and having two openings: the upper, towards the left side, by which it receives food from the gullet, called the cardia; and the lower, on the left side, called the pylorus or janitor, by which the food passes into the intestines. Its inner surface consists of a soft membrane, called the mucous, or villous coat, which is carried through the whole alimentary canal; its middle coat is muscular, and, by means of this coat, the stomach has the power of emptying its contents; its outer is a membraneous covering, common to the stomach, intestines, and all the other organs contained in the cavity of the abdomen. At the pylorus is a contraction, which prevents the too ready passage of the food downwards. Between its coats are various small glands which secrete, and pour into the stomach, a fluid called the gastric juice, which dissolves the substances taken into the stomach, converts them into a uniform, greyish,
pulpy mass, called chyme, and thus fits them for becoming nourishment. Digestion is totally independent of any pressure which is exercised by the coats of the stomach, for it has been found that if portions of food were placed in silver balls, and these swallowed, such portions would be dissolved.
When food has undergone the change which it is meant to suffer in the stomach, it passes through the pylorus or lower orifice, into the intestines. When the food has passed into the intestines, it receives the bile, which is a secretion from the liver; and the pancreatic juice, which is the secretion of the pancreas, or sweetbread. By the mixture of these substances, the food is so far altered in its nature as to be capable of affording chyle, which is a fluid like milk. This fluid is taken up by small vessels, called lacteals, spread upon the surface of the intestines. These lacteals, uniting together, convey their contents into one of the large blood-vessels of the body, and thus supply the means of nourishment to the whole system. That part of the food which cannot afford nourishment, is carried off as excrementitious matter.
All carnivorous animals have stomachs of the same kind: and in them the digestive organs are of the more simple kind, as animal food is more easily converted into chyle. Many birds not only take in portions of gravel to assist their digestion; but, as they have no teeth, and can divide their food in but a very imperfect manner with their bills, the gizzard is given them for the purpose of doing so. The gizzard is a muscle in the stomach with two bodies, called therefore the digastric, calculated to press any substance very strongly between the two parts of which it consists. But as the gizzard could not perform the whole of the duty at once, there is a bag, or enlargement of the gullet given to many birds, called the crop, which is situated in the front of the chest, at some distance from the gizzard. In this the hard and dry food is macerated; it is then let into the gizzard, where it is bruised and divided, and mixed with the gastric juice, which is secreted by glands near the entrance of the
gizzard; and thus the changes are produced upon the food, which fit it for nourishment.
The crop, in such birds as have it, is principally to be viewed as a repository, in which the food is first softened, and then transferred to the gizzard. But in all birds of the dove kind, and it is supposed in parrots, macaws, and cocatoos, the crop, both in the male and female, is endowed with the power of secreting a fluid, which coagulates into a whitish curd, and is employed to feed the young for two or three days after hatching. It is then found to be mixed with some of the common food; and as the pigeon grows older, the proportion of common food is increased; so that by the time it is eight or nine days old, and able to digest common food, the secretion of the food in the old bird ceases.
In some of the crustaceous animals, as the lobster and crab, the division of the food is accomplished by means of teeth placed in the stomach. These teeth are of the molaris or grinding shape, and are one on each side. Immediately behind them, is a single projecting tooth, which answers the purpose of preventing the food from passing on till it is sufficiently divided. The stomach of these animals is also lined with a hard substance, similar to the external coat, so that it is never collapsed: and it is a curious circumstance, that this coat, as well as the hard covering of the teeth, are parted with, when these animals cast their shells. The tooth-like processes at the entrance of the mouth, which are sometimes represented as teeth, are nothing more than a kind of pincers, to grasp the food, and convey it into the mouth.
Teeth are likewise met with in some of the worm tribe; and such is also the case with various insects, particularly the Cape grasshopper, and mole cricket.
'The most curious apparatus for the conversion of vegetable food into nourishment, is that which belongs to the cow, the sheep, the deer, the camel, and other animals which usually chew their cud. In these animals there are four stomachs, which are concerned in digestion. The first stomach receives the food after a slight mastication; thence it goes into the second,
called the honey-comb; and when it has been macerated for some time, it is carried up into the mouth. It is then chewed, and passes into the third stomach, or many-plies, whence it goes into the fourth, or read, the proper digesting stomach, where its conversion into chyme is completed. The animal seems to have the power of sending the food at once into the second, thi or fourth; and this they do according to the facility with which the different kinds of food may be digested.. For instance, cows in the north of Scotland, and the Hebrides, are occasionally fed on fish, which does not require a second mastication, and is therefore received at once into the third stomach; and calves, when fed on milk, receive it into the fourth stomach. In the camel, the second stomach consists of cells, and is solely appropriated to the reception of water. By means of a curious muscular structure the orifices of these cells are closed, and the water preserved from being mixed with the food. It is this peculiar structure, which in the camel, dromedary, and lama, fits them to live in sandy deserts, where the supplies of water are so precarious. Bruce mentions that four gallons were taken out of the stomach of a camel, during one of his journeys in the desert, when there was much distress for want of water.
The heart is the grand reservoir of the blood, whence it flows through the arteries to the utmost extremities of the body, and is conveyed back again by the veins. This organ is situated in the thorax, or chest, between the two lobes of the lungs. In man it is placed almost cross-wise.-The base, or broad part, is directed towards the right side, and the point towards the left. It is securely enclosed in a membraneous sac, or pouch, which contains a fluid that gives smoothness to its surface, and ease to its motions.
The substance of the heart is entirely fleshy or muscular. Its basis, from which the great blood vessels originate, is covered with fat, and it has two hollow appendages, called auricles. Within, it is divided into two cavities, or ventricles, separated from each other by a fleshy partition. The use of these ventricles and auricles is to circulate the blood through the whole body, by means of the power of contraction and enlargement which the heart possesses from its numerous fibres, that surround it in a spiral direction. When these fibres are contracted, the sides of the muscular cavities are necessarily squeezed together, so as to force out of them any fluid which they may contain. By the relaxation of the same fibres, the cavities become dilated, and of course prepared to admit any fluid which may be poured into them. The great trunks, both of the arteries, which carry out the blood, and of the veins, which bring it back, are inserted in these cavities. By dilating the fibres, which anatomists call diastole, the cavity of the ventricles is opened to receive the blood from the auricles: on the contrary, when the ventricles are contracted, which is called systole, the auricles are expanded; and by this alternate action, they carry on the wonderful operation of supplying with blood the most distant parts of the body.
The blood, which has been ejected from the auricles and ventricles, is prevented from returning, by valves, or little doors, placed between the auricles and ventricles, and at the mouths of the great arteries. These valves open inwards, but not in the contrary direction; of course when the blood has passed through them the valves close, and a return is thus rendered impossible.
You may perceive, by this account, that there is a continual exchange of the blood that fills the heart. It is no sooner emptied into the arteries, than it is filled again from the veins; and this contraction and dilatation succeed each other with great rapidity; and by its re-action causes that beating at the wrist, and other parts, that is called the pulse.
It is supposed that the quantity of blood contained