ORGANS OF DIGESTION AND RESPIRATION.
Digestion and respiration are the processes, by which the food is converted into blood for the nourishment of the body. The engraving on the next page (Fig. [16]) shows the organs by which these operations are performed.
In the lower part of the engraving, is the stomach, marked S, which receives the food through the gullet, marked G. The latter, though in the engraving it is cut off at G, in reality continues upwards to the throat. The stomach is a bag composed of muscles, nerves, and blood-vessels, united by a material similar to that which forms the skin. As soon as food enters the stomach, its nerves are excited to perform their proper function of stimulating the muscles. A muscular (called the peristaltic) motion immediately commences, by which the stomach propels its contents around the whole of its circumference, once in every three minutes.
This movement of the muscles attracts the blood from other parts of the system; for the blood always hastens to administer its supplies to any organ which is called to work. The blood-vessels of the stomach are soon distended with blood, from which the gastric juice is secreted by minute vessels in the coat of the stomach. This mixes with the food, and reduces it to a soft pulpy mass, called chyme. It then passes through the lower end of the stomach, into the intestines, which are folded up in the abdomen, and the upper portion, only, of which, is shown in the engraving, at A, A. The organ marked L, L, is the liver, which, as the blood passes through its many vessels, secretes a substance called bile, which accumulates in the gall-bladder, marked B. After the food passes out of the stomach, it receives from the liver a portion of bile, and from the pancreas the pancreatic juice. The pancreas does not appear in this drawing, being concealed behind the stomach. These two liquids separate the substance which has passed from the stomach, into two different portions. One is a light liquid, very much like cream in appearance, and called chyle, of which the blood is formed; the other is a more solid substance, which contains the refuse and useless matter, with a smaller portion of nourishment; and this, after being further separated from the nourishing matter which it contains, is thrown out of the body. There are multitudes of small vessels, called lacteals, which, as these two mixed substances pass through the long and winding folds of the intestines in the abdomen, absorb the chyle, and convey it to the thoracic duct, which runs up close by the spine, and carries the chyle, thus received, into a branch of the vena cava superior, at t, whence it is mingled with the blood going into the heart. In this engraving, the lacteals and thoracic duct are not shown; but their position is indicated by the dotted lines, marked X, Y; X, being the lacteals, and Y, the thoracic duct.
In the upper half of the engraving, H represents the heart; a, the commencement of the aorta; v c s, the termination of the vena cava superior. On each side of the heart, are the lungs; l l, being the left lobe, and r l, the right lobe. They are composed of a network of air-vessels, blood-vessels, and nerves. W, represents the trachea, or windpipe, through which, the air we breathe is conducted to the lungs. It branches out into myriads of minute vessels, which are thus filled with air every time we breathe. From the heart, run the pulmonary arteries, marked p a. These enter the lungs and spread out along-side of the branches of the air-vessels, so that every air-vessel has a small artery running side by side with it. When the two vena cavas empty the blood into the heart, the latter contracts, and sends this blood, through these pulmonary arteries, into the lungs.
As the air and blood meander, side by side, through the lungs, the superabundant carbon and hydrogen of the blood combine with the oxygen of the air, forming carbonic acid gas, and water, which are thrown out of the lungs at every expiration. This is the process by which the chyle is converted into arterial blood, and the venous blood purified of its excess of carbon and hydrogen. When the blood is thus prepared, in the lungs, for its duties, it is received by the small pulmonary veins, which gradually unite, and bring the blood back to the heart, through the large pulmonary veins, marked p v, p v.
On receiving this purified blood from the lungs, the heart contracts, and sends it out again, through the aorta, to all parts of the body. It then makes another circuit through every part, ministering to the wants of all, and is afterwards again brought back by the veins to receive the fresh chyle from the stomach, and to be purified by the lungs.