CHAPTER XXXIV.

A

THE CIRCULATION AND DIGESTION.

WORK of this sort, in which so much has been said as to

the bodily functions, should not omit some general mention of the special functions of the circulation of the blood and the digestion of food. Accordingly, before proceeding to our concluding chapters on miscellaneous matters, cosmetic articles, household recipes, etc., we give a brief description of these processes.

The popular notion of respiration is that it is represented only by an alternate expansion and contraction of the chest, corresponding to the inspiration and expiration of air. But, respiration being fundamentally the consumption of oxygen and the liberation of carbonic-acid gas and aqueous vapor, it takes place in other ways. The gills of the fish absorb the oxygen present in water and liberate carbonic-acid gas. The human skin aids the lungs to some degree by respiration. The unborn child, as we have already incidentally mentioned, respires entirely by means of the placenta of the mother, a temporary organ of vascular character, which, known as the after-birth, follows the course which the latter name indicates.

Between birth and death the human organism breathes, as is well known, by means of the lungs, save, as has been indicated, to a certain extent by the skin. The vital functions of digestion and circulation, of which respiration is a part, proceed as follows: The food taken into the stomach forms a pulp, which has absorbed saliva and other fluids as the food finds its way to the more potent digestive fluids of the stomach proper. This pulp, known as chyme, is, then, a mass of food that has gone through some chemical change representing partial digestion; the process

of digestion being that which renders food fit for the renovation of the blood, and through it of the general system. In this condition the chyme passes from the stomach into the first part of the intestines, a short sac of greater diameter than that of the small intestine, which is continuous with it; the small intestine, in turn, leading into the larger intestine, which closes the alimentary tract.

This part of the bowels into which the food first enters from the stomach is called the duodenum. The duodenum is entered by ducts leading from the gall-bladder and the pancreas. The gall-bladder is the liver's reservoir for bile. The pancreas, which has been likened to internal salivary glands, secretes the so-called pancreatic fluid. At the point of time when the chyme enters the duodenum, nerve-signals transmit the fact to the gall-bladder and the pancreas, and they pour their fluids (bile and pancreatic juices) into the duodenum, and thereby a further chemical change is produced in the chyme, and it is converted into a product called chyle, representing the ultimate digestive elaboration of the food. The process continues by the absorption of the chyle by the blood through the intermediation of a multitude of ducts of exquisite fineness belonging to the lymphatic system. The main body of the chyle rises from the duodenum through a duct. called the thoracic duct, to an opening, under the left collar-bone, of a vein called the left subclavian vein, and also, to a lesser degree, by another duct into the right subclavian vein, and thence through the heart and the lungs into the general circulation, thus renewing the blood.

The product of lymph, as saturated with chyle and absorbed by veins and capillaries, may be regarded as a sublimated sort of blood. The process of the absorption of this renovating fluid does not cease at the point mentioned. Throughout the body is continued the absorption of the chyme, down to the small intestine, followed by the succeeding portion of the intestines, the

larger intestine, or colon,-which becomes finally, in the rectum, a duct releasing the excrementitious matter to be voided by the system.

The mode in which sapid, or soluble, matters are utilized in the system having been thus briefly described, it remains only to speak of the general circulation of the blood, by which they are carried to different portions of the body. The heart, as is well known, is a force-pump. It consists of four chambers,-the right auricle and ventricle, and the left auricle and ventricle, with valves adapted to the functions now to be described. The blood is, by its automatic contraction of the right auricle, propelled from it into the right ventricle, and thence into the lungs, there receiving oxygenation. From the lungs it is received by the left auricle of the heart, which forces it into the left ventricle, which in turn transmits it to the greatest artery of the body, the aorta, whence by innumerable ramifications of smaller arteries, and through the net-work of capillaries connecting the arterial with the venous system, it reaches every part of the human organism, finally returning, through the venous system, to the right auricle of the heart. The blood-vessel system is provided with valves, so that the blood can flow in only one determinate direction, from the right side of the heart to the lungs, from the lungs to the left side of the heart, thence to the extremities of the body, and thence, on its return to the right side of the heart, to repeat the same round.

As might well be supposed of an animal so highly organized as man is, who owes his pre-eminence to a nervous system of extremely great relative power and delicacy, the organism is so adapted to its needs that the best blood goes to the chief portion of the nervous system, the controlling brain. This condition is based on a circulatory system of a higher order than that possessed by many animals. Whereas, in ranks of creation below that of birds, the arterial and venous bloods often commingle in

the heart; in the heart of the most perfectly differentiated beings the arterial is separated from the venous blood. In some invertebrates, or spineless animals, the circulatory apparatus is so low in order that, instead of their possessing capillaries to convey the blood from the arterial to the venous system, these are represented in structure and function by lakelets, or what anatomists call sinuses, into which the arteries flow, and from which the blood is taken up directly by veins instead of by intermediate capillaries.

The significance of these differences is easily to be gathered from the consideration of the fact that the blood, according to its purity, is to be regarded as a more or less life-giving stream, and that the purer and more disseminated it is the higher the life will be. When the blood flows out of the left side of the heart into the aorta, it is a highly oxygenated, crimson tide, bearing its life-giving properties to every part of the organism through the minutest capillaries. As it returns to the heart, after a tour, which on the average lasts twenty seconds, it is laden with carbonic-acid gas and other effete matters representing waste of the body, has assumed a dulled look evidencing the fact of their presence, and requires again immediate renovation by a return through the heart to the lungs.