Medicinal articles are generally exhibited by the mouth in this way; their subsequent action depends upon their immediate effect upon the mucous membrane lining the stomach and intestinal tube, or on the action being transferred to distant parts, or to the whole system, through the medium of the nerves; and lastly, they may be taken up and introduced into the blood, and subsequently make their appearance in one or other of the secretions. For example, many find their way to the kidneys, and produce an increased flow of urine; some are determined to the skin, promoting transpiration and inducing sweat; others direct their action to the lining membrane of the air-passages, and facilitate expectoration, while others exert an influence more or less marked upon the body at large. Sulphur, when taken internally, shortly discovers itself by the odour it imparts to the breath and emanations of the skin. When mercury has been exhibited for some time in any quantity, it displays its action on various secreting organs, and exerts an influence directly or indirectly on every organ and tissue in the body; in some instances it has shewn itself by silvering coins, or the watch in the pocket, or rings and other ornaments worn by a person undergoing what used to be called a course of mercury. In these instances it unites with the other metals, as gold and silver, forming what is termed an amalgam. In other cases it has been observed deposited in its metallic state, even when taken in another form, shewing that it had suffered reduction in the system. Several salts pass from the stomach into the circulation, and are again expelled without suffering decomposition or change. This is the case with saltpetre or nitrate of potash, for on evaporating the urine of an individual who has previously taken doses of this salt, it is found in a proportional quantity in the residue. Several other saline substances may be shewn to pass by the same channel, and with similar integrity, as proved by their appropriate tests and reagents; others again are altered or decom

posed either while in the stomach or intestines, or after they have been absorbed. The salts of iron, from the facility with which they are decomposed by tannin, the astringent and tanning principle of most vegetables, are often reduced to the tan-gallate of iron, the colouring matter of ink. When exhibited as medicine, and when they meet with tannin and gallic acid in their course, either from the food or other medicines which may have been taken during their employment, the discharges become of a black inky colour, frequently to the no small alarm and consternation of the patient and his friends. Calomel, which is composed of chlorine and mercury, is also liable to be changed in the bowels, by meeting with substances capable of acting upon it. Of these sulphuretted hydrogen is one, a gas which, in some diseased states of the intestine, especially of the colon, is generated in considerable quantity. When calomel is exposed to this gas, it is converted into a black substance, which imparts a dark colour to the alvine discharges. Some physicians, ignorant of this fact, have recommended that the calomel should be continued as long as dark-coloured discharges are produced. Now, in such cases sulphuret of mercury is the cause of that colour, and as long as the cause continues, so long will the effect. Various alkaline salts, formed with the vegetable acids, are liable to decomposition, the alkali and the acid being separated either in the first passages or in the course through the circulation, or lastly, while being excreted from glands, especially the kidneys. This is apt to occur where potash or soda is combined with carbonic, acetic, citric, and tartaric acids.

We have already shewn that chyle is taken up from the intestinal canal by the lacteal absorbents, and conveyed to the thoracic duct, but that occasionally a little of it may find its way to the liver, either in consequence of some of the lacteals having joined branches of the vena portæ, or from these veins themselves acting as absorbents;

for, as we have stated, the lacteals appear to exercise the power of selection or choice with respect to what they receive. The veins, on the other hand, are much more indiscriminate in what they take up. Now there is a wise provision in this; for whatever the absorbents receive, they must convey into the general mass of circulating fluids, although it may be retarded or modified in its course; but when any substance has been absorbed by the branches of the vena portæ, there is a subsequent opportunity for its being cast out, while the contents of the porta are filtered through the liver. In this way the liver acts as an important organ in connexion with digestion and sanguification, and thus not only is the bile secreted in order to be subservient to chylification, and to stimulate the peristaltic motion of the bowels, but the liver acting as a strainer in the formation of bile, drains off from the venous blood what might have proved highly noxious; and hence the total suppression of the action of the liver speedily proves fatal.

It has been stated that drinks are rapidly taken up after their introduction, that they are not subjected to alteration previous to absorption, and that the veins from the stomach and intestines appear to be the chief agents of their removal. They are in this way mixed with the venous blood, instead of being immediately taken into the absorbents. It has been farther stated that the spleen is calculated to admit of considerable accumulation of fluids, that a very large quantity of blood circulates through it, that after a meal it swells, and that the numerous absorbents arising from it, and terminating in the receptacle of the chyle and the thoracic duct, convey a coloured fluid, displaying several of the characters of more perfect blood. Fluids, therefore, enter the circulation from the alimentary canal, pursuing three different routes in their passage; first, directly to the liver, through the veins, mixing immediately with the contents of the veins; secondly, they may be arrested for some time in the spleen, where

certain changes may be effected upon them. Besides, their slow introduction being partly insured in this way, it becomes more in accordance with the gradual changes which occur in organized bodies in general. From the spleen one portion flows by the absorbents to the thoracic duct, the other is conveyed by the porta, and filtered through the liver. It may be observed that as the porta is the only vein which appears expressly intended to convey new materials into the general system, so is it the only one which discharges from its blood a secretion whereby an opportunity is afforded of throwing off what might have been superfluous or noxious; and, as just now observed, the bile may thus far be considered as excrementitious.

Colouring, odorous, medicinal, and chemical substances, such as have been adverted to, though occasionally observed in the absorbents, are principally carried off by the veins through the liver. The speedy appearance which some of them make, as garlic and alcohol, in the breath, and turpentine and prussiate of potash in the urine, has led to the inference that such matters may enter the circulation, and be conveyed more directly to those parts than by the more circuitous course through the heart and arteries. The observations, however, are as yet too few and indefinite to warrant any certain conclusions from them.

To recapitulate, the chyle enters by the lacteal absorbents, to be carried into the thoracic duct, and by the veins to be conveyed into the system chiefly through the spleen; and all those substances which find their way into the circulation without change, and which are characterized by their peculiar qualities, such as their colour, odour, chemical properties, or the effects they produce upon the body, being taken up by the veins, are introduced principally through the liver.

CHAPTER IV.

THE BLOOD.

Blood-Physical Properties of-Colour-The Red Globules-Their Form and Constitution-Smaller and more numerous in Mammalia-Odour-Characteristic of the Animal to which it belongs-Consistence-Temperature-Specific Gravity-Quantity of in the Body-Coagulation-Relative Proportion of Serum and Clot-Serum-Fibrin-Oil of the Blood-Table of Analysis of-Vital Properties.

ONE of the most characteristic properties of organized bodies is the power which they possess of appropriating to themselves foreign matter, and so altering it as to adapt it to their various necessities in the growth and nourishment of their own individual structures, and for the continuance of their race.

In the last chapter we have treated of the various successive steps which occur in the conversion of food into a fluid in its most perfect state of adaptation to the highest organized beings. The name blood is applied to this elaborated fluid in animals, the term sap being employed to designate the corresponding fluid in plants. We have noticed what a multiplicity of organs and complexity of apparatus are engaged in forming and perfecting the arterial blood, the characters of which we have now to inquire into.

The examination of this interesting and important fluid may be conveniently conducted under the following heads: first, its physical properties; secondly, its chemical constitution; and thirdly, its vital characters.

The physical properties may be considered in respect