The hypothesis of the Animists, of which Stahl was the great defender in the seventeenth century, and which has been received, with some modifications, by Hunter, Blu menbach, Darwin, and others in our own times, will not require any long discussion, because the remarks that have been offered on the other parts of this doctrine will equally apply to the subject of secretion. Even supposing the ex istence of the intelligent or vital principle, as assumed by these physiologists, we have no account given us of the mode of its operation, nor are we informed whether it acts upon chemical or mechanical principles.
The fourth hypothesis, that of chemical affinity, seems to have been suggested by Keill, but it. has received its principal support from the numerous discoveries that have been made of late years in the science of chemistry, and especially from our knowledge of the great variety of changes which a compound body may undergo, by having its components combined in different proportions, or ex posed to the action of various external agents. Thus oxy gen and azote, when united in one proportion, form at mospheric air, in other proportions, nitrous and nitric oxide, and in others, nitrous and nitric acid, substances which differ from each other most materially in both their physical and chemical properties. Now the blood is com posed of several ingredients, which are held together by a weak affinity, and are very liable to be disunited by slight causes. There is reason to suppose that its consti tution varies in the course of the circulation, partly from the action of its components upon each other, and partly from the addition and substraction of various substances. If, to these circumstances, we add the mechanical ac tions indicated by Haller, and, perhaps, some additional ones of a similar kind, which may affect the passage of the fluids through the vessels, and the degree of their action upon them, we may conceive of as great a variety of pro ducts as there are secretions, and differing as much from each other. How far any considerations of this kind may be thought capable of affording a satisfactory solution of the difficulty that attends this question, we are not dis posed to decide ; but, upon the whole, we consider this hy pothesis as more tenable than the mechanical doctrine, and we conceive that the objections which have been rais ed against it arise rather from our ignorance than from its own insufficiency.
The last hypothesis of secretion, is that which attempts to account for it by the action of the nerves. It may be regarded in two points of view : 1st. As professing to explain the nature of the function generally ; and, 2dly, As showing how particular glands prepare their specific fluids from the blood. There are many facts which show that there is an intimate connexion between the influence of the nerves and the action of the glands. Certain secre tions are increased in quantity, and have their quality af fected by the various agents, hoth mental and corporeal, which operate through the medium of the nerves. This is so well known with respect to different articles of diet and medicine as to require no illustration, and the power of the mental affections is likewise sufficiently obvious ; for example, every one knows that the flow of the saliva is increased by the sight of food, a fact which may be ob served in the lower animals as well as in man. It is there fore to be regarded as a well established point, that nerv ous excitement promotes secretion ; but although this may prove the connexion which subsists between the muscular and the nervous systems, it does not throw any light upon the nature of secretion. An increase of nervous energy
produces an increased contractility of all the muscles, and, among the rest, of those which are immediately concern ed in the circulation ; hence the blood is propelled more freely to all parts of the body, and the glands being more plentifully supplied with it, are more disposed to produce their appropriate secretions.
The hypothesis which refers secretion to the action of the nerves, has been lately made the subject of an elabo rate and very ingenious set of experiments by Dr. Philip; and from their results it is fully established, that, by divid ing the par vagum in such a manner as to prevent the transmission of the nervous influence, the digestive power of the stomach is almost entirely destroyed. The conclu sion deduced from the experiment by Dr. Philip is, that secretion is a nervous function, or one, in the due per formance of which the nervous influence is the prime and essential agent. But, although this might, at first view, appear the natural and necessary inference from the facts, it has been remarked by Dr. Alison and others, that it does not necessarily follow from them. They prove, indeed, that the nerves which go to the stomach have the power of affecting the secretion of the gastric juices, but they do not prove that they are essential to the formation of this fluid, much less to the function of secretion generally. There arc, indeed, certain facts which would seem to de monstrate that secretion is, under ordinary circumstances, altogether independent of the nervous system, especially those to which we have already referred, where fwtuses have been born in a state approaching to maturity, in which the brain and spinal marrow were entirely wanting.
Some of the modern physiologists have not been satis fied with the general reference of secretion to the power of the nerves, but have endeavoured to spew how particu• lar glands produce particular secretions, by imparting to the different organs what they term specific sensibilities, so as to enable them to select from the blood those ele ments which may produce the different secreted substan ces. This we regard as one of those refi cd speculations which it is not easy to put to the test of experiment, and which must rest upon analogy and its intrinsic plausibili ty ; upon the whole, however, it appears to us to be too vague and indefinite to require any more minute examina tion.
With respect to the nature of secretion, it appears to us that there are two leading points to be ascertained before we can arrive at any perfect theory upon the subject. We must learn, in the first place, how the elements of the blood can be so changed as to produce the secretion ; and, in the second place, how the substance, when formed, can be separated from the blood. Although, in the present state of our knowledge, we arc not able to give a decisive an swer to either of these questions, yet we may, perhaps, go so far as to conjecture, that the first of these effects depends more upon chemical, and the second upon mechanical causes. We cannot form any conception of a mere me chanical action producing a new chemical affinity, although it may favour the union of elements which had a previous affinity for each other. But, when the compound is once formed, we may conceive how the size or shape of the ves sels, or the velocity with which the fluid passes through them, may tend to favour its separation from the blood. With respect to any share which the action of the nerves may have in secretion, we consider it as depending alto gether upon their influence on the living system generally, and not upon any specific operation on the glands.