With regard, however, to the elements of other secretions, the evidence is less clear as to the state in which they exist previously to their elimination by the secretinrr apparatus. The fact would appear to be, Cowever, that the solid constituents of most of them are little else than constituents of the blood itself, either pure or but slightly altered. Thus in the lachrymal fluid, the saliva, the gastric and pancreatic juices, and the serous fluid of areolar tissue and of serous and synovial membranes, we find little else than saline matters, which are normal consti tuents of the serum of the blood, with one or more organic compounds, that seem like albumen in a state of change. The repres sion of these secretions does not produce any deleterious effect upon the general system, otherwise than as impairing or preventing the performance of the function to which they are subservient; whence it may be inferred, that the selection of the secreted products from the blood is made in these cases, not for the sake of purifying the circulating fluid from any matter that would be noxious if retained, but merely for some minor purposes in the economy, to which these simple fluids are adequate.
Metastasis of secretion. — Although the number and variety of the secretions become greater in proportion to the increased com plexity. of the nutritive processes in the higher classes, and although each appears as if it could be formed by its own organ alone, yet we may observe, even in the highest animals, some traces of the community of function which characterises the general surface of the lowest. It has been shown that, although the products of secretion are so different, the elementary structure of all glands is the same; that wherever there is a free secreting surface it may be regarded as an extension of the gene ral envelope of the body, or of the reflexion of it which lines the digestive cavity ; that its epithelium is continuous with the epider mis of the integument, or with the epithelium of the mucous membran6 from which it is prolonged; and that the peculiar principles of the secreted products pre-exist in the blood, in a form at least closely allied to that which they assume after their separation. Now, it may be stated as a general law in physiology, that in cases where the different functions are highly specialised (that is, where every one has its special and distinct organ for its own pur pose alone), the general structure retains, more or less, the primitive community of function which characterised it in the lowest grade of develop ment.* Thus, although the functions of absorption and respiration have special organs provided for them in the higher animals, they are not altogether restricted to these, but may be performed in part by the general sur face, which (although the special organ for exhalation) permits the passage of fluid into the interior of the system, and allows the in terchange of gases between the blood and the air. In the same manner, we find that the
functions of secretion being equally performed in the lowest animals by the whole surface, whilst in the highest there is a complicated apparatus of glandular organs, to each of which some special division of the function is assigned, either the general muco-cutaneous surface, or some one of its subdivisions or prolongations, is able to take on in some degree the function of another gland whose functions may be suspended. This truth was well known to Haller, who asserted that almost all secretions may, under the influence of disease, be formed by each and every secreting organ..f This statement, however, needs to be received with some limitation, and it would be probably safest to restrict it to the excretions, whose elements pre-exist in the blood, and accumulate there when the elimination of them by their natural channel is suspended. We shall now consider some of the more remarkable examples of the me tastasis of secretion.
It seems to be established by a great mass of observations, that urine, or a fluid present ing its essential characters, may pass off by the mucous membrane of the intestinal canal, by the salivary, lachrymal, and rnam mary glands, by the testes, by the ears, nose, and navel, by parts of the ordinary cutaneous surface, and even by serous membranes, such as the arachnoid lining the ventricles of the brain, the pleura, and the peritoneum. A considerable number of such cases was col lected by Haller I : many more were brought together by Nysten § ; more recently Bur dach has furnished a full summary of the most important phenomena of the kind 11 ; and Dr. Laycock has compiled a valuable summary of cases of urinary metastasis occurring as com plications of hysteriall The following table of the different forms of this curious affec tion :— of cases referred to by the last of these authors will give some idea of the relative frequency ft is to be borne in mind, however, that cases of hysterical ischuria are frequently compli cated with that strange moral perversaan, which leads to the most persevering and in genious attempts at deceit ; and there can be little doubt that a good many of the instances on record, especially of urinous vomiting, are by no means veritable examples of metastasis. The proofs of the fact we are seeking to establish are, therefore, much more satisfac tory when drawn from experiments upon animals, or from pathological observations, about which, from their very nature, there can be no mistake.