233 CHAPTER VI. SECRETION. Preliminary Observations-Classification of Secretions-Matter of PerspirationScarf-skin serves as a barrier between the Body and the External World-The vast extent of the two general Mucous Membranes-Only one Mucous Membrane in Animals below the class of Mammalia-Gaseous Exhalations-Follicular Secretions-Unctuous Secretion of the Skin-Necessity of Personal Cleanliness -Liquor of the Eye-lids-Wax of the Ears-Musk, and other odorous Secretions of Animals-Glandular Secretions-Arrangement of the Glands-Tears-Effects of the passions and emotions of the Mind upon Lachrymal Glands-Why Women and Children shed Tears easily-Secretions connected with Digestion adverted to-Structure and use of the Kidneys-The very compound nature of the UrineUrine carries off the superfluous, useless, and worn-out particles from the System -The quantity of Urine regulates the balance of Fluids in the Body-Sugar in the Urine Urinary Concretions-Urinary Bladder-its Structure and UsePartly under the control of the Will-Glandular Bodies, whose use has not been ascertained-Interstitial Secretion-Nutrition, Growth, and Renewal of the various Textures of the Body. HAVING considered the agents by which the blood is distributed to the various parts of the body, the changes it is subjected to in its circulation through the lungs and system at large, the means of compensation for its expenditure, its character and constitution, and the manner in which both the old and new materials are taken up and introduced, we may now direct our attention to the nature and source of the different substances elaborated from it by the function of secretion. No physical laws, either mechanical or chemical, as hitherto ascertained, will enable us to explain the mode of formation from the same fluid of the various gaseous, fluid, and solid substances of which living bodies consist, these products materially differing, as they do, not only from each other, but likewise from that from which they are derived. In the sap taken up by different plants, and in which, from whatever plant derived, chemists can detect not the slightest shade of difference, we see narcotic opium formed by the seed capsules of the poppy, the poisonous prussic acid by the leaves of the cherrylaurel, an emetic principle by the root of ipecacuan, a pure bitter in the strobiles of the hop. Acids are obtained from some, alkalies from others, sweet juices, nutritious fecula, and fixed or volatile oils from others. Even different parts of the same plants furnish peculiar secretions: the fleshy part of the fruit of the olive abounds with a bland oil, no trace of which can be detected in the sap of the vessels proceeding to the part in which it is formed, and the most minute examination of the tissue in which it is prepared has failed to furnish an explanation of the manner of its production. So likewise in animals we cannot tell how the nutritious milk is secreted in one organ, the bitter bile in another, the acrid urine in a third, and from others an offspring endowed with independent life for the continuance of the race; nor can we tell how the same individual, under the influence of disease, has established new secretions possessed of properties whereby they become capable of propagating themselves to an indefinite extent, such as from small pox, measles, and contagious fevers. The most careful and elaborate examination of the biood, from which these secretions are formed, throws no light on the processes by which they are effected, nor will the most delicate and searching analysis explain the origin of their essential qualities. We say that the ultimate elements of animal bodies are chiefly hydrogen, carbon, oxygen, and nitrogen: they are termed elements because they have resisted all the means hitherto had recourse to for decomposing them; we are therefore entitled to hold them as elementary until they be shewn to be compound. Yet for any thing that is known they may in reality be composed of still more subtile principies, and regulated by laws of which at present we can entertain no conception. The compound nature of such as water and atmospheric air, not long ago considered as elements; the still more recent decomposition of the fixed alkalies and earths, as well as the simple constitution of others, such as chlorine, till lately esteemed compound; the powers possessed by electricity, as modified by friction, chemical action, magnetism, heat, and organic life; and the effects which heat and light are capable of producing, the investigation of which adds every day to our acquirements, all tend to show the uncertainty of human knowledge, and teach a lesson of humility in estimating the stability of the most universally received and generally established doctrines respecting the laws and operations of nature. Although the means by which living organs effect these transmutations, and the immediate source of these seemingly creative powers baffle every attempt to unravel them; although the proximate causes of the peculiar qualities of their products, and in what these essentially consist, be entirely unknown ;--admitting that these are provinces which cannot be cultivated with any prospect of success, that they are secrets which man cannot penetrate, mysteries which for the present at least are hid from his eyes, still the examination of the construction of the machinery by which they are produced, and the investigation of the properties characterizing them, are both legitimate and interesting fields of inquiry, and well repay the labour bestowed on their cultivation. be For the present we shall confine ourselves to the examination of those secretions which contribute to the preservation and wants of the individual, leaving for a future occasion the consideration of such as are subservient to the continuance of the race. Secretions may arranged under four general divisions: first, exhalations where the thinner part of the blood is discharged from extended surfaces, including those from the skin, mucous, synovial, serous, and cellular membranes ; second, follicu lar secretions from small crypts dispersed on the skin, and mucous membranes ; third, glandular secretions, comprising the tears, saliva, pancreatic juice, bile, and urine; and, fourth, interstitial secretions, such as constitute the peculiar matter to each tissue, as that of bone, cartilage, ligament, muscle, nerve, &c. Exhalations. We have already had occasion to state (p. 85) that a large quantity of the watery part of the blood is daily discharged from the skin, forming the matter of sensible and insensible perspiration, and that it is of great importance in the regulation of animal heat. This discharge may depend on two causes-mechanical transudation, or exosmose, and vital action of the vessels, whereby a portion of the serous part of the blood escapes in quantity varying according to the state of the system and condition of the atmosphere. The cuticle or scarf-skin, spreading over the whole external surface of the body, affords a covering of greater or less porousness and density in different situations, forms a barrier between all that belongs to the body of the individual and the external world around him, and regulates to a certain extent perspiration. It is devoid both of blood-vessels and nerves, and is the product of the subjacent vessels; it forms numerous branched and reticulated lines or ridges, sometimes symmetrically arranged, as at the points of the fingers, and having intermediate fissures conspicuous in the palms of the hands. It is pierced with an infinite number of exceedingly minute perforations, termed pores, some of which give exit to the hair, through which the vapour exhales; but even independently of these pores, as shewn by the phenomena of endosmose and exosmose, membranes do not offer a complete obstruction to the passage of fluids. Being extravascular, its influence is entirely mechanical, and continues to operate even in the dead body. In temperate climates, it lessens evaporation from the dead body, and to a certain extent retards decomposition; but so rapidly does evaporation take place in the arid deserts of Africa and Arabia, that when the devoted caravan has been overtaken and destroyed by the blast of the burning simoom, the fluids are so instantly drunk up that the light and parched mummy which remains may continue for years without undergoing further change. The true skin, which lies immediately under the cuticle, is highly vascular, and possessed of acute sensibility, and although defended by its covering, still it is not beyond the influence of external agents which come in contact with the external surface of the body, and which affect the condition of its vessels; for, on the application of several substances, these become excited, and exhale a greater quantity than usual of the serous part of the blood. Thus heat, friction, and various other stimulants, promote this discharge, and the quantity poured out may be so abundant that it cannot obtain a sufficiently ready escape through the scarf-skin, and therefore accumulates beneath it, raising blisters which may appear in a few seconds, as from the application of boiling water. The action of the vessels distributed on the surface of the true skin is also affected by the state of the system, in all its various conditions of health and disease, or as under the influence of medicinal and other agents. The quantity and quality of the blood likewise have their effect. Some parts of the body also perspire more freely than others, as the palms of the hands, and the soles of the feet. we perceive that perspiration, whether sensible or insensible, is regulated by the condition of the atmosphere, the state of the cuticle, of the true skin, and system at large. So far as it depends on the two last, it may be considered as more affected by vital than mechanical agency. Thus The vessels of the skin likewise secrete carbonic acid, as proved by keeping the arm in a jar, so managed as to prevent the renewal of air in the jar; after some time an additional quantity of carbonic acid will be found to have made its appearance. This secretion of carbonic acid, and |