skin is avoided by inserting the matter of the disease beneath it with a lancet. You must not suppose, however, that the scarf-skin entirely prevents the entrance of morbific matter by the skin; on the contrary, the skin is frequently the medium of infection, although not so much so as is the lining membrane of the lungs. There are endless proofs and illustrations of the process of membranous absorption which I have not noticed. I have here treated principally of the three great surfaces. I have stated what I deem the most essential and needful to be known by you, that you may the more readily understand the remaining letters I have to address to you respecting health and disease. Before I enter on the task of further reply to your many inquiries, let me beg of you to consider well the concise account I have now given you of the function of absorption, in all its varieties; for it is assuredly very closely concerned in every stage of life, health, and disease. I must leave the subject for your meditation. FIVE GREAT ORGANS OF EXCRETION-EFFECTS OF OBSTRUCTED EX CRETION. MY DEAR SIR, THERE are two organic functions of the human body, of which I have not written to you. They are intimately connected with the chief subjects of these letters. In meeting with the names of these two processes non-medical readers are frequently puzzled to understand their true nature and different uses. I allude to secretion and excretion. I can only undertake, in my limited space, to make a few brief remarks concerning them, yet they will suffice for the end in view. The word secretion is from the Latin word secerno, to separate, to sever, to put asunder or apart. The entire matter of the human body is literally secreted or formed from the blood, which is formed from food, with the aid of air, water, heat, electricity, and light. These things have been repeatedly explained to you. But the term secretion is used in a more special sense, and is applied to certain matters, both fluid and solid, which are secreted or formed by certain parts of the body, called secreting organs or instruments. The principal ones are glands of various sizes, and for various purposes; and serous and mucous membranes, also, form extensive secreting surfaces, which perform functions of great importance to the health and well-being of the animal economy. I must not forget to tell you that the word secretion was first used on the erroneous supposition that it was literally and only a separation of those different matters from the blood which existed in it. The truth is, that they are formed or elaborated out of its constituent elements. The liver secretes, or elaborates, or forms bile from the carbonised and impure venous blood, which is carried through it for the purpose. Again, the breasts of the mother secrete or elaborate milk from the nutritious arterial blood circulating through them for the purpose. The stomach, by the apparatus on its lining membrane, secretes the gastric juice. The kidneys secrete urine. The salivary glands in and around the mouth secrete saliva. The wax of the ear is secreted by a suitable apparatus on its internal surface. All matters, both fluid and solid, which are produced to serve some useful purpose in the human body, are the products of secretion. And mark you, all matters which are separated from the body, to be removed as useless or noxious, are also the produce of secretion. But they are called excretions, because they are separated for the purpose of their removal from the system. Excretion is a particular form of secretion. The distinguishing difference between the two processes is, that in excretion the matter separated is either noxious or useless, and must be removed from the body. The matter separated by secretion has to serve some useful purpose in the body. I cannot explain to you the real nature or mode of performance of secretion; for, like some other things in the animal system, it is unknown. It is certainly performed by the joint means of arteries, veins, nerves, and absorbents. The chief agency is, doubtless, that of the nervous power, and may be of a modified kind for the purpose. Membranes become the seat of secretion, and we see a great variety of its products. The membranes which line the large and closed cavities of the human body are termed serous membranes, because the fluid which they secrete, and which preserves their proper and moistened condition, is serum. I mean the cavities of the chest and of the abdomen. Again, there is the synovial membrane, which lines the interior surface of the joints, and secretes synovia, or joint-oil, a glairy fluid which is useful and necessary for locomotion. Then, you must consider the most extensive membrane of the body, which is called mucous membrane. It lines the open cavities and canals of the body-the mouth, the stomach, and intestines also the air passages, and the lungs. This mucous membrane secretes mucus, which adheres to its surface, and keeps it in a suitable state of moisture. : An Italian physiologist called Malpighi, and another called Ruysh, who flourished at Amsterdam at the same time, and Müller of Berlin, all these narrowly investigated the minute structure of the secreting apparatus, and formed certain doctrines on the nature and uses of secreting sacs, and follicles, and tubes but they differed in their opinions. Nevertheless, it is now established that follicles, cells, and tubes constitute the principal apparatus of secretion, with some variety of arrangement of the same. : Sometimes the apparatus or means of secretion consists in simply extended membrane: and a fine network of minute or capillary arteries, nerves, and absorbents, is stretched over its secreting surface; and by the specific action of these the matter secreted is separated from the blood. F Yet you are to keep in mind, that there are the other forms of apparatus for the purpose; namely, cryptæ, or small pits; follicles, or small bags; cæca, or small pouches; tubuli, or small tubes; which also serve for retaining the matter for a while, to be supplied according to the wants of the system. When these cryptæ, follicles, cæca, and tubuli are collected into close contact, and have their necessary arteries and nerves enclosed with them in a common membrane, and as one mass, they constitute a secreting gland. You are to consider a secreting gland as a collection of these secreting bodies connected by cellular tissue, and enveloped in a common membranous covering, and thus forming a distinct organ of secretion. Such are the liver, the pancreas, the spleen, and the. kidneys. There are many very interesting particulars in connection with the present subject; but my limited space allows not of further particularization. I must not leave it, however, without some notice to you of the extent of secreting apparatus. Think, that wherever nutrition is carried on, there, also, is secretion and its apparatus. The extent cannot be correctly stated. All the internal surfaces are studded with secreting bodies. The skin is covered with them for the secretion of insensible perspiration; also for that of the oily matter which. gives to it its softness. But further, think of the great organs of the body, the liver, the lungs, the pancreas, and spleen: also the brain. Again, think of the organs of the senses, the eyes, the nose, the tongue, and the ears: nay, every point of the body, and even the bones, have innumerable organs of secretion. The great and indispensable agent in this process or function of secretion is organic nervous power; this is undoubted. Just in familiar illustration of the fact let me call your attention to what you are already aware of. You know that the sight, and even the thought itself, of agreeable food, fills the mouth with a secretion of saliva; in common parlance, it makes the mouth water. Again, music, or agreeable society at dinner, or other meal, increases the appetite, and favours digestion. You well know, and I well know from experience, that disagreeables of any kind, affecting the mind through the brain and nerves of animal life, and through these, the nerves of organic life, destroy the appetite of the most hungry man: the secretion of gastric juice is excited by the causes of agreeable kind, whilst it is arrested by the contrary. Again, you well know that grief causes a flow of tears, in other words, it causes an increased secretion of tears by the lachrymal glands. Fear will cause an increased secretion of urine, and an inability to retain it. Often have I known the fine maternal feelings which bind woman's heart to her offspring, called into active exercise by the cry of her child! That cry, or the sight of her child, will at once fill her breasts with milk. Nay, it is a well-known fact that the woman of strong maternal feelings has had the secretion of milk produced by an infant's cry when she has heard it in after years, and long past her own period of child-bearing. The imagination can affect secretion. Dr S. Smith mentions the case of a female who had a great aversion to calomel. She was taking it in minute doses, unknown to herself. She was told of it, and was immediately salivated. On being persuaded that she had not taken any, the salivation ceased. Again she was told that she was taking it, the salivary glands were again excited to excessive action and salivation. In proof that it was purely the work of imagination, there was no redness of the gums or swelling of them, which always is the case in the action of calomel. Allow me to conclude this very interesting subject of secretion, to which your earnest letters have led me, by a few remarks on another point which crosses my mind regarding it. I cannot give you any satisfactory explanation of the fact. But how is it, that from the same vital fluid, the blood, the same kind of secreting apparatus produces the secreted matters according to the specific purposes to be served by them, and produced, too, by the same nervous power? How is it, I mean, that the breasts of the mother secrete the bland and nutritious fluid called milk, which is so suitable for infancy, and not |