[31] The cause of the clotting of blood is not yet fully understood. Although the process has been thoroughly investigated we have not yet a satisfactory explanation why the circulating blood does not clot in healthy blood-vessels. The ablest physiologists of our day do not, as formerly, regard the process as a so-called vital, but a purely chemical one. [32] Serous Membranes.—The serous membranes form shut sacs, of which one portion is applied to the walls of the cavity which it lines; the other is reflected over the surface of the organ or organs contained in the cavity. The sac is completely closed, so that no communication exists between the serous cavity and the parts in its neighborhood. The various serous membranes are the _pleura_ which envelops the lungs; the _pericardium_ which surrounds the heart; the _peritoneum_ which invests the viscera of the abdomen, and the _arachnoid_ in the spinal canal and cranial cavity. In health the serous membranes secrete only sufficient fluid to lubricate and keep soft and smooth the opposing surfaces. [33] A correct idea may be formed of the arrangement of the pericardium around the heart by recalling how a boy puts on and wears his toboggan cap. The pericardium encloses the heart exactly as this cap covers the boy’s head. [34] “Alcohol taken in small and single doses, acts almost exclusively on the brain and the blood-vessels of the brain, whereas taken in large and repeated doses its chief effects are always nervous effects. The first effects of alcohol on the function of inhibition are to paralyze the controlling nerves, so that the blood-centers are dilated, and more blood is let into the brain. In consequence of this flushing of the brain, its nerve centers are asked to do more work.”—Dr. T. S. Clouston, Medical Superintendent of the Royal Asylum, Edinburgh. “Alcoholic drinks prevent the natural changes going on in the blood, and obstruct the nutritive and reparative functions.”—Professor E. L. Youmans, well-known scientist and author of _Class Book of Chemistry_. [35] The word “cell” is not used in this connection in its technical signification of a histological unit of the body (sec. 12), but merely in its primary sense of a small cavity. [36] “The student must guard himself against the idea that arterial blood contains no carbonic acid, and venous blood no oxygen. In passing through the lungs venous blood loses only a part of its carbonic acid; and arterial blood, in passing through the tissues, loses only a part of its oxygen. In blood, however venous, there is in health always some oxygen; and in even the brightest arterial blood there is actually more carbonic acid than oxygen.”—T. H. Huxley.