Sinuses are venous channels, which in their structure and mode of distribution differ altogether from the veins. They are found only in the interior of the skull, and consist of channels formed by a separation of the two layers of the dura mater. [Illustration: FIG. 11—Human blood.] =Blood.=—The blood of the body is contained in a practically closed system of tubes, the blood vessels, within which it is kept circulating by force of the heart beat. It is usually spoken of as the nutritive liquid of the body, but the functions may be stated explicitly, although still in quite general terms, by saying that it carries to the tissues food stuffs after they have been properly prepared by the digestive organs; that it transports to the tissues oxygen, absorbed from the air by the lungs; that it carries from the tissues various waste products formed in the processes of dissimilation; that it is the medium for the transmission of the internal secretion of certain glands; that it aids in equalizing the temperature and water contents of the body. The total quantity of blood in the body has been determined approximately for man as one-thirteenth of the body weight. The specific gravity of human blood in the adult may vary from 1.041 to 1.067, the average being about 1.055. The blood is composed of a liquid part, the plasma, in which float a vast number of microscopical bodies, the blood corpuscles, known respectively as the red corpuscles, the white corpuscles or leucocytes, of which in turn there are a great many different kinds, and the blood plates.