Among the nerves lie concealed the arteries, which are so many passages for the spirit; and upon these float the veins, as conduits for the blood. The pulsation of the arteries is more especially perceptible on the surface of the limbs, and afford indications of nearly every disease, being either stationary, quickened, or retarded, conformably to certain measures and metrical laws, which depend on the age of the patient, and which have been described with remarkable skill by Herophilus, who has been looked upon as a prophet in the wondrous art of medicine. These indications, however, have been hitherto neglected, in consequence of their remarkable subtilty and minuteness, though, at the same time, it is by the observation of the pulse, as being fast or slow, that the health of the body, as regulating life, is ascertained. CHAP. 89.—THE ARTERIES; THE VEINS: ANIMALS WITHOUT ARTERIES OR VEINS. THE BLOOD AND THE SWEAT. The arteries are destitute of sensation, for they are devoid of blood. They do not, all of them, however, contain the vital spirit, and when one of them has been cut, it is only that part of the body that is reduced to a torpid state. Birds have neither veins nor arteries, which is the case also with serpents, tortoises, and lizards; and they have but a very small proportion of blood. The veins, which are dispersed beneath the whole skin in filaments of extreme thinness, terminate with such remarkable fineness, that the blood is able to penetrate no further, or, indeed, anything else, except an extremely subtle humour which oozes forth from the skin in innumerable small drops, and is known to us as “sweat.” The knot, and place of union of the veins, is the navel. CHAP. 90. (38.)—ANIMALS, THE BLOOD OF WHICH COAGULATES WITH THE GREATEST RAPIDITY: OTHER ANIMALS, THE BLOOD OF WHICH DOES NOT COAGULATE. ANIMALS WHICH HAVE THE THICKEST BLOOD: THOSE THE BLOOD OF WHICH IS THE THINNEST: ANIMALS WHICH HAVE NO BLOOD.