4. The aorta leaves the heart as a large hollow passage, but as it advances it becomes narrower and more sinewy. From the aorta, veins extend also to the mesenterium, like those from the great vein, but far inferior in size, for they are narrow and muscular. They terminate in small hollow muscular veins. No branch of the aorta extends to the liver and the spleen, but the branches of either vein extend to each hip, and both touch upon the bone. Branches reach the kidney both from the great vein and the aorta; they do not, however, enter the cavity, but are taken up in the substance of the kidney. 5. Two other strong and continuous passages reach from the aorta to the bladder, and others from the cavity of the kidney; but these do not communicate with the great vein. From the centre of each kidney a hollow sinewy vein passes through the other veins to the spinal column; first of all they disappear upon each hip, and then appear again in branches towards the hip; their extremities are distributed upon the bladder and penis in the male, and upon the uterus in the female; no branch of the great vein passes to the uterus, but many and thick ones reach it from the aorta. 6. From the aorta and great vein branches are distributed to the nates; at first they are large and hollow, afterwards they pass through the legs, ending upon the feet and toes; and others again pass through the nates and thighs, alternately from right to left, and they join with other veins below the knees. 7. The nature and origin of the veins are evident from this description. In all sanguineous animals, the nature and origin of the principal veins are the same, but the multitude of smaller veins is not alike in all, for neither are the parts of the same nature, nor do all possess the same parts. Nor are the veins equally apparent in all animals; but they are more manifest in those which have most blood, and in the largest creatures; but in those animals which are small, and have not much blood, either by nature or from excess in fat, they are not so easily investigated, for some of the passages are confused, like rivulets that are lost in beds of mud; and there are some animals which have but few, and these fibres instead of veins. The great vein is very conspicuous in all, even the smallest animals. CHAPTER V. 1. The following is the nature of the sinews of animals. The origin of these, also, is in the breast, for there is a sinew in the principal cavity of the heart itself; and that which is called the aorta is a sinewy vein, for its terminations are always sinewy, for they are not hollow, and are extensible, like the sinews which end upon the bending of the bones: for it is not the nature of sinews to be continuous from one origin, like the veins, for the veins have the whole form of the body as in outline sketches, so that in emaciated subjects the whole mass appears full of veins, for the same place is occupied by veins in lean persons that in fat ones is flesh.