I come now to certain phenomena which are not appreciable to us on mere external examination; but which I am able with certainty to predicate. And the first of these is the proportion of arterial to venous blood in the capillaries. In infancy, the arterial capillaries contain far more blood than the capillary veins; in old age, the proportion is exactly reversed; whereas, in maturity, the ratio is just equal. Now, here there is a very small preponderance of arterial blood, indicating a period but slightly remote from maturity on the side of youth; well agreeing with the conclusion arrived at from previous premises, of some twenty to five-and-twenty years. Other and more marked manifestations occur in the condition of the skeleton. In the spine, I find _the spinous and transverse processes_ of the several vertebrae are completed by separate _epiphyses_, the ossification of which does not commence till after puberty, and the final union of which with the body of the bone does not occur till about the age of twenty-five years. Each _vertebra_, moreover, has attained a smooth annular _plate_ of solid bone, covering a surface that was previously rough and fissured, which is invariably added at the same period. The _ossification of the sacrum_ also has reached its culminating point. At the age of puberty, the component vertebrae began to unite from below upwards, and the two highest have now coalesced; which also marks a period of life not earlier than the twenty-fifth year. The whole united mass, moreover, is furnished on each side with thin bony plates, the appearance of which is no less characteristic of the same age. Each of the _ribs_ is here furnished with two _epiphyses_, one for the head and the other for the tubercle; the ossification of these began soon after puberty; but their union with the body of the bone, as presented here, has taken several years to accomplish.