In man and the higher animals the whole mechanism of the heart is greatly complicated by the action of the nervous system upon it and upon the contraction or expansion of the blood vessels. In this way the rate of the beat of the heart is affected and brought into relation with the needs of the blood circulation in remote parts of the body. The beat of the heart in the human species is more rapid in children than in adults, and more rapid in women than in men, and it differs in all individuals under differing conditions. Before birth it is 140 per minute, in the first month after birth 130, and gradually diminishes to 90 at nine years of age, and at twenty-one to 70 in man and to 80 in woman. But these figures only represent a general average; there are healthy men whose pulse usually is less than 45 per minute, and there are individuals who, without being invalids, yet have the movement of the heart so liable to increase in rapidity through mental or other excitement, acting by nerves directly on the heart muscle, that the pulse often goes up to 120. In the horse and the ox the pulse or heart beat is 36 to 40 a minute; in the sheep 60 to 80; in the dog 100 to 120; in the rabbit 150; and in small creatures, like mice and moles, 200, and even more! I do not know what is the record for the elephant, but as it seems that the larger the mammal the slower the pulse, one would not expect more than 20 to 25 beats a minute in his case. It is easy to watch the beating of the heart of a flea or other small insects—under the microscope—since the skin is sufficiently transparent. It is not usually much more rapid than in man, but in the very transparent little fresh-water shrimps which are called water-fleas (_Entomostraca_) I have seen the heart beating so rapidly that I could not count its rate. The heart in insects and shrimps and their like is remarkable for the fact that whilst it pumps out blood through arteries both in front and behind, it has no actual veins opening into it. All the veins, which in their ancestors entered the heart in a row on each side of it, have united, and their walls broken down, so that the heart lies in a sac full of venous blood from which it draws its fill, when it dilates, through a series of valve-bearing openings on its surface, openings which, in an earlier stage of development, were connected with individual veins.