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Large-muscle Activities: The heart's pacemaker is located where the superior vena cava, the large vein carrying blood to the heart from the upper parts of the body, joins the right atrium. It is made up of a collection of muscle cells, blood vessels, and nerves and is technically known as the sinoatrial node, or the node of Keith and Flack, the two men who discovered it in 1907. Every second, or even more often, an electrical impulse originates in the pacemaker and travels down over the two atria in a wave of about Vio of a millivolt, causing the atrial muscle to contract.
Paralleling these large-muscle activities is a gradual increase in the effective use of the hands. From the aimless hand movements which he makes at birth, the baby progresses to squeezing an object, then to grasping it with his hand, and finally to using his thumb and forefinger. By the end of the sixth month, the majority of children who have been studied can grasp an object of suitable size, hold it, and let go of it. By the end of the ninth month, practically all babies can accurately reach for and pick up an object. "Give and Take" becomes a favorite game.
The molars in some cases (as in the beavers) also grow from persistent pulps and possess undivided fangs. There is always a succession of milk and permanent dentitions. The chief chewing muscle is the masseter which is greatly developed, while the temporal muscle is small. The usual number of toes is five on both feet, but the pollex may be rudimentary or absent, and in the hind feet the number may be reduced to four, as in the hares, or to three, as in the agouti and jerboa. |
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