19. THE CAVITIES OF THE HEART.--The heart is hollow, and so partitioned as to contain four chambers or cavities; two at the base, known as the _auricles_, from a fancied resemblance to the ear of a dog, and two at the apex or point, called _ventricles_. An auricle and a ventricle on the same side, communicate with each other, but there is no opening from side to side. It is customary to regard the heart as a double organ, and to speak of its division into the right and left heart. For while both halves act together in point of time, each half sustains an entirely distinct portion of the labor of the circulation. Thus, the right heart always carries the dark or venous blood, and the left always circulates the bright or arterial blood. [Sidenote: 20. Capacity of the chambers of the heart? What wise provision is mentioned? The auricles?] 20. If we examine the heart, we at once notice that though its various chambers have about the same capacity, the walls of the ventricles are thicker and stronger than those of the auricles. This is a wise provision, for it is by the powerful action of the former that the blood is forced to the most remote regions of the body. The auricles, on the contrary, need much less power, for they simply discharge their contents into the cavities of the heart near at hand and below them--into the ventricles. {111} [Sidenote: 21. Substance of the heart? Its fibres? Its movements? The advantage of such movements? Action of the heart? Its period of repose?] 21. ACTION OF THE HEART.--The substance of the heart is of a deep red color, and its fibres resemble those of the voluntary muscles by which we move our bodies. But the heart's movements are entirely involuntary. The advantage of this is evident; for if it depended upon us to will each movement, our entire attention would be thus engaged, and we would find no time for study, pleasure, or even sleep. The action of the heart consists in alternate contractions and dilatations. During contraction the walls come forcibly together, and thus drive out the blood. In dilatation, they expand and receive a renewed supply. These movements are called _systole_ and _diastole_. The latter may be called the heart's period of repose; and although it lasts only during two-fifths of a heart-beat, or about a third of a second, yet during the day it amounts to more than nine hours of total rest. [Sidenote: 22. Remarkable property of the tissue of the heart? How shown? How interesting? In cold-blooded animals? Heart of a turtle? Of a frog? Alligator?]