. Animal physiology. Physiology, Comparative; Physiology, Comparative. ARRANGEMENT OP THE PERICARDIUM. 199. -7* Fig. 116.—Diagram of the Pericardium. aa, auricles ; vv, ventricles; b, pulmonary artery; c, aorta; pp', pericardium. cause it affords a simple type or specimen of the mode in which other membranes of similar character are arranged round other organs, such as the lungs, bowels, and brain, and also in the joints. In Fig. 116 is seen the heart sus- pended freely in its cavity, by the large vessels proceeding from its top. This cavity is completely lined by the membrane p', which closel
. Animal physiology. Physiology, Comparative; Physiology, Comparative. ARRANGEMENT OP THE PERICARDIUM. 199. -7* Fig. 116.—Diagram of the Pericardium. aa, auricles ; vv, ventricles; b, pulmonary artery; c, aorta; pp', pericardium. cause it affords a simple type or specimen of the mode in which other membranes of similar character are arranged round other organs, such as the lungs, bowels, and brain, and also in the joints. In Fig. 116 is seen the heart sus- pended freely in its cavity, by the large vessels proceeding from its top. This cavity is completely lined by the membrane p', which closely embraces the vessels, and which then bends down over the surface of the heart, so as to enclose it in the envelope, p. Hence it will be seen that the membrane forms a completely closed bay or sac; whilst it nevertheless includes the heart, and allows it to communicate with its vessels. To adopt a homely simile, it is exactly analogous to a common double cotton or woollen nightcap; for this too is composed of a closed bag; and the head is really on the outside of its cavity (which intervenes between its two layers), whilst seeming to be included within it. It is by a similar arrangement, that the other organs just mentioned are kept in their places; and thus their vessels, nerves, &c. are allowed to reach them, without perforating their envelope; whilst they are securely retained in their places, in a mode which nevertheless permits sufficient freedom of movement. 257. The heart may be described as a hollow muscle, which, in Birds and Mammalia, as in Man, is divided into four dis- tinct chambers. This division is effected by a strong vertical partition, that divides the entire heart into two halves, which are almost exactly similar to each other, excepting in the greater thickness of the walls on the left side; and each of these halves (which do not communicate with one another) is again subdivided by a transverse partition, into two cavities, of which the upper one is t
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