. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. he dorsal vessel, (Fig. 84,)by fissures at its sides. 236. In all the higher animals there is a central organ,the heait, which forces the blood through the arteries to-wards the periphery, and receives it again on its HEART is a hollow, muscular organ, of a conical form,which dilates and contracts at regular intervals, independ-ently of the w
. Principles of zoölogy : touching the structure, development, distribution, and natural arrangement of the races of animals, living and extinct with numerous illustrations : Part 1, Comparative physiology : for the use of schools and colleges. he dorsal vessel, (Fig. 84,)by fissures at its sides. 236. In all the higher animals there is a central organ,the heait, which forces the blood through the arteries to-wards the periphery, and receives it again on its HEART is a hollow, muscular organ, of a conical form,which dilates and contracts at regular intervals, independ-ently of the will. It is either a single cavity, or is dividedby walls into two, three, or four compartments, as seen inthe following diagrams. These modifications are importantm their connection with the respiratory organs, and indicatethe higher or lower rank of an animal, as determined by thequality of the blood distributed in those organs. 237. In the mammals and birds the heart is divided by avertical partition into two cavities, each of which is againdivided into two compartments, one above the other, as seen\-\ the diagram, Fig. 85.) The two upper cavities are called AND CIRCULATION. 115 aurichs, and the two lower ventricles. Reptiles have two. Fig. 86. Fig. 87. auricles and one ventricle, (Fig. 86.) Fishes have one auri-cle and one ventricle only, (Fig. 87.) 238. The auricles do not communicate with each other,in adult animals, nor do the ventricles. The former receivethe blood from the body and the respiratory organs, throughveins, and each auricle sends it into the ventricle beneath,through an opening guarded by a valve, to prevent its reflux ;while the ventricles, by their contractions, force the bloodthrough arteries into the lungs, and through the body gen-erally. 239. The two auricles dilate at the same instant, and alsocontract simultaneously ; so also do the ventricles. Thesesuccessive contractions and dilatations constitute the pulsa-tions of the heart. The contraction is c
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