Dynamic Theory. 

 tractility and circulation may also survive a partial unwinding of heart 

 development. The heart is primarily only a portion of the system of 

 vessels, which, from its position in the body, is exposed to more power- 

 ful or more frequent stimulations than other parts, and so has come to 

 acquire a more constant habit and a greater facility for contraction, 

 which has become settled and intensified by ages of use, while the con- 

 tractility of most the other vessels has, by disuse, remained in a back- 

 ward condition. 



In some .of the mollusks, contractile vesicles are found in different 

 parts of the circulatory system. The Amphioxus has no heart, but its 

 blood vessel contracts rythmically, thus propelling the blood. In the 

 Ascidian, the heart is a dilation of the principal tube containing the 

 blood, and by its contractions the blood is sent in both directions, a part 

 going forward to the gills and becoming oxidized, and a part going 

 backward into the tail end of the body and being deoxidized, the two 

 bloods meeting in the heart when it dilates, and, to some extent, chang- 

 ing places for the next pulsation. No doubt the intermingling of these 

 two, generates the greater part of the electrieity which causes the con- 

 tractions of the organ. The heart of the fishes is two-chambered, and 

 the blood flows in at the auricle, which, by its contraction, drives it into 

 the ventricle, which, in turn, drives it to the gills for oxidation. ( See 

 fig. 26.) The heart of the frog has three chambers, two auricles and 

 one ventricle. The blood from all the veins, ( two superior and one in- 

 ferior venae cavse ) enters the right auricle through an ante-chamber 

 called the venous sinus. The blood from the lungs enters the left auri- 

 cle. Both auricles drive their blood into the ventricle, which in turn 

 drives it out into the aorta through a muscular swelling called the 

 bulbus arterwsus. A branch of the aorta takes part of the blood to the 

 lungs, while the rest is conveyed to all parts of the body. There are 

 three nerve ganglions in this heart, one in the venous sinus and the 

 others on the auricles. They are connected with each other and with 

 the central nervous system by way of the vagus nerve. They no doubt 

 correlate the movements of the different chambers of the heart so as to 

 preserve the due sequence of contractions, although they were not orig- 

 inally essential to cause the contractions. Any part of the heart can be 

 made to pulsate by direct stimulation by pricking with a needle, electric 

 shocks, &c. The reptile heart has two auricles, one receiving blood 

 from the body and the other from the lungs. It has one ventricle, but 

 it is partly divided into two, and the aorta and the lung artery leave 

 these separately. In the crocodile, the ventricle is fully divided. 



The human heart, like that of the birds and mammals, has four fully 

 formed chambers. The right auricle and ventricle are connected with 

 each other, but are wholly separated from and independent of the left 



