ORIGIN OF THE HEART. 



135 



ic veins, which empty into the inferior vena cava, and so it reaches 

 the general circulation. The physical peculiarity of the portal cir- 

 culation is, that it commences in a capillary system, and ends in one, 

 without the intervention of any central organ of impulse, or heart. At a 

 very early period, comparative anatomists were struck with Portal circuia- 

 the analogy between the portal circulation in man and the J^^j^t^fa* 1 

 systemic circulation of fishes, both being carried on in the fish, 

 same way, that is, without a heart. In fishes, the heart is a branchial, 

 respiratory, or pulmonary one. Their systemic circulation, or circula- 

 tion of crimson blood, commences in the capillaries of the respiratory ap- 

 paratus, the gills ; a convergence takes place into an aorta, which ramifies 

 into systemic capillaries. So the great circulation in these tubes is ac- 

 complished without any heart. It is scarcely necessary to point out the 

 bearing of such a fact on the theories of the movement of the blood. 



In Fig. 54 is a diagram of the circulation of a fish ; a, 

 is the auricle ; , the ventricle ; c, the branchial or pulmo- 

 nary artery; , e, the branchial or pulmonary veins, bring- 

 ing blood from d, the branchiae, and converging directly to 

 /", the aorta, which distributes the systemic blood. This 

 is collected into a vena cava, <?, and so brought to the au- 

 ricle, a. There is therefore no systemic heart. 



The further discussion of this subject will be continued 

 as follows : We shall describe, 1st, the construction and 

 action of the heart ; 2d, of the arteries ; 3d, of the capil- 

 laries ; 4th, of the veins. We shall then present a view 

 of the combined result of these various mechanisms. 



1st, The Heart. The first appearance of the heart is as 

 a cavity arising in a collection of cells, by cleli- 



J , AX The heart. 



quescence or separation of the central ones. At 

 this early period, and even before the cavity has fairly formed, pulsation 

 may be observed. The organ soon assumes a tubular form ; and this, 



becoming curved, as shown in Fig. 

 55, differentiates into three compart- 

 ments, with arterial and venous con- 

 nections ; 1, the venous trunks ; 2, 

 the auricle ; 3, the ventricle ; 4, the 

 bulbus arteriosus. The form to be 

 eventually assumed is foreshadowed in the manner in which the curved 

 tube develops, the arch of the curve, 2, bulging so as to form a conical 

 ventricle. This tri-chambered heart remains permanent in fishes, as seen 

 in the preceding figure (54), of which c is the third chamber. But in birds 

 and mammals, the aortic bulb merges into the ventricle, through which, 

 as well as through the auricle, a septum or partition is established, and 



Diagram of fisli 

 circulation. 



Fig. 55. 



Rudimentary heart 



