NATURAL IIISTOKY OF OKGANIZED BODIES. 



9nr. 



Among tlie meclianical pliciioin(.'na of the circulation of the hh)o<l there are 

 quite a number which may be imitated in a perfect manner. A schema, well 

 known in Germany, is that of AVeber : it shows how the circular movement of 

 the blood is accomplished in that vast self-re-entering system represented by the 

 heart and blood-vessels, [jig. 3.) 



We take an elastic tube, curved upon itself, so as to form a complete circuit. 

 which may be filled with a liquid by means of the funnel, e. At a point in 

 this tube a part, marked c, is bounded by two valves, both of which open in the 

 same direction. This portion of the circuit corresponds to the heart. At the 

 point directly opposite the portion c is placed, at c, a tube of glass, in which a 

 sponge is infixed tightly, forming, of course, an obstacle to the passage of the 

 liquid, in regard to which it exerts a resistance like that opposed l)y Uie capil- 

 lary vessels to the course of the blood. The apparatus being now filled with 

 liquid is ready for operating. If intermitting pressures be exerted on the part 

 c which represents the heart, the enclosed liquid is propelled and made to pass 

 into the portion of the tube where the ]ilay of the valves permits its beinf intro- 

 duced, nameh', into a, a'. Under the influence of compressions frequently 

 repeated, the portion into which the liquid flows becomes distended. Now, it is 

 in this condition that the arterial system subsists in animals, since there the blood 

 is continually pressed forward 1)y the systoles of the left heart. Hence the 

 liquid acquires in this part of the tube a considerable amount of pressure Avhich 

 imitates, with sufficient exactness, the pressure of the blood in the arteries. The 

 sponge, c, allows the liquid to pass gradually from the arterial part of the tnbc 

 into the venous part, that is to say, into the portion v' v of the apparatus. This 

 passage of the liquid takes place in a continuous manner, notwithstanding the 

 interuiission of the impulses given to the liquid. Here, then, we realize an imi- 

 tation of the phenomenon produced in the circulatory apparatus : ihe regularity, 

 namely, of the course of the blood in the small vessels. In both cases this 

 result is obtained through the effect of the elasticity of the conduits in which 

 the liquid has circulated. Further, it is the same cause which producers in fire- 

 engines the regularity of the jet, notwithstanding the successive checks in the 

 play of the pump. In apparatus of the latter kind resort is had to a bell-shaped 

 receiver, under which the liquid arrives on issuing from the i)unq>, and which 

 counteracts the irregularities of the motive force. 



It should likewise be remarked, that, under the inflaence of successive impul- 

 sions given to the liquid by pressing on the part r, it will l)e found that the 

 arterial and the venous portion of tlie circuit present opposite conditions of 

 repletion : the arterial pressure constantly tending to distention at the expense 

 of the venous portion which is at the same time partially depleted. It is thus 

 also in the economy of the living animal, the repletion of the arterial svstem 

 taking place at the expense of the contents of the veins. Finall}', it will l>e 

 observed that each impulsion given to the liquid, by the compression of the tube 

 at c, communicates to the whole of the arterial column a pulsation analogous to 

 that i)resented by the arteries of a living animal, and that this iiupulsicm is 

 anmilled at the extremity of the arterial part, so as to fail entirely in the \enous 

 portion. On the whole, then, this schema of Webei-'s reproduces in a very sim- 



