{410 Analogy between Vegetables and Animals^ 



entirely mechanical, and that the progression of the blood 



through them is promoted by the m a tergo, [propulsion from 

 J^ehind], and by the pressure of the muscles during their con- 

 (traction. Among the individuals who hold that the arteries 

 ,?exhibit no manifestations of contractility, are Haller, Bichat, 

 liCraigie, Nysten, Pang, and Dollinger; while Hunter, Whytt, 

 jiSenac, Thompson, Philip, Bikker, and Rossi are among those 

 'jwho entertain a contrary opinion. Wilson supposes that the 

 ^force of the heart projects the blood through the arteries, and 

 jeven into the veins, but that it is impelled along the venous 

 itrunks into the heart by what he terms derivation, Wilson's, 

 -^theory has, with considerable modifications, been adopted by 

 oCarson. 

 s. Dutrochet, a physiologist of great eminence, conceives that 



every hypothesis hitherto offered to explain the circulation 



.of the blood is inadequate to the purpose. He has therefore 

 ^proposed a new one. He seems, indeed, to allow that the 

 i4)Ower of the heart transmits the blood through the large arterial 

 «, trunks ; and, when there are no capillary branches between the 



arteries and veins, that this power extends from the former 



into the latter, and alone propels their contents forwards. He 

 ^maintains, however, that the progression of the fluids through 

 jjihe capillaries is perfectly independent of the impulse of the 

 -Ij^eart ; at the same time, he denies that these vessels circulate 

 stheir fluids by the agency of contractility, because this is a 

 ^/acuity, he asserts, which they have never yet been proved to 

 ipossess. Neither does he admit that the pressure of the contract- 

 lifeig muscles is the chief cause of the motion of the blood through 

 i4;he veins and lymphatics. Dutrochet is of opinion that the 

 qfiirculation of the blood is conducted by the operation of 

 ^^endosmose [impulse inward]. I shall first endeavour to explain 

 -^hovf this principle carries on the circulation of fluid through 

 .the lacteais. The extremities of these vessels, as I have before 

 , observed, commence in what are called ampuUulae, which are 



composed of small cells, filled with a dense organic fluid ; 

 ha condition peculiarly adapted for the exercise of endosmose. 



Jn consequence of their endosmosmic power, the ampuUulae 

 ,. of the lacteais cause the chyle, by which they are surrounded, 

 \ to flow towards them, and to enter their cells. This ingress 

 ^jproduces an accumulation of chyle within these organs, which 

 Jrenders them very turgid. In consequence of this turgidity, 

 htheir natural elastic power is called into play, which, reacting 



upon the accumulating chyle, forces it into the lacteal tubes.. 

 bBythe introduction of fresh chyle into the cells of the ampul- 

 alulae, the current is unceasingly kept up. The lacteal tubes 



are furnished with glands, which possess the same structure, 



