OF ANIMAL BODIES. 99 



branous purse or bag, made of strong tough materials, is 

 provided for it ; holding the heart within its cavity ; sitting 

 loosely and easily about it ; guarding its substance, without 

 confining its motion ; and containing likewise a spoonful 

 or two of water, just sufficient to keep the surface of the 

 heart in a state of suppleness and moisture. How should 

 such a loose covering be generated by the action of the 

 heart ? Does not the enclosing of it in a sack, answering 

 no other purpose but that of enclosure, show the care that 

 has been taken of its preservation ? 



One use of the circulation of the blood (probably amongst 

 other uses) is to distribute nourishment to the different 

 parts of the body. How minute and multiplied the ramifi- 

 cations of the blood-vessels, for that purpose, are ; and 

 how thickly spread, over at least the superfices of the body, 

 is proved by the single observation, that we cannot prick 

 the point of a pin into the flesh, without drawing blood, 

 i. e. without finding a blood-vessel. Nor, internally, is their 

 diffusion less universal. Blood-vessels run along the sur- 

 face of membranes, pervade the substance of muscles, pen- 

 etrate the bones. Even into every tooth, we trace, through 

 a small hole in the root, an artery to feed the bone, as well 

 as a vein to bring back the spare blood from it ; both which, 

 with the addition of an accompanying nerve, form a thread 

 only a little thicker than a horse-hair. 



Wherefore, when the nourishment taken in at the mouth, 

 has once reached, and mixed itself with the blood, every 

 part of the body is in the way of being supplied with it. 

 And this introduces another grand topic, namely, the man- 

 ner in which the aliment gets into the blood; which is a 

 subject distinct from the preceding, and brings us to the 

 consideration of another entire system of vessels. 



I. For this necessary part of the animal economy an 

 apparatus is provided, in a great measure, capable of being, 

 what anatomist call, demonstrated, that is, shown in the 

 dead body ; and a line or course of conveyance, which we 

 can pursue by our examinations. 



First, The food descends by wide passages into the in- 

 testines, undergoing two great preparations on its way, 

 one, in the mouth by mastication and moisture, (can it be 

 doubted with what design the teeth were placed in the 

 road to the stomach, or that there was choice in fixing them 

 in this situation?) The other by digestion in the stomach 

 itself. Of this last surprising dissolution I say nothing ; 

 because it is chemistry, and I am endeavouring to display 



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