106 ^'ATURAL THEOLOar. 



vevy middle of the substance of the bone. This takes place 

 in the lower jaw, and is found where there would other- 

 wise be danger of compression by sudden curvature. All 

 this care is wonderful, yet not more than what the impor- 

 tance of the case required. To those who venture their 

 lives in a ship, it has been often said, that thers is only an 

 inch-board between them and death ; but in the body itself, 

 especially in the arterial system, there is, in many parts, only 

 a membrane, a skin, a thread. For which reason, this sys 

 tern lies deep under the integuments ; whereas the veins, in 

 which the mischief that ensues from injuring the coats is 

 much less, he in general above the arteries, come nearer to 

 the surface, and are more exposed. 



It may be further observed concerning the two systems 

 taken together, that though the arterial, with its trunk and 

 branches and small twigs, may be imagined to issue or pro- 

 ceed, in other words, to gwiv from the heart, like a plant 

 from its root, or the fibres of a leaf from its footstalk — 

 which, however, were it so, would be only to resolve one 

 mechanism into another — yet the venal, the returning system, 

 can never be formed in this manner. The arteries might go 

 on shooting out from their extremities, that is, lengthening 

 and subdividing indefinitely ; but an inverted system, con- 

 tinually uniting its streams instead of dividing, and thus 

 carrying back what the other system carried out, could not 

 be referred to the same process. 



II. The next thing to be considered is the engine which 

 works this machinery, namely, the heart. For our purpose 

 it is unnecessary to ascertain the principle upon which the 

 heart acts. Whether it be irritation excited by the contact 

 of the blood, by the influx of the nervous fluid, or whatever 

 else be the cause of its motion, it is something which is capa- 

 ble of producing, in a living muscular fibre, reciprocal con- 

 traction and relaxation. This is the power we have to Avork 

 with ; and the inquiry is, how this power is jipplied in the 

 instance before us. There is provided, in the central part o^ 



