103 OF THE VESSELS 



propulsion which the arteries possess, is furnished with a 

 succession of valves to check the ascending fluid, when 

 once it has passed them, from falling back. These valves 

 look upwards, so as to leave the ascent free, but to prevent 

 the return of the chyle, if, for want of sufficient force to 

 push it on, its weight should at any time cause it to de- 

 scend. Fifthly, The chyle enters the blood in an odd place, 

 but perhaps the most commodious place possible, viz. at a 

 large vein in the neck, so situated with respect to the cir- 

 culation, as speedily to bring the mixture to the heart. 

 And this seems to be a circumstance of great moment; for 

 had the chyle entered the blood at an artery, or at a dis- 

 tant vein, the fluid, composed of the old and the new ma- 

 terials, must have performed a considerable part of the cir- 

 culation, before it received that churning in the lungs, which 

 is, probably, necessary for the intimate and perfect union of 

 the old blood with the recent chyle. Who could have 

 dreamt of a communication between the cavity of the in- 

 testines and the left great vein of the neck 1 Who could 

 have suspected that this communication should be the me- 

 dium through which all nourishment is derived to the body ; 

 or this the place, where, by a side inlet, the important junc- 

 tion is formed between the blood and the material which 

 feeds it? 



II. We postponed the consideration o^ digestion, lest it 

 should interrupt us in tracing the course of the food to the 

 blood ; but, in treating of the alimentary system, so prin- 

 cipal a part of the process cannot be omitted. 



Of the gastric juice, the immediate agent by which that 

 change which food undergoes in our stomachs is effected, 

 we shall take our account, from the numerous, careful, and 

 varied experiments of the Abbe Spallanzani. 



1. It is not a simple diluent, but a real solvent. A 

 quarter of an ounce of beef had scarcely touched the sto- 

 mach of a crow, when the solution began. 



2. It has not the nature of saliva ; it has not the nature 

 of bile ; but is distinct from both. By experiments out of 

 the body it appears, that neither of these secretions act up- 

 on alimentary substances, in the same manner as the gas- 

 tric juice acts. 



3. Digestion is not putrefaction ; for the digesting fluid 

 resists putrefaction most pertinaciously ; nay, not only 

 checks its further progress, but restores putrid substances. 



4. It is not a fermentative process : for the solution 

 begins at the surface, and proceeds towards the centre, 



