VESSELS OF ANIMALS. ]]7 



means or other, to be passed tlirough them. Accordmgly, 

 we find the number of the lacteals exceeding all powers of 

 computation, and their pipes so fine and slender as not to be 

 visible, unless filled, to the naked eye, and their orifices, 

 which open into the intestines, so small as not to be d:scern» 

 ible even by the best microscope. Fourthly, the main pipe, 

 which carries the chyle from the reservoir to the blood, 

 namely, the thoracic duct, being fixed in an almost upright 

 position, and wanting that advantage of 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. The 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 descend. Fifthly, the chyle enters the 

 blood in an odd place, but perhaps the most commodious 

 place possible, namely, at a large vein in the neck, so situ- 

 ated with respect to the circulation 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 distant vein, the fluid composed of the old 

 and the new materials must have performed a considerable 

 part of the circulation 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. V^ ho 

 could have dreamed of a communication between the cavity 

 of the intestines and the left great vein of the 7icck ? Who 

 could have suspected that this communication should be llie 

 medium through which all nourishment is derived to the 

 body, or this the place where, by a side inlet, the important 

 junction is formed between the blood and the material which 

 feeds it ? 



We postponed the consideration oi digestio?!, lest it should 

 interrupt us in tracing the course of the food to the blood ; 

 but in treating of the alimentary system, so principal a part 

 of the process cannot be omitted. 



