198 Dr. T. Williams on the Mechanism of Aquatic 



governs the movements of the nutritive fluid. If the same pas- 

 sages served everywhere for the blood and for the trachece, and 

 if their parallelism was unexceptional and universal^ wherever 

 the trachese could be seen, there also should be observed the cor- 

 puscles of the blood. This is the case only in the primary and 

 secondary, never in the capillary trachese (fig. 11, 6). The blood- 

 corpuscles (fig. 10,^) in the Myriapod, Insect, and Arachnid 

 exceed by several times in diameter that of the extreme capillary 

 membranous trachese. It is perfectly marvellous to what incon- 

 ceivable minuteness the air-current is reduced in travelling along 

 these tubes. It affords a captivating example of the illimitable 

 divisibleness of matter. 



If everywhere the blood and the air travelled together, branched 

 together, capillated in concert, — if everywhere a double blood-cur- 

 rent to one air-tube could meet the eye, the inference could not 

 be resisted, that the sole, entire and exclusive design of the 

 tracheal apparatus of the Insect consisted in aerating the fluids. 



Since, however, the blood returns far before the trachese reach 

 their remote penetralia ; — since the comitance between the blood 

 and the air is broken abruptly at a limit proximal to the ex- 

 tremes of the organism, it is cei^tain that the tracheal system in 

 the Insect fulfils some other function— answers some other end — 

 than that merely of aerating the fluids. What can be the mean- 

 ing of those incomparable pneumatic plexuses — veritable retia mi- 

 rabilia — which embrace immediately the very ultimate elements of 

 the solid organs of the body ; — those microscopic air-tubes, which 

 carry oxygen in its gaseous form, unfluidified by any intervening 

 liquid, to the very seats of the fixed solids which constitute the 

 fabric of the organism ? There is an immeasurable difiference be- 

 tween oxygen dissolved and oxygen free. In the former case, all 

 the forces liberated during the moment of condensation from the 

 elastic to the fluid form are expended upon the blood, and that, 

 too^ remote from the scene at which that blood is to be utilized ; 

 in the latter case, free, gaseous and uncombined, it is delivered 

 immediately at the spot where the oxygen is to be employed; 

 it electrizes by direct combination the last sedentary elements 

 of the organism; by such an arrangement those forces at- 

 tendant on chemical action vivify undissipated the very ulti- 

 mate components of the body at the very moment of their 

 disengagement. This then is the real difference between an 

 insect and every other living animal. This is the unequalled 

 mechanism which renders the insect a multum inparvo, the un- 

 solved riddle of creation. In all other animals the quickening 

 action of oxygen is first exclusively exhausted upon the fluids ; in 

 the insect, the fluids are only partially influenced as the vitalizing 



