OF THE TRACHEA IN INSECTS. ' 27 



from any other source; and it seems evident tlierefore that in these cases the air must 

 pass to and fro in the same channel. 



Dr. Williams's fourth assertion seems to me as little tenable as any of the three pre- 

 ceding. I do not, of course, deny that in some organs the tubules may inosculate, or, in 

 other words, that the trachese may end in loops ; but certainly this does not seem to me to 

 be universally or even generally the case. E-eferring again to the trachete belonging to the 

 ovary of Aphrophora, we see that each branch tapers from -g-^^oth do-\vn to 2000 o ^ of ^'^ 

 inch without giving off a single branchlet. Does, then, the tine end run into the similar 

 termination of some similar branch, and thus form an elongated ellipse ? if so, we should 

 have expected to find the calibre nearly equal throughout; but the ends were so ex- 

 cessively delicate that any such inosculation could be of Httle importance. 



A glance at the trachese of the Malpighian vessels in JEschna seems to me to be almost 

 equally conclusive, as the tracheoe there run nearly half an inch, becoming very fine, and 

 yet sometimes only give out a single branchlet. I readily admit that these long blind 

 tixbes seem at iirst sight but little fitted to ensure a constant supply of fresh air to the 

 organs along their whole course : on the other hand, it must be remembered that the 

 interchange of gases is very rapid ; and as we do not find among the larger trachese any 

 separate afferent and efferent branches, we ought not hastily to conclude that they must 

 certainly exist among the branchlets. 



It occurred to me, in reflecting upon this siibject, that the removal of the carbonic acid 

 from the trachese, and the continual supply of oxygen, were probably effected in con- 

 sequence of the diffusion of gases. Prof. Huxley also suggested to me that in fact the 

 same is the case in man and the higher animals generally. Upon mentioning this to 

 Prof. Graham, he referred me to his paper in the ' Philosoplrical Magazine ' for 1833, 

 where he has expressed the very same opinion. 



As his suggestions appear to have been almost, if not altogether, overlooked by physio- 

 logists, I may perhaps be permitted to quote a few of his remarks. He says, " I may be 

 allowed to mention an application of the law of diffusion in explanation of the mechanism 

 of respiration. The cavity into which air enters during respii'ation consists, first, of a 

 large tube, the windpipe ; secondly, of smaller tubes, into which the windpipe diverges ; 

 and thirdly, of a series of still smaller tubes, diverging from the last, themselves rami- 

 fying to an indeterminate extent, till at last the tubes cease to be of sensible magnitude, 

 but are believed to terminate in shut sacs. The capacity of the whole cavity cannot easily 

 be determined ; but we may estimate it at 800 cubic inches. In a natural expiration, 

 about 20 cubic inches or ^jth of the contents are thrown out, from the application of a 

 general pressure to the whole ; but it is evident that these 20 cubic inches will be the 20 

 cubic inches nearest the outlet, or the contents of the larger tubes. The contents of the 

 second-sized tubes will advance at the same time into the largest tubes, but no further, 

 and will recede again into their original depositories on the next inspiration, which Avill 

 fill the larger tubes with fresh air, wliich identical quantity will again be expelled in tlie 

 next expiration. 



" This illustration is perhaps too strongly stated ; but it is evident, that, in ordinary 

 respiration, the slight mechanical compression will have little or no effect in emptying 



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