TRACHEiE 
27 
from any other source ; and it seems evident therefore 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 tracheae may end in loops ; but certainly this does not seem to me to 
be universally or even generally the case. Referring again to the tracheae belonging to the 
ovary of Aphrophora, we see that each branch tapers from soW n down to Toioo tn °f au 
inch without giving off a single branchlet. Does, then, the fine 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 little importance. 
A glance at the tracliMB of the Malpighian vessels Y&JEschna seems to me to be almost 
equally conclusive, as the tracheae 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 
tubes seem at first 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 trachea? 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 subject, that the removal of the carbonic acid 
from the trachea?, 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 * Philosophical 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 respiration 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 300 cubic inches. In a natural expiration, 
about 20 cubic inches or -j^th of the contents are thrown out, from the application of a 
o-eneral 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 will 
fill the larger tubes with fresh air, which identical quantity will again be expelled in the 
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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