234 
Dr. Kidd on the 
in the vitreous humor of the eye becoming expanded, pre- 
serves the spherical form of the organ until the whole of the 
moisture has been evaporated ; and it is then sufficiently firm 
to support itself. I have traced most of the tracheae to the 
parts on which they are respectively distributed ; but as no 
adequate object, nor indeed any object of importance, would 
be gained by the description of a distribution which is not 
marked by any physiological peculiarity, I shall only insist 
on such 'points as appear to me to be either new, or hitherto 
not sufficiently elucidated. 
The tracheae of insects are generally described as tubes 
constructed of a spiral thread, the successive coils of which 
are closely in opposition with each other ; such a structure 
is represented in Swammerdam's plates, and I have no doubt 
from his acknowledged accuracy, that he represents what he 
observed. It has not however happened to me, with the 
exception of one equivocal instance, to perceive such a struc- 
ture in the mole-cricket, the character of the tracheae of which 
varies in different parts of the insect ; for sometimes they re- 
semble the pulmonary tracheae of the higher classes of ani- 
mals, in having an annulated structure ; and sometimes they 
appear as tubes of a perfectly uniform substance like cuticle, 
or some very thin and unorganized membrane. It is gene- 
rally understood, that the tracheae of insects penetrate each 
organ and every part of the body : and certainly the case 
is such in the instance before us. Thus, in that brush of 
capillary yellow tubes supposed to constitute the hepatic 
system, the total number of which amounts to 150 or 200, 
there is reason to believe that each tube is accompanied by 
a distinct trachea coiled round it in a long spiral. Again ; the 
