TEEMINATIONS OF THE TEACHEAL TUBES. 323 



interfering with its flexibility, or obstructing its movements ; and this 

 fibre, delicate as it is, may be traced, with the microscope, even through 

 the utmost ramifications of the tracheae, a character whereby these 

 tubes may be readily distinguished. 



(845.) There is a limit, observes Dr. Williams*, different in different 

 structures, at which the spiral thread ceases ; and at this point the mem- 

 branous trachea begins. It is not the external covering which ceases, 

 but the spiral which, growing less and less visible, graduates insensibly 

 into a continuous tube. The diameter of the " spiral" trachea constantly 

 decreases as it divides ; that of the membranous observes, throughout 

 its entire course, whether it multiply into a network, or wavy brushes, 

 or into the muriform plexus which exists in the substance of muscles, 

 a uniformity which can compare only with that of the true blood- 

 capillaries of the vertebrate animal. The tracheae terminate differently, 

 and form different plexuses, in different organs, according to the varying 

 mechanical arrangements of the ultimate parts of the latter. They are 

 evidently air-tubes throughout, even to their final extremes. 



(846.) The primary, secondary, and tertiary air-tubes divide and sub- 

 divide arborescently, the branches never uniting, but the ultimate rami- 

 fications dividing and subdividing in the same profuse retiform manner as 

 the blood- capillaries of the vertebrate animal, supplying the muscles, 

 the glands, the mucous membranes, the brain, and every other viscus. 

 The large air-tubes which travel along the axes of the spacious blood- 

 channels detach from their sides here and there minute wavy branches 

 which float in the fluid, and appear to be expressly intended to aerate 

 the fluids. 



(847.) In all the transparent structures of insects, such as the wings, 

 antennae, branchiae, &c., the blood- currents travel in the same passages 

 as the tracheae. On closer scrutiny it will be seen that a channel, such 

 as that of the nervure of the wings, bearing in its centre a large tracheal 

 tube, exhibits on one side a current going in one direction; on the other, 

 another bearing in an opposite course. These are afferent and effe- 

 rent, arterial and venous blood-streams. They are bounded by separate 

 walls. The afferent current is circumscribed by its own proper coats, 

 the efferent by its own ; and the trachea is placed intermediately, having 

 parietes quite distinct from, although contiguous with, those of the 

 blood-channels. This coincidence between the tracheae and the blood- 

 currents can be traced in the wings nowhere beyond the limits of the 

 nervures into the scaly spaces that they circumscribe. The returning 

 of the corpuscles at a certain point renders this fact quite unquestion- 

 able. Beyond this limit, only the fluid elements, not the corpuscles of 

 the blood, penetrate. In this extra- vascular region it is cyclosis, not 

 circulation, which governs the movements of the nutritive fluid. If, 

 says Dr. Williams, everywhere the blood and the air travelled toge- 



* Loc. cit. 



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