Page:Popular Science Monthly Volume 9.djvu/599

Rh without a flaw anywhere. The tubes are composed of an outer and an inner coat, containing between them the spiral coil, to which they are closely attached; a delicate membrane also connects the turns of the spiral with each other. It is interesting to compare these animal breathing tubes with their analogues the spiral vessels of the vegetable kingdom; the latter are easily extracted from the young shoots of asparagus, or from the leaves of the hyacinth. The spring-like coil insures a free open passage for the air which rushes in by the spiracular orifices, expiration being effected by the contraction of these elastic channels, by which the effete air is forcibly expelled through the openings by which it originally entered.

The main tracheæ pass down the axes of the blood-channels, floating in the vital fluid, which they revivify with the oxygen which they thus carry to and through the life-stream. We are told that the air-pipe does not terminate where the wiry-looking spiral comes to an end; the latter dwindles away imperceptibly to nothing, but the trachea thence becomes membranous, and, dividing into innumerable branches, which bear to the main trunks the same relations that the capillaries bear to the arteries, penetrates the substance of the muscles, inconceivably fine branches having been traced accompanying the nerves, while the ultimate plexiform extremes of the system aërate immediately the solids. "In all the transparent structures of insects every observer may prove for himself that the blood-currents travel in the same passages as the tracheæ, but this is only the case with the primary and secondary branches, never in the capillary tracheae; the blood-corpuscles of the Myriapod exceed by several times in diameter that of the extreme capillary membranous tracheæ; it is perfectly marvelous to what inconceivable minuteness the air-current is reduced in traveling along these tubes." What a simple and efficient plan, what an economy of space is this arrangement of tube within tube, for aërating the blood in a class