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POPULAR SCIENCE REVIEW. 
towards the head of the larva its nature becomes more apparent. More 
than a century ago Lyonnet showed that the dorsal vessel of the goat-moth- 
caterpillar has only a single outlet for the passage of the fluids, namely, 
where it terminates with a flaring opening at the extreme front of the head, 
just above the base of the oesophagus, into a cavity closed by membrane. 
The direct passage of the fluids thence has never been observed, but they 
must in some way at once enter the passages surrounding the tracheae, since 
they are next found circulating in these all over the body. Other writers 
have described the anterior extremity as branching, but in like manner have 
not traced the passage of the fluids beyond the main trunk. 
Mr. Scudder remarks that the only large tracheal vessel connecting the 
two sides of the body lies in the first segment behind the head ; and that 
according to the little-known investigations of Barthelemy, * this trans- 
verse tracheal conduit appears at the earliest formation of the system in the 
embryo, before any regular distribution of fluids over the body can be 
observed ; from the middle line of this transverse trachea half a dozen or 
more prominent branches pass directly forward, and some, at least, of their 
branches penetrate the cavity into which the dorsal vessel empties its con- 
tents. It is, therefore, highly probable that through them the fluids enter 
the peri-tracheal system, for by this channel the fluids would pass in the 
easiest possible manner to every part of the body from the moment that the 
pulsations of the dorsal vessel commence in the embryo. This, therefore, is 
the principal point towards which it is desirable that future investigations 
should be directed ; and especially the structure of the tracheal threads 
which enter this cavity should be minutely studied. 
The tracheae, as shown by several anatomists, consist of two entirely dis- 
tinct parts, and although authors do not wholly agree in their explanation 
of the structure of the tracheae, there appears to be little doubt that it is 
substantially as follows : — First, a main stem and its branches, in which, 
between investing tissues, lies a closely-coiled spiral thread ; and outside of 
and enveloping which is the peri-tracheal vessel, whose outer walls are not 
limited by the extent of the tracheae proper, but extend beyond the spiral 
coil to form the second and distinct part of the system, namely, capillary 
tubes, penetrating every portion and organ of the body, and terminating in 
a meshwork of interlacing branches. In other words, this portion of the 
circulatory system consists of branching tubes, which enclose within all but 
their ultimate ramifications the similarly branching tubes of the respiratory 
system. The fluids, therefore, forced by the dorsal vessel into the peri- 
tracheal cavities, become thoroughly aerated before passing into the tissues 
of the body to perform their functions ; when they have done their work, 
they empty themselves into the general cavity of the body, and mingling 
with the fluids newly expressed from the alimentary canal, join the general 
currents, which, as first shown by Oarus, appear to set in regular channels 
at the sides and floor of the body, often, however, with no vascular boun- 
daries, towards the hinder extremity of the body, or towards the sides of 
the dorsal vessel, to enter again the initial point of the circulation. 
* 11 Recherches d’Anatomie et de Physiologie Generates sur la classe des 
Lepidopteres.” Toulouse, 1864. 
