THE TRACHEAL SVSTEM OF THE BLOW-FLY. 367 
become obvious, during the dissection of these vessels, as the 
air can be driven by pressure in one direction only, towards the 
smaller. branches, but never backwards from the smaller to 
the larger trunks. 
Relations of the Trachee to the Blood-Sinuses,—All the large 
tracheal sacs lie in blood-sinuses, which can be injected by 
inserting the needle of a hypodermic syringe into the dorsal 
thoracic sinus ; the legs should be cut off to allow the blood to 
be replaced by the injection. The air-sacs may be described, 
therefore, as being surrounded by narrow blood-sinuses. In 
this sense there is a true peritracheal circulation, and the air is 
only separated from the blood by a thin cuticular plicated 
membrane. The larger cylindrical trunks also lie in the walls 
of blood-sinuses. 
Peritracheal Circulation.—The manner in which the blood- 
currents follow the course of the tracheal vessels has been 
repeatedly observed, and Blanchard* described a blood-passage 
between the membranes of the trachee and the spiral thread, 
which he believed to be a Separate structure. Blanchard’s 
theory of an intra-tracheal circulation was the result of a mis- 
conception as to the structure of the trachez, and is, of course, 
not possible, but in the sense explained above the circulation is 
essentially peritracheal. 
It is certain that all the trachez, even the finest capillaries, 
are bathed by the blood, and it is probably to the blood only 
that they give their oxygen, from which it is absorbed in 
turn by the tissue elements, and stored as intra-molecular 
oxygen. 
The Arrangement of the Capillary Network indicates that there 
is no continuous air circulation. This is especially seen in the 
rectal papilla, in which the capillaries form a tuft separated 
from those of other parts by a chitinous capsule. These 
organs have only a single tracheal trunk, and not an afferent 
and an efferent vessel. 
The capillaries all ramify in the reticular connective tissue of 
the ccelom, and are therefore surrounded by the nutrient fluid; 
* Comptes rendus,’ tom. xxviii, 1840. 
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