THE TRACHEAL SYSTEM 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 Tracheae 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 tracheas 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 tracheae, and is, of course, 

 not possible, but in the sense explained above the circulation is 

 essentially peritracheal. 



It is certain that all the tracheae, 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 papillae, 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,' torn, xxviii., 1849. 



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