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TEXT-BOOK OF ENTOMOLOGY 



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section through the entire body (Fig. 374) : // is the true dorsal vessel ; 8, the 

 diaphragm. A surface view is seen at 373, C, b, where it appears as a plate with 

 the edge regularly curved outwards on each side. Its precise mode of working is 

 thus : from each dorsal band of the sides of the abdomen arises a pair of muscles 

 spreading out fan-like, and extending to the heart, so that the fibres of one side 

 pass directly over to those of the other, often splitting apart, or, between the 

 two, extends outwards a perforated, thin web, like an elastic, fibrous sheet 

 (Fig. 373, A, a), with numerous perforations, forming a diaphragm. 



Graber has thus explained the action of the pericardial diaphragm and cham- 

 ber, as freely translated by Miall and Denny: " When the alary muscles con- 

 tract, they depress the diaphragm, which is arched upwards when at rest. A 

 rush of blood towards the heart is thereby set up, and the blood streams through 

 the perforated diaphragm into the pericardial chamber. Here it bathes a 

 spongy or cavernous tissue (the fat-cells), which is largely supplied with air- 

 tubes, and having been thus aerated, passes immediately forwards to the heart, 



entering it at the moment of diastole, 

 which is simultaneous with the sinking 

 of the diaphragm." 



In the cockroach, however, Miall 

 and Denny think that the facts of 

 structure do not altogether justify this 

 explanation: "The fenestne of the 

 diaphragm are mere openings without - 

 valves. The descent of a perforated 

 non-valvular plate can bring no pres- 

 sure to bear upon the blood, for it is 

 not contended that the alary muscles 

 are powerful enough to change the 

 figure of the abdominal rings. . . . 

 The diaphragm appears to give me- 

 chanical support to the heart, resisting 

 pressure from a distended alimentary canal, while the sheets of fat-cells, in 

 addition to their proper physiological office, may equalize small local pressures, 

 and prevent displacement. The movement of the blood towards the heart must 

 (we think) depend, not upon the alary muscles, but upon the far more powerful 

 muscles of the abdominal wall, and upon the pumping action of the heart itself." 

 "The peculiar office," says Graber, "performed by the heart has already 

 been stated. It is nothing more than a regulator ; than an organ for directing 

 the blood in a determinate course in order that this may not wholly stagnate, or 

 only be the plaything of a force acting in another way, as, for example, through 

 that afforded by the body-cavity and the inner digestive canal. At regular 

 intervals a portion of the blood is sucked through the same, and then by means 

 of the anterior supply tube it is pushed onward into the head, whence it passes 

 into the cavities of the tissues. The different conditions of tension under which 

 the mass of blood stands in the different regions of the body then causes a 

 farther circulation. Besides this, the blood passes through separate smaller 

 pumping apparatuses, and through vessel-like modifications of cavities, also 

 through hollow spaces between the muscles, as, for example, in the appendages 

 win -re a regular backward and forward flow of the blood, especially in the limbs, 

 wings, antcnnie, and certain abdominal appendages takes place. Here and there 

 ma.y occasionally occur a narrow place where the flow of blood is obstructed by 

 the accumulation of the blood corpuscles, causing a considerable stagnation." 

 (Graber.) 



FK;. 374. Diagram of transverse section of 

 perirarilial sinus of ;Eil ijioila coe-rulexcena : H, 

 heart ; s, septum ; ///, muscles, the upper sus- 

 ]>ens iry, tin 1 lower alary. Alter Graber, from 

 Sharp.' (See also Fig. 377.) 



