150 ANATOMY. 



layer of muscle, which is still thicker and stronger in the middle of 

 each chamber. Perhaps both membranes are but the different layers 

 of one muscular membrane, and then we might, by the analogy of all 

 blood-vessels, entertain the idea of the presence of an innermost struc- 

 tureless mucous membrane, which escapes observation by its delicacy. 

 It is from the presence of these muscular layers that it is possible for 

 the heart to contract and dilate. By both membranes simultaneously 

 contracting the heart becomes straitened, and this distends again as 

 soon as the membranes become flaccid after the contraction,, when the 

 muscles of the lobes contract themselves. 



119. 



To the posterior portion of the dorsal vessel which we find provided 

 with apertures and valves, and which we must consider as the true 

 heart, several triangular, flat, membranous muscles are affixed, the 

 points of which pass on to a dorsal plate of the abdomen, and there 

 attach themselves (PI. XXII. f. 9). If these wings (fliigel) of the 

 heart, as they are called, are short, or consequently of the shape of an 

 equilateral triangle, other muscles of the form of a band originate at 

 the apex of this triangle, and pass in a diverging direction from each 

 other, and insert themselves upon the abdominal plate, where this 

 becomes membranous (Lamia (Ecl'dis}. Generally, however, the wings 

 are so long as not to require the muscles of attachment (Melolontha, 

 &c.), and they then take the shape of a very acute triangle. The 

 conjunction of these muscular wings with the heart, which they merely 

 retain in its place, is very intimate, without its being possible to say 

 where ; whether it be by fibres passing from these wings into those of 

 the heart, or whether the membrane of the heart sends forth lateral 

 folds it is impossible to say. They lie in a row upon the two opposite 

 sides of the heart, precisely where the anterior aperture of each cham- 

 ber is found. They pass over these apertures, the fibres attaching 

 themselves to a small membranous arch which crosses these orifices 

 transversely ; consequently, in front of each orifice, there is a small 

 semicircular hole in these wings, which are thus prevented from inter- 

 rupting the flow of blood. 



These wings are wanting to the dorsal vessel of the Libellula, and 

 Phasma has but one pairin the sixth abdominal segment. Besides 

 this we find a pair of muscles passing from the posterior margin of the 



