DORSAL VESSEL OF THE IMAGO OF THE BLOW-FLY. 645 



Graber in various Coleoptera, Orthoptera etc., as consisting 

 of circular fibres about 8 /A to 10 ^ in diameter, distinctly 

 striated transversely, and easily separated from each other by 

 the action of dilute acids ; and he states that the dorsal vessel 

 of Musca has similar characters. Longitudinal sections of the 

 dorsal vessel do not, however, support the view that it consists 

 o r circular muscle fibres. The whole section exhibits no trace 

 of such fibres, but is uniformly covered by areas of Cohnheim. 

 The regular position of the nuclei, and the division of the entire 

 tube into segments is adverse to the muscle fibre theory of its 

 structure ; but is quite consistent with Weismann's view. 



Longitudinal muscle fibres have been described forming an 

 external layer. Graber says : ' Longitudinal muscle fibres can 

 seldom be demonstrated in the middle or muscular coat. I 

 have only been able to make out such with certainty in Musca, 

 and the longitudinal fibres in the hearts of Gryllotalpa, Calop- 

 tenus and other Orthoptera belong to the connective tissue 

 adventitia.' 



In Musca I have certainly convinced myself that the longi- 

 tudinal layer of muscles is entirely pericardial. I have re- 

 peatedly seen it as it is represented by Graber in his figures, 

 but on examining preparations of the entire vessel with a 

 binocular, it is easy to see that these fibres all lie on the ventral 

 surface of the vessel, and there is no trace of any longitudinal 

 fibres in sections, with the exception of those which lie in the 

 pericardial septum. 



The Tunica Intima. Graber has described a fine membranous 

 intima, and I have seen indications of an extremely fine cuti- 

 cular layer on the inner surface of the muscle layer. Graber 

 is doubtful whether it is a mere cuticular layer or a connective 

 tissue, as Leydig holds it to be. He admits that it is 

 extremely difficult to observe any trace of structure. The 

 complete absence of connective elements in the muscular coat 

 is quite destructive of Leydig's view. Graber has failed to 

 demonstrate an endothelial lining by the application of silver 

 nitrate, and my own researches in this direction have only 

 given negative results. 



