DORSAL VESSEL OF. THE IMAGO OF THE BLOW-FLY. 645 
Graber in various Coleoptera, Orthoptera etc., as consisting 
of circular fibres about 8 mw to Io mw 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 
of 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. 
