644 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
but in the posterior part, and especially in the region of the 
last chamber, numerous exceedingly fine muscular fibrillze 
extend from the dorsal vessel to the lateral and dorsal walls of 
the cavity, whilst the ventral wall of the dorsal vessel and the 
pericardial septum are intimately united. 
Histology of the Dorsal Vessel.—The older writers on insect 
anatomy all held that the dorsal vessel is a compound organ, 
consisting of muscle fibres, cells and connective tissue. This 
opinion is supported by Leydig, Graber, and many others. 
Weismann, however, regarded it as a single hollow muscle 
fibre, consisting of a sheath, contractile elements, and nuclei. 
Although this view has received comparatively little support, 
I am inclined to regard it as correct. 
Graber describes three coats in the dorsal vessel, and com- 
pares it with an artery; Villanes, as has been already remarked, 
compared it with a capillary bloodvessel. Graber described 
an external tunica adventitia, an intermediate tunica musculosa, 
and an internal tunica intima, or endocardium. 
The Tunica Adventitia.—The existence of a true adventitia 
was denied by Weismann [2], although Graber [818] says it is 
easily demonstrated in the majority of the insects which he 
investigated, and especially in such large insects as Locusta 
viridissima, and Carabus cancellatus. The figures given by 
him apparently settle the question, and indicate that his 
adventitia is merely a portion of the pericardial septum. 
Owing to the great elasticity of this structure, it easily wraps 
itself round the dorsal vessel when the latter is removed, and 
may readily be mistaken for a true adventitia; and it is 
extremely difficult to remove the vessel without a portion, at 
least, of the pericardial septum. Sections show that there is 
no connective tissue in immediate relation with the external 
structureless muscle sheath, although it is probable that the 
whole pericardial sinus is lined with an endothelial layer, 
which is reflected over the dorsal vessel itself. Branches from 
these endothelial cells form the reticulum, uniting the dorsal 
vessel with the pericardial septum. 
The Tunica Media, or Muscular Layer.—This is described by 
