644 CIRCULATION Ol- Till-: JiLOOD AND DORSAL VESSEL. 



but in the posterior part, and especially in the region of the 

 last chamber, numerous exceedingly fine muscular fibrilhe 

 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. 

 \Veismann, 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 [313] 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 



