650 CIRCULATION OF THE BLOOD AND DORSAL VESSEL. 
or bipolar; each has a large vesicular nucleus and a distinct 
nucleolus ; they are loaded with brown granular pigment, and 
exhibit distinct capsules surrounding the cell protoplasm. I 
am inclined to regard some of Graber’s figures of pericardial 
cells [318, Figs. 13 and 23] as representing ganglion cells. He, 
however, figures the branches of the cells in Fig. 13 as trans- 
versely striated, and the cells in Fig. 23 as binucleate ; other- 
wise I should have said that they are typical ganglion cells. 
In the Blow-fly the cells of the pericardial plexus average 30 » 
in their long diameter, and are exactly like the nerve-cells of the 
sympathetic ganglia of fishes. 
The Trachew of the pericardium and dorsal vessel are very 
numerous. The principal vessels form fan-shaped groups 
(Pl. XLV.), which diverge from a single large trachea, and 
resemble in their general arrangement the alar muscles of 
those insects in which fan-shaped alar muscles occur. When 
seen with the naked eye or a pocket lens of insufficient power, 
it would be easy to mistake the tracheal vessels for lateral 
muscular alee. 
3. PHYSIOLOGY OF THE DORSAL VESSEL. 
Movements of the Dorsal Vessel.—Pulsations of the dorsal 
vessel, or rather, of the pericardial sinus which surrounds it, 
were first observed by Malpighi in the larva of the silk-moth, 
Bombyx Mori [148]. These movements, which are visible 
through the transparent skin in many larve, have been 
frequently seen and described since. The general view of the 
manner in which the dorsal vessel acts was for a long time 
derived from the writings of Straus Durckheim [40], who, after 
describing the dorsal vessel of the Cockchafer, says : 
‘When the posterior chamber of the dorsal vessel dilates, 
the blood in the abdominal cavity rushes into it through 
two auriculo-ventricular openings. When the posterior 
chamber contracts, the semilunar valves of these openings 
close and prevent the exit of the blood, which by its pres- 
sure opens the inter-ventricular valves between the posterior 
v's 
