6 5 o ciRCCi..-irio.\ OF mi-: ni.ooD A.\D 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 [313, 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 Tracheae of the pericardium and dorsal vessel are very 

 numerous. The principal vessels form fan-shaped groups 

 (PI. 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 alae. 



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 arc visible 

 through the transparent skin in many larvae, 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 

 cle>r.rihing 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 

 i h.imbcr 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 



