IV CIRCULATION 1 35 



its floor or septum less arched, as shown in our diagram (Fig. 73), 

 representing a transverse section through the pericardial chamber, 

 H being the dorsal vessel with m its suspensory muscles, and s its 

 septum, with m the alary muscles. The contraction of these latter 

 would draw the septum into the position of the dotted line, thus 

 increasing the area of the sinus above ; but as this floor or septum 

 is a fenestrated structure, its contraction allows fluid to pass 

 through it to the chamber above ; thus this arrangement may 

 be looked on as a means of keeping up a supply of fluid to the 

 dorsal vessel, the perforated septum, when it contracts, exerting 

 pressure on the tissues below ; these are saturated with fluid, 

 which passes through the apertures to the enlarged pericardial 

 chamber. 



Some misconception has prevailed, too, as to the function of 

 the pericardial chamber. This space frequently contains a large 

 quantity of fat-body pericardial tissue together with tracheae, 

 and this has given rise to the idea that it might be lung-like 

 in function ; but, as Miall and Denny ^ have pointed out, this is 

 erroneous ; the tissues in Insects have their own ample sup- 

 plies of air. It has also been supposed that the alary muscles 

 cause the contraction of the heart, but this is not directly the 

 case, for they are not attached to it, and it pulsates after they 

 have been severed. It has been suggested that the contractions 

 of this vessel are regulated by small ganglia placed on, or in, its 

 substance. However this may be, these contractions vary enor- 

 mously according to the condition of the Insect*, they may be 

 as many, it is said, as 100 or more in a minute, or they may be 

 very slow and feeble, if not altogether absent, without the death 

 of the Insect ensuing. 



The expulsion of the blood from the front of the dorsal 

 vessel seems to be due to the rhythm of the contrac- 

 tion of the vessel as well as to its mechanical structure. 

 Bataillon says,^ confirming an observation of Eeaumur, that at 

 the period when the silkworm is about to change to the chrysalis 

 condition, the circulation undergoes periodical changes, the fluid 

 moving during some intervals of about ten minutes' duration in 

 a reversed direction, while at other times the blood is expelled 

 in front and backwards simultaneously, owing apparently to a 

 rhythmical change in the mode of contraction of the dorsal vessel. 

 ^ Cockroach, p. 140. - Bull. Sci. France Belgique, xxv. 1893, p. 22. 



