132 ME. J. M. BROWN ON THE ANATOMY OF 



above. They appear as pocket- or pouch-like expansions of the wall of the heart, a 

 median almost vertical slit (o.) occurring in the centre, of the pouch. During pulsations 

 these pockets are alternately turned inwards (i. e. projecting into the lumen of the heart) 

 when the valves are open, and outwards when the valves close. At the free edges of 

 the slit the outer (cuticle) layer is thickened, hut otherwise they possess no special 

 structure. The margins of the pouches are, however, supported by strands from the 

 diaphragm. No special opening or closing muscles could be detected, and their action 

 also appears to be automatic, depending on the movements of the heart-wall and on the 

 external aud internal blood-pressure. 



The pericardial diaphragm is in structure much like the wall of the heart. 



It consists of a base of striated muscle in the form of fan-shaped bands, converging 

 to the point of attachment to the lateral body-wall (alary muscles). In the middle line 

 they spread out and serve as supports to the heart, with which they are in places in 

 connection. They further give support to the transparent fibrous membrane, which is 

 overlaid by the loose tissue formed by the "pericardial cells.'" These are of peculiar 

 form. Their shape is variable, but generally irregular, with outstretching processes by 

 which one cell becomes associated with its neighbours. Many of these processes become 

 long and thread-like. Fine branches of the tracheal system occur amongst the cells in 

 large numbers. Each cell has generally two large and prominent nuclei. Often the 

 cells present the appearance of vacuolization. 



The pericardial cells occur (1) on the surfaces of the diaphragm, (2) on the surfaces 

 of the heart. 



The function of the pericardial cells, which differ in structure absolutely from the 

 fat-body, seems to be, largely at least, excretory. 



Animals injected with carmine powder, and left for a period and then fixed, showed 

 the pericardial cells laden with fine granules of carmine. This was seen both in the 

 pericardial cells on the wall of the heart, and in those on the pericardium itself. The 

 presence of numerous tracheal branches also suggests an excretory function *. 



The posterior end of heart. — Inmost insects the dorsal vessel ends blindly, but in this 

 case it is not so. In the hindmost body-segment the heart becomes deeper from the 

 dorsal to the ventral side, and so forms a much enlarged posterior chamber. The hind 

 face of this opens backwards by means of a vertical slit, the edges of the slit folding 

 inwards and acting as a valve. During pulsation an exceptionally strong flow of fluid 

 enters through this opening and plays an important part in "sucking" the blood 

 through the posterior respiratory region of the body. 



Pulsations of the Heart. 



During pulsation a gradual wave of contraction passes along the dorsal vessel from 

 behind forwards. 



* Kowalewsky states tliat the carmine is absorbed by tlie leucocytes and passed on by these to the pericardial 

 cells after solution, the pericardial cells themselves being unable to ingest. My preparations did not, however, 

 show the leucocytes containing carmine, but the pericardial cells were everywhere crowded with minute particles, 

 as were the cells of the Malpighiau tubes. 



