128 MEDICAL ENTOMOLOGY 



THE HEART, VESSELS, AND PERICARDIAL CELLS 



The circulation of the blood of insects plays apparently only a 

 secondary part in their physiology. The heart is a longitudinal dorsal 

 tube, situated in the middle line and closely pressed against the tergal 

 plates ; it extends from the posterior part of the abdomen to the thorax, 

 where it is replaced by a thinner vessel, the aorta, which passes through 

 the thorax and neck, and divides into two lateral branches about the 

 occipital foramen. The aorta and the head vessels are very hard to 

 trace, and can as a rule only be found in the larger flies. The heart 

 itself is best seen by dissecting off the whole of the dorsal wall of the 

 abdomen in one piece, and then drawing two tergites apart in order to 

 rupture the vessel. One end of it usually remains projecting from 

 one of the fragments, and can be recognized under a low power by its 

 pulsation. 



The heart is not a simple tube, but is divided into a number of small 

 chambers, which correspond more or less with the abdominal segments. 

 Between each two segments of the heart there is a 

 simple valve, which prevents the blood from flowing 

 backwards when the walls of the chamber contract, and on each side of each 

 segment there is an opening through which the heart is put in com- 

 munication with the body cavity. There are no veins, and no arteries 

 other than the forward prolongation of the heart referred to as the 

 aorta. 



The wall of the heart consists of a fine elastic intima, an intermediate 

 layer of loosely arranged cells, and a muscular layer, the fibres of 

 which are arranged as a rule in an obliquely circular direction and inter- 

 lace with one another ; outside these there is said to be a thin layer of 

 fibrous tissue. The heart is separated from the rest of the body con- 

 tents by a layer of muscle and membrane which constitutes a sort 

 of diaphragm. (Plate XXVII, fig. 11.) On each side of the middle 

 line there is a set of fine fan-shaped muscles, one pair to each tergite, 

 which arise near the lateral borders and expand as they pass inwards ; 

 they unite with one another across the middle line as a fine sheet of 

 membrane, which lies immediately below the heart. The space between 

 the membrane and the insect's heart is filled in with large cells, termed 

 the pericardial cells, and there are apertures in the membrane below 

 these, through which the pericardial chamber communicates with the 

 rest of the body cavity. The heart and pericardial cells are well sup- 

 plied with tracheae ; branches from all the abdominal spiracles pass 



