138 THE COCKROACH: 



difficulty, and we have not observed nuclei belonging to them. 

 Each extends over more than one bundle of annular muscles. 



The difficulty of investigating a structure so minute and 

 delicate as the heart of an Insect may explain a good deal of the 

 discrepancy noted on comparing various published descriptions. 

 Perhaps the most obvious peculiarity which distinguishes the 

 heart of the Cockroach, is the subdivision of the thoracic por- 

 tions into three chambers, which, though less prominent in side- 

 view than the abdominal chambers, are, nevertheless, perfectly 

 distinct. The number of abdominal chambers is also unusually 

 high ; but it is so easy to overlook the small chambers at the 

 posterior end of the abdomen, that the number given in some 

 of the species may have been under-estimated. 



V 



Pericardial Diaphragm and Space. 



The heart lies in a pericardial chamber, which is bounded 

 above by the terga and the longitudinal tergal muscles ; below 

 by a fenestrated membrane, the pericardial diaphragm. The 

 intermediate space, which is of inconsiderable depth, is nearly 

 filled by a cellular mass laden with fat, and resembling the 

 fat-body. 



The pericardial diaphragm, or floor of the pericardium, is 

 continuous, except for small oval openings scattered over its 

 surface. It consists of loosely interwoven fibres, interspersed 

 with elongate nuclei (connective-tissue corpuscles) and con- 

 nected by a transparent membrane. Into the diaphragm are 

 inserted pairs of muscles, which, from their shape and supposed 

 continuity with the heart, have been named alee cordis, or alary 

 muscles.* These are bundles of striated muscle,, about '003 in. 

 wide, which arise from the anterior margin of each tergum. 

 In the middle of the abdomen every alary muscle passes 

 inwards for about *04 in., without breaking-up or widening, 

 and then spreads out fanwise upon the diaphragm. The 

 fibres unite below the heart with those of the fellow-muscle, and 

 also join, close to the heart, those of the muscles in front and 

 behind. The alary muscles are often said to distend the heart 

 rhythmically by drawing its walls apart, but this cannot be 



* Ly on net. 



