THE ORGANS OF CIRCULATION AND RESPIRATION. 



137 



intermittent stream to enter from the pericardial space, but 

 preventing regurgitation. 



The wall of the heart includes several distinct layers. There 

 are (1) a transparent, structureless intima, only visible when 

 thrown into folds ; (2) a partial endocardium, of scattered, 

 nucleated cells, which passes into the interventricular valves ; 

 (3) a muscular layer, consisting of close-set annular, and distant 

 longitudinal fibres. The annular muscles are slightly inter- 

 rupted at regular and frequent intervals, and are imperfectly 



Fig. 75. Junction of two chambers of the Heart, seen from above. ML, median 



lobe ; /, lateral inlet. 



joined along the middle line above and below, so as to indicate 

 (what has been independently proved) that the heart arises as two 

 half-tubes, which afterwards join along the middle. Elongate 

 nuclei are to be seen here and there among the muscles. The 

 adventitia (4), or connective tissue layer, is but slightly developed 

 in the adult Cockroach. 



Within the muscular layer is a structure which we have 

 failed to make out to our own satisfaction. It presents the 

 appearance of regular but imperfect rings, which do not extend 

 over the upper third of the heart. They probably meet in a 

 ventral suture, but this and other details are hard to make out, 

 owing to the transparency of the parts. The rings stain with 



