THE ANATOMY OF INSECTS. 33 



In many insects, instead of a rapid alternation of contraction and 

 expansion of the chambers, the heart begins to contract at its caudal 

 end, and a wave of contraction passes towards the head along the 

 entire length of the organ ; frequently one wave will pass the entire 

 length of the heart before another begins. 



The cephalic prolongation of the heart, which extends through 

 the thorax and into the head, is a simple tube, the aorta (Plate I., 

 3 ; Fig. 40, c and a). The aorta ends in the head, near the brain, 

 where it is usually somewhat branched. The branches are very 

 shorthand the blood passes from them directly into the body-cavity. 

 Here it bathes the viscera, receiving the products of digestion from 

 the alimentary canal, giving up to the various glands their secretions, 

 and carrying nourishment to all parts of the body. In its course 

 through the body the blood flows in regular channels, without walls, 

 like the currents of the ocean. 



The blood is usually colorless, or slightly tinged with green ; but 

 its circulation is made conspicuous by the movements of the large 

 corpuscles with which it abounds. In transparent insects it can be 

 seen pouring forth from the cephalic end of the aorta, bathing first 

 the brain, and then passing to all parts of the body, even out into 

 the appendages. By tracing the course of any one of these currents 

 it will be found to flow sooner or later to the sinus in which the 

 heart rests, and from which it receives its blood. 



The Pericdrdial Sinus, to which reference has just been made, is 

 separated from the general cavity of the body by a membrane, the 

 pericardial diaphragm. This diaphragm is perforated by many open- 

 ings, through which the blood passes on its return to the heart. 

 Into the diaphragm there are inserted a double series of triangular 

 muscles (Plate I., 6 ; and Fig. 40, c). These meet on the mid- 

 dle line, and are attached by their smaller ends to the lateral walls 

 of the body. They were formerly supposed to be attached to the 

 sides of the heart, and to aid in the expansion of that organ ; they 

 were, therefore, named by Lyonet the wings of the heart. 

 The relation of these muscles to the heart is shown in 

 Fig. 43, which is a diagram of a cross-section of the body. \^ 

 In this w represents the position of the wings of the heart. FlG . 43 ._Dia- 

 I do not think that the use of these muscles has yet been wfngrofthe 

 fully determined. One function, though probably a 

 subordinate one, is doubtless to protect the heart from pressure. 

 One has only to watch the peristaltic movements of the alimentary 

 canal in a transparent larva to appreciate the importance of this. 



