RESPIRATORY ORGANS 



235 



reaches their cells (Fig. 171). While some carbon dioxide is lost 

 through the tracheal system most diffuses directly through the 

 skin. 



BLOOD VESSELS 



The direct supply of air to the tissues is no doubt the reason 

 for the simple condition of the blood-vascular system, which 

 consists of a long heart (Fig. 169), lying along the mid-dorsal 

 line of the abdomen and thorax, an anterior aorta, and a system 

 of ill-defined sinuses, of which the principal is the perivisceral 

 cavity. The heart is enclosed in a pericardial space and is divided 



Fig. 171. — Tracheoles running to a muscle fibre. — From Imms, after Wigglesworth. 



A, Muscle at rest ; the terminal parts of the tracheoles (shown dotted) contain fluid ; B, muscle fatigued; 



air extends far into the tracheoles. 



into thirteen chambers corresponding to the segments. Each 

 chamber communicates by a pair of ostia at its sides with the 

 pericardial space. Blood from outlying parts of the body flows 

 to the perivisceral cavity, thence into the pericardial cavity 

 through openings in the floor of the latter, and so through the 

 ostia into the heart, which, contracting from behind forwards, 

 drives it through the aorta into the sinus system, by way of the 

 sinuses of the head. Paired triangular alary muscles, whose outer 

 ends are attached to the terga, move the pericardial floor, and 

 thus cause the flow of blood from the perivisceral cavity into the 

 pericardial. Both these cavities are hsemoccelic (p. 189). They 

 contain a white tissue known as the fat body (Fig. 172), some of 

 whose cells hold reserves of fats, carbohydrate, and proteins, 

 others at least temporarily retain nitrogenous excreta as ucic 

 acid, and others harbour micro-organisms (bacteroids) which are 



