ANATOMY AND PHYSIOLOGY 



137 



FIG. 174. 



Thus, in the stag-beetle, a crescentic bozv (Fig. 173, /;) extends 

 half around the trachea, and the rest of the circumference is 

 spanned by a lever (/) and a band (bd) ; these three diitiimus 

 parts, articulated together, form a ring around the trachea. 

 Furthermore, a muscle (m) connects the lever and the band. 

 As the muscle shortens, the lever 

 turning upon the end of the band as 

 a fulcrum, pulls the bow toward the 

 lever and band until the enclosed 

 trachea is pinched together. When 

 the muscle relaxes, the trachea opens 

 by its own elasticity. 



Structure of Tracheae. - - The 

 trachea? originate in the embryo as 

 simple in-pocketings of the outer 

 germ layer, or ectoderm, and from 

 these the countless tracheal branches 

 are derived by the same process 

 of invagination. The lining mem- 

 brane of a trachea is, then, con- 

 tinuous with the external cuticula, 



and the cellular wall of a trachea is continuous with the 

 rest of the hypodermis. This wall consists of a layer of 

 polygonal cells (Fig. 174) fitting closely together as a pave- 

 ment epithelium. The chitinous lining, or intima, is thick- 

 ened at regular intervals to form thread-like ridges, which x 

 course around the inner circumference in essentially a spiral 

 manner, though the continuity of the so-called spiral thread is 

 frequently interrupted. These elastic threads, or ttcnidia, 

 serve to keep the trachea open without affecting its flexibility. 



The ultimate tracheal branches (Fig. 175) are extremely 

 delicate tubes, which do not end blindly, but anastomose with 

 one another, forming a capillary network of confluent tubes. 

 Some authors have held that the finest tracheal filaments pene- 

 trate epithelial or other cells. 



Respiration. The external signs of respiration are the 



Structure of a trachea, h, 

 tracheal hypodermis; i, intima; 

 t, taenidium. 



