ANATOMY AND PHYSIOLOGY 



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ticulated 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 



FIG. 177. Structure of a 

 trachea, h, trachea! hyj3o- 

 dermis; i, intima; t tasnidium. 



PIG. 178. Tracheolar end-network from silk gland 

 of Porthetria dispar. p, peritracheal membrane; t, 

 tracheal capillary. After WISTINGHAUSEN. 



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 tracheae originate in the embryo as 

 simple in-pocketings of the outer germ layer, or ectoderm, and from 



FIG. 179. Transverse sections of abdominal segments to illustrate respiratory move- 

 ments. A, cockroach (Blatta); B, bee (Bombus}\ s, sternum; t, te'rgum. The dotted lines 

 indicate positions of terga and sterna after expiration; the continuous lines, after inspira- 

 tion. After PLATEAU. 



these the countless tracheal branches are derived by the same process of 

 invagination. The lining membrane of a trachea is, then, continuous 

 with the external cuticula, and the cellular wall of a trachea is continu- 

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

 polygonal cells (Fig. 177) fitting closely together as a pavement epithe- 

 lium. The chitinous lining, or intima, is thickened at regular intervals 

 to form thread-like ridges, which course around the inner circumference 



