Ciasi- 5. Aves. 



447 



Insects, constitute not only a respiratory, but also an aerostatic 

 apparatus. 



The syrinx has usually the following structure : the two bronchi are separated 

 at their upper ends, where they pass into the trachea, hy a median hony rod, 

 or septum, connected with the last tracheal ring. The inner wall of the 

 anterior portion of each bronchus is membranous, and is termed the internal 

 tympanic membrane. The outer side of the wall of the bronchus is 

 strengthened by semicircles of cartilage or bone, and there is often here also a 

 membranous portion, the external 

 tympanic membrane, which, in 

 other cases, may be replaced by a thicken- 

 ing of connective tissue, projecting into 

 the lumen from one of the haK hoops of 

 cartilage. The lower end of the trachea 

 itself, which may be termed the tym- 

 panum, is usually modified for the pro- 

 duction of voice, the last rings, for 

 instance, may be fused, or this portion 

 is compressed or widened, etc. In the 

 males of Merganser (Mergus) and most 

 Ducks, the tympanum possesses a lateral 

 saccular outgrowth with ossified walls, 

 the resonator (or the labyrinth) . 



To these structures accessory to the 

 respiratoiy system, the following may be 

 added : in the Whistling Swan, the 

 Crane, and others, the keel of the stermmi 

 is thick and hollowed, with an opening 

 above ; within this cavity the trachea 

 makes a long loop before passing back 

 into the body-cavity ; in some other 

 birds similar coils of the trachea lie 

 below the skin or in the body-cavity : 

 the tubular continuation of the bronchus 

 which i-uns through the lung (see above), 



gives off branches from which numerous long parallel tubes arise, from these 

 are given off close-set radial, somewhat racemose, tubules, which end blindly ; 

 these are all bound together by connective tissue, and thus form a thick 

 layer round each of the parallel tubes: the air-sacs in the bones of the skull 

 are extensions frbm the tympanic and nasal cavities : several other air-sacs 

 of the head are connected with the latter {e.g., one in the orbit below the eye), 

 these reach back into the neck, and, in some birds, communicate with the 

 pulmonary air-sacs. Inspiration is brought about by movements of the ribs, 

 resulting in a, forward motion of the sternum and the widening of the body- 

 cavity. Certain muscles, which ai-ise from the inner side of the wall of the 

 body-cavity (from the ribs and sternum), and are attached to a membrane 

 which extends over the ventral surface of the lung, assist in the process, since 

 by their contraction, the lungs are expanded. 



The heart and the large arterial trunks arising from it, offer 

 relations which .may be shown to be a modification of those obtaining 

 in the Crocodiles. Both auricle and ventricle are completely divided 

 into right and left halves. The conns is wanting. The left aortic 



Fig. 374. Section through the lower 

 end of the trachea and the upper ends of 

 the two large bronchi of a Bird; dia- 

 grammatic, b bronchi, me external, nd 

 internal tympanic membranes, s bony sep- 

 tum, t drum, tr trachea. I — IV the four 

 lower rings of the trachea ; 1 uppermost 

 half-hoop of a bronchus. — Orig. 



