SEC. IV 



ANATOMY OF BIRDS 



299 



admirable account. When contracted, the rings look like an 

 alternating series of lateral half-hoops, as in Fig. 96, a; when 

 stretched to the utmost, as in Fig. 96, h, they are clearly seen to be 

 annular, or completely circular. The curious bevelling of the right 

 and left sides of each ring alternately is 

 shown in Fig. 97, ^ 2. and Fig. 97, \ \ 

 represents the same two rings put to- 

 gether. The principle by which any 

 two rings slip partly over each other on 

 alternate sides is something like that 

 upon which a cooper fastens the ends of 

 any one barrel-hoop without any nailing 

 or tying. The rings are in some birds 

 perfectly cartilaginous : in most they 

 become osseous. The trachea is moved 

 by lateral muscles, which not only 

 shorten the tube by approximating the 

 rings, but also drag the whole structure 

 backward, by their attachment to the 

 clavicle and sternum. The strip, or two 

 strips, of muscle lying upon each side 

 of the trachea, is the contractor trachece 

 (Fig. 101, ^, ss, ss) ; the most anterior, 

 when there are two, as soon as it 

 leaves the tube to go to the clavicle, 

 becomes the clidotrachealis, or clidohyoicl. Fig. 101, ^, f, f ; the 

 other is similarly the sternotrachealis. The latter may be a 

 direct continuation of the contractor, as in Fig. 101, ^, the loose 

 strips under q, or apparently arise separately from the side of the 

 lower end of the tube, as in Fig. 101, ^^, e. (Other muscles are to 



be described with the 

 larynx superior and 

 inferior.) The trachea 

 is long in birds, pro- 

 portionate to the exten- 

 sion of the neck ; it is 

 very flexuous, follow- 

 ing with ease the bends 

 of the neck in which 

 it lies so loosely. Its cross section is oval or circular ; but all that 

 relates to the configuration and course of the pipe requires special 

 description, — so variable is the organ in different birds. It is 

 subject to dilatations and contractions in any part of its extent, and 

 to deviations from its usual direct course to the lungs. Minor 

 modifications must be passed over. The most remarkable expan- 



Fio. 90. — a, an inch of trachea, 

 contracted to the utmost, the rings 

 looking like alternating half-rings ; 6, 

 the same, stretched to two inches, 

 the rings evidently complete, with 

 intervening membrane. (After Mac- 

 gillivray.) 



Fig. 97. — 1, 2, left-hand, two tracheal rings, separate, as 

 in Fig. 9t3, 6 ; 1, 2, right hand, the same put together, as in 

 Fig. 90, a. (After Macgillivray.) 



