RESPIRATORY SYSTEM 63 



fig. 42), the former presence of the muscle is testified to by 

 a thin ligament, which occupies the position that the muscle 

 would occupy were it present. 



The syrinx of the hoatzin shows an intermediate stage ; 

 the muscle is absent for the lower part of its course, but 

 present above ; it is represented below by a fibrous band. It 

 seems from what w r e know of the relation of muscle to 

 tendon generally that this change is in the direction indi- 

 cated and not in the converse direction. The fibrous band of 

 the syrinx of Balceniceps has, so to speak, been muscle ; it is 

 not on its way to become muscle. The ostrich affords an 

 example of a further degeneration of the syrinx, or a reten- 

 tion of a very primitive and unspecialised syrinx, according 

 as we view the facts. The syrinx of this bird has been care- 

 fully described and figured by FORBES/ whose words we will 

 quote. ' The trachea inferior to the insertion of the sterno- 

 tracheales slightly narrows, having above the antepenultimate 

 ring a diameter of about one inch. The tracheal rings are here, 

 as elsewhere, entire simple rings of an average depth of about 

 15 inch, and are separated only by very slight interannular 

 intervals. The trachea is slightly compressed and posteriorly 

 carinated for about the last seven rings. The last ring but 

 four is somewhat produced downwards in the middle line 

 both anteriorly and posteriorly ; it is in consequence nar- 

 rower laterally than elsewhere. The antepenultimate ring- 

 presents the same features more strongly developed. In 

 two of the four specimens examined it sent down a small 

 prssuliform process of cartilage in the middle line behind, 

 filling the chink left between the posterior extremities of 

 the last two (incomplete) rings. The penultimate ring is 

 narrower and more cylindrical than its predecessors ; it is 

 also wider transversely, and incomplete behind in the middle 

 line, its extremities, however, being closely approximated to 

 each other. The last tracheal ring is still wider transversely, 

 and more cylindrical ; and it, too, is incomplete posteriorly 

 to a greater extent than its predecessor ; viewed from the 



' ' On the Conformation of the Thoracic End of the Trachea in the "Eatite " 

 Birds,' P. Z. S. 1881, p. 778. 



