INTRODUCTION. 
XXI 
and qiiadrupedes, but is loosely attached, to enable it to follow the flexions 
of their necks with ease, and without the danger of suffocating them. 
This, together with the formation of the cartilages which compose this 
tube, and extend all around it, enables birds to turn their heads so flexibly. 
On dividing the breast-bone, and laying open the stomach, we perceive the 
trachea, “ in most birds, greatly contracted from the point at which it 
separates into two parts ; but in some birds, as the Black-capped Gull, each 
of its divisions is larger at the point at which they branch off, than the 
trachea itself. The heart of birds lies in the thorax, with its apex toward 
the sternum, and, instead of the valvulae tricuspides, the right ventricle is 
provided with a strong muscular flap. Their sternum, or breast-bone, 
unlike that in man, has a high ridge from the middle, similar to the keel of 
a ship ; which, in general, is deeper in wild birds than in domestic fowls. 
This ridge, or vertical process, is called the crista, and in that of the Wild 
Swan, and of some of the Crane kinds, is formed a cavity for the reception 
of a part of the trachea. In the Puffin the sternum is of such a length, that 
it almost extends to the rectum, which ends in the cloaca, and this contains 
the terminations of the ureters, the genital organs, and what has been 
'9 The cartilaginous annuli, or little rings of the trachea of birds, are complete circles 
slightly overlapping each other at their superior and inferior edges. 
The trachea of different birds varies In formation. In the males of the Duck kind, 
particularly in the Golden-eye, it has long been known that the trachea takes a large globous form 
(called the labyrinth) at the point of divarication ; and Willughby adds, “ et praeterea superius 
in ventrem intumescit.” And the still more wonderful structure of this part of the Crane, and the 
Wild Swan, long since attracted the notice of the naturalist. In the crista of the sternum of 
these birds is a deep recess, like a sheath, formed for the reception of the trachea, which makes 
a large convolution in this cavity. The use of this extraordinary apparatus, called the labyrinth, 
is still unknown. Voci intendendae (inquit Willughbeius) non inservire argumento est, quod aves 
ejusdem tribus quse eo carent, ut v. g. Anates domesticae vocem validam et vehementissimam 
edunt; quodque nec ad urinandum conducat, illud suadet quod colymbi dicti, qui prae aliis 
omnibus frequentissime urinantur, diutissime quae sub aquis manent, eo careant. 
