SKELETON OF THE SWAN. 139 



stance of tlie back part of the sternum. Small pneumatic 

 foramina are situated at the anterior and inner surface of 

 the bone, and perforate the articular surfaces for the ster- 

 nal ribs. 



In the skeleton of the wild swan {Cygnus ferus) (Fig. 

 24), here selected as an illustration of the ornithic modi- 

 fication of the vertebrate type, there are not fewer than 

 twenty-eight vertebrae, CSD, between the skull and the 

 sacrum, the last six of which, dd, support movable ribs; 

 of these, the first and second pairs are free; the next four 

 are articulated to the sternum by bony hasmapophyses ; 

 the last five pairs of ribs are attached to the sacrum, and 

 also to the sternum; but the tenth, or last rib on the left 

 side, is very rudimentary, being only about one inch in 

 length. There are eight caudal vertebrae, Od. The trachea, 

 or windpipe, penetrates the sternum, and bends and mnds 

 in the interior of the bone before returning to enter the 

 chest. The apex of the furculum, 58, bends upwards, 

 and forms a hoop over the windpipe as it enters into the 

 keel of the breast-bone. The furculum, sometimes called 

 "merrythought," consists of the two clavicles confluent 

 at tljeir lower free ends. If a portion of the one side of 

 the sternum be removed, the tortuous trachea which it 

 incloses will be exposed. To the great length and pecu- 

 liar course of the windpipe in this species is to be attri- 

 buted its remarkably loud and harsh voice ; whence the 

 name hooper, or whistling swan, has been derived ; and 

 is applied in contradistinction to the domestic or mute 

 swan, in which, as in most other birds, the trachea pro- 

 ceeds at once to the lungs, without entering the sternum. 

 In the female of the wild species, the course of the tra- 

 chea is much more limited than in the male, seldom 



