204 THE VOYA.GE OF H.M.S. CHALLENGER. 



another. The greater portion of this muscle is concealed by the sphincter laryngis. The 

 action of this muscle is apparently that of a dilator of the laryngeal aperture. This is 

 accomplished by the muscle taking its fixed point externally, and acting upon the readily 

 moveable arytenoid cartilages. 



The upper larynx in every species of Penguin agrees, except in size, with that above 

 described in Eudyptes chrysocome from Tristan d'Acunha. 



The Trachea. 



The trachea of Eudyptes chrysocome from Tristan d'Acunha (PI. VIII.) measures 

 7 inches in length. In its course along the neck it lies at first in front of the vertebral 

 column, but like the oesophagus, becomes displaced, so as to lie to the right of the 

 posterior cervical vertebrae. This displacement is due to the fact that the posterior 

 cervical vertebrae project forwards to such an extent that they come into contact with 

 the furcular angle, and consequently the trachea and oesophagus are displaced to the 

 right before passing into the thorax. After entering the thorax, the trachea regains the 

 middle line, and almost at once divides into the two bronchial tubes. The thoracic 

 portion of the trachea lies in front of the oesophagus, and occupies the fork formed 

 by the two innominate arteries. The tube is flattened from above downwards, and 

 presents externally a slight trace of a longitudinal mesial groove, which indicates the 

 position of a septum, which in the majority of the Penguins divides the tracheal tube 

 more or less completely into two distinct compartments. The structure of the trachea 

 in the Penguins does not differ from that of other birds. The tracheal rings, which in 

 birds in general are cartilaginous, are prone to ossify in the Penguins. In Eudyptes 

 chrysocome from Tristan, these rings are 107 in number. They are freely moveable upon 

 one another in the region of the neck, and accommodate the trachea to the varying 

 flexures of that portion of the vertebral column. In the thorax, on the other hand, they 

 are much less mobile, and give to the thoracic portion of the tube a rigidity which 

 is wanting in the cervical segment of the tube. The thoracic portion of the trachea 

 as a whole is much less flexible than the cervical, still a certain amount of movement 

 is permitted between the rings composing it, with the exception of the last nine, which, 

 although not actually soldered together, are so arranged as to prevent the possibility of 

 any bending of that portion of the trachea of which they are component elements. 



The most striking peculiarity of the trachea in the Penguins lies in the j)resence of a 

 mesial tracheal septum, which extends from the point of bifurcation forwards, to a greater 

 or less extent in different species. This septum divides the lumen of the trachea more or 

 less completely into two lateral channels. In Eudyp>tes chrysocome from Tristan, it is 

 more limited in extent than in any other species which I have examined. Indeed in 

 one specimen, the trachea presented not the sUghtest trace of a septum, while in 



