ON THE ANATOMY OF THE NORTHERN BELUGA. 421 
cricoid cartilage, which, less differentiated than usual from the succeeding 
tracheal rings to which it is united, is like them deficient ventrally. The 
interspace thus left is occupied by a membrane continuous with that which 
fills up the interval left by reason of a similar incompleteness of the anterior 
tracheal rings. The arytenoid cartilages are prolonged backwards and down- 
wards to form posterior horns, which, reaching the lower part of the larynx, 
turn inwards and approach each other, leaving but a slight interval between 
their extremities. This interval in the recent state is occupied by a ligamentous 
band which connects the two posterior.horns. Thus a distinct arch is formed, 
which, taking the place of the deficient ventral portion of the cricoid, apparently 
compensates for its loss. 
So far as we can ascertain, the cricoid is never so incomplete ventrally in 
the toothed whales ; on the contrary, it usually forms a distinct ring. In some 
species, however, ey., Beluga, Phocena communis, the ring is not quite 
perfect, although almost so. In no species of toothed whale yet described are 
the arytenoids prolonged into posterior cornua, as in the whalebone whales. 
To summarise, in respect of the larynx Beluga conforms to the characteristics 
of the toothed whales in general; at the same time, possessing as it does a 
small laryngeal sac and a cricoid cartilage ventrally incomplete, it manifests a 
tendency toward the possession of that form of larynx met with in the group 
of whalebone whales. 
Trachea.—The windpipe, from the posterior border of the cricoid cartilage 
to the point of bifurcation into its terminal bronchi, measures 6 inches in length. 
Somewhat flattened from above downwards, its greatest diameter is transverse, 
and measures 2 inches. Of the cartilaginous rings forming part of its wall there 
are fifteen, seven of which are situated in front of an accessory bronchus to be 
presently described. The rings, as a rule, are complete, and entirely surround 
the trachea ; there is consequently an absence of the so-calied membranous 
portion of the trachea usually met with in other mammals. The three anterior 
rings, however, form an exception to the general rule, their ventral segments 
being deficient ; the corresponding portion of the tracheal wall just behind the 
cricoid cartilage is therefore entirely membranous. 
Bronchi.—The bronchi are three in number, of which two correspond to 
those commonly resulting from the bifurcation of the trachea ; whilst the third 
(azygos) is given off from the trachea about 21 inches in front of the bifurcation. 
This accessory bronchus passes to the right lung, and enters its inner surface 
4 inches in front of the entrance of the principal or terminal bronchus. It is 
the smallest of the bronchi, measuring when flattened only # of an inch in 
breadth. Of the principal bronchi, the right measures 4 inches in length from 
the tracheal bifurcation to the point at which the first intra-pulmonic air-tube 
is given off; whilst the left, between the corresponding points, measures but 
VOL. XXIX. PART I. 5 Q 
