140 
GREENLAND VOYAGE, 
ing, expanding immediately within the skin into 
a sac, or air-vessel, six or eight inches wide, and 
extending laterally and forward into two cavities 
(a a of the annexed figure, which is the appear¬ 
ance of the upper part of the head of the narwal, 
when the skin and fat are sliced off horizontally), 
one on each side, the extremities of which are 
about twelve inches apart. These contain a por¬ 
tion of a mucous substance. The whole sac is 
lined with a thin greenish-black skin. At the 
posterior extremity of the cavity, or sac, are seen 
