OCR Text |
Show 1901.] LARYNX OP CERTAIN WHALES. 291 arytenoid cartilages, and below the articular facets the cricoid passes as a nearly horizontal band, much narrower than before, round the side and across the ventral surface to the other side. At the lower angle formed by the lateral and dorsal moieties is the facet for the thyroid cornu, and from here a slight ridge passes obliquely forwards and ventral wards to reach the anterior margin of the latero-ventral moiety of the cricoid ; this is much thinner and of less diameter than the dorsal moiety. The measurements of the cricoid are as follows:- Length along the median dorsal ridge 11 inches. Width of each limb of V 11 inches. Separation of thyroid facets 2 inches. Width of the latero-ventral moiety g inch (except in ventral mid-line, where it is | inch). In the Pilot Whale the cricoid has a much greater posterior (dorsal) surface, for according to Murie (1871) it is a " trifle longer than the body of the thyroid," while on the ventral surface it is produced into completely posterior cornua which embrace the trachea and " wellnigh meet in the mid-line." In the Porpoise, too, the cartilage is incomplete ventrally (Owen). The arytenoid cartilage (PI. X X V I I . fig. 16) is of considerable length, as in other Odontocetes, and though the same regions may be recognized as in the arytenoid of Balcenoptera, these are less distinctly marked off from one another. But the descriptive terms used in that Whale are no longer appropriate here. In general form it closely resembles the corresponding cartilage in the Porpoise. Each arytenoid is a long flat rod, oval in transverse section through the greater part of its extent, broader and thicker inferiorly, thin and flat superiorly. On the dorsal side is a distinct shoulder, at a point about two-thirds of its length from the summit; this makes nearly a right angle with the narrow (superior) moiety, and from this point, which projects about \ inch, the dorsal (or posterior) border slopes very gradually downwards ; on this margin, just below the shoulder, is the articular facet from the cricoid. The lower end of the cartilage is broad and rounded and very thick. The upper moiety or supra-arytenoid (Thompson) becomes quite thin as the extremity is approached, and the plane of this narrow plate becomes twisted near the extremity, so as to take on a position finally which makes an angle with the plane of the broad face lower down. The margin of this upper extremity is recurved, and supports the mucous membrane that constitutes the arytenoid body, and the two cartilages touch one another here. The external surface of the posterior moiety or " processus vocalis " is irregularly convex and serves for the attachment of muscles. |