OCR Text |
Show 1901.] LARYNX OP CERTAIN WHALES. 283 The cricoid cartilage differs from the form usual in mammals in that it is incomplete ventrally; it consists of a great dorsal plate, which curves round the sides and is produced backwards, towards the ventral face, into two " horns " or processes. (Plate X X V I . fig. 7.) W e may, therefore, distinguish a body and a pair of cornua. The body is nearly square ; wheu seen from the dorsal surface, its anterior margin is nearly straight in the middle line, though the corners are obliquely truncated to bear the arytenoid cartilage ; the posterior margin is produced backwards in the middle line, to form a somewhat rounded prominence, with which, in this young individual, 4 or 5 of the upper tracheal rings are continuous. The dorsal surface is almost flat, slightly concave in the middle. As this broad plate of the cricoid curves round the side of the larynx its longitudinal diameter diminishes. The anterior margin, beginning at the arytenoid facet, commences to slope gently backwards, and the inclination increases as the ventral surface is reached, till it makes an abrupt backwardly-directed curve near the middle line, giving rise to a rounded angle ; the margin then continues nearly straight backwards to constitute the ventral or inner edge of the cornu of the cricoid. The posterior margin, meanwhile, is inclined forwards from the mid-dorsal line towards the thyroid facet, but the inclination is slight; beyond this point it is continued forwards for a short distance and theu curves backwards, forming a shallow lateral bag in the cricoid ; their margin then passes nearly directly backwards to form the dorsal or outer edge of the cornu. The cornu itself is not so definitely marked off from the cricoid as is the cornu of the thyroid from its body, it is rather the ventral posterior angle drawn out backwards to form on each side a short parallel bar for the support of the peculiar " sublaryngeal pouch " of the Mystacocete. As to measurements, the cricoid is 3 inches long in the mid-dorsal line and 3 inches across, taken from the lower edges of the arytenoid facets, and the same between the thyroid facets, while the space between the two arytenoid facets is 1| inches. The lateral margin, as seen from the back, i. e. the distance from the outer edge of the arytenoid facet to beyond the thyroid facet, is 2 inches. The ventral margin -of the cricoid (or rather of its cornu) is 2 inches ; the dorsal or outer edge of the cornu is 1| inches. In the text-books, both of O w e n and Huxley, the ventral incompleteness of the cricoid is mentioned. Carte and Macalister give no clear figure of the cricoid, and do not represent the ventro-posterior cornua; but in the text this " tongue-shaped process " is described as reaching to the first ring of the trachea. In the present youthful specimen it extends backwards to the sixth ring. The arytenoid cartilage consists of a somewhat conical " body " or processus inuscularis, of a stout cylindro-conical posterior |