568 ORGANS OF RESPIRATION. 



attachment of the cricoid cartilage, and the same posterior 

 margin is prolonged upwards to form a superior cornu for the 

 attachment to the os hyoides. The thyroid cartilage is still 

 sometimes divided vertically on the median plain, by a 

 distinct suture, into two lateral halves, as it is in many of 

 the reptiles ; and although it no longer manifests a division 

 into its primitive separate tracheal rings, the perforation for 

 the inferior laryngeal artery is considered by Henle as 

 indicating an earlier separation at that point. The thyroid 

 cartilage of the ornithorhynchus is a small, simple, flat, arched 

 piece, without either superior or inferior cornua, and its 

 firm ligamentous connection with the hyoid bone, was sup- 

 posed by Meckel to be a peculiar extension of the thyroid 

 cartilage, embracing the oesophagus, in this animal. 



The cricoid cartilage of mammalia, always most developed 

 behind, where it rises upwards between the posterior edges 

 of the thyroid, and tapering forwards, where it is often 

 deficient, as in many carnivora and cetacea, is formed by the 

 union of the small cricoid of chelonia and birds with the 

 detached posterior, or lateral pieces of the thyroid seen 

 in the latter class. It here commonly forms a complete 

 ring, united though small in front, resting on the first tracheal 

 ring, attached to the inferior cornua of the thyroid, and by 

 a small vertical median ridge behind to the oesophagus, and 

 supporting on its upper convex posterior articular surfaces, 

 the two arytaenoid cartilages. There are commonly two 

 small round inter articular cartilages, discovered by Brandt, 

 like sesamoid bones, between the upper edge of the thyroid 

 and the articular surfaces of the arytsenoid ; these are largest 

 and triangular in the hedgehog, and are transversely elongated 

 in the vampire. Similar small inter articular pieces are also 

 frequently found between the posterior and inner edges of 

 the arytaenoid cartilages, which in the ornithorhynchus, the 

 opossum, and some other mammalia, are consolidated into a 

 single piece on the median plain. 



The two arytaenoid cartilages preserve their elongated 

 triangular form and their general connections, most con- 

 stantly throughout the pulmonated vertebrata, being less 

 extended longitudinally in mammalia than in birds. Resting 

 by their grooved and smooth articular bases on the back 

 part of the cricoid, and extending forwards, with their flat 



