202 THE VOYAGE OF H.M.S. CHALLENGER. 



RESPIRATORY ORGANS. 



The Upper Larynx. 



The framework of the upper larynx of eA^ery species of Penguin presents the same 

 form, and consists of four separate cartilages — the thyroid, cricoid, and two arytenoids. 



The Thyroid Cartilage (PI. XVIII. figs. 1, 2) is usually extensively ossified, and some- 

 what resembles in form that of a shoe. Its lower surface is convex from side to side, and 

 is covered by the thyro-hyoid muscles, while the upper or laryngeal surface is deeply 

 concave. The lateral portions of the cartilage are prolonged backwards in the form 

 of two processes or horns, the posterior extremities of which come into relation to, and 

 articulate with, the lateral borders of the intermediate cricoid cartilage. The anterior 

 extremity of the thyroid cartilage is somewhat pointed and convex. The posterior 

 margin, on the other hand, is uniformly concave, with the exception of a small projecting 

 nodule which occupies the middle line of the cartilage and projects horizontally 

 backwards. The concavity itself is adapted to the reception of the anterior tracheal 

 rings, which are incomplete, and limited to the lower aspect of the windpipe. From 

 the middle line of the upper or laryngeal surface of the thyroid cartilage there projects 

 upwards a vertical plate, which equals in length the cartilage itself. This forms an 

 incomplete septum laryngis, which, projecting into the box of the larynx, divides the 

 latter incompletely into two lateral compartments, each of which is bounded externally 

 by the arytenoid cartilage and internally by the septum in question. The arytenoid 

 cartilages are connected by means of ligament to the upper border of the thyroid 

 cartilage, but nowhere directly articulate with it. 



The Cricoid Cartilage (PI. XVIII. fig. 2), or rather bone, for it is usually entirely 

 ossified, completes the laryngeal box posteriorly, being wedged in between the extremities 

 of the cornua of the thyroid cartQage. Its form is somewhat irregular, but two surfaces 

 and two lateral margins may be distinguished. The anterior surface is smooth and covered 

 by the laryngeal mucous membrane, while the posterior surface, deeply hollowed, afibrds 

 attachment to the sphincter-like muscles of the upper larjmx. The lateral margins are 

 deeply concave, the concavity being due to the presence of a concave articular surface 

 adapted to the reception of the posterior extremity of the corresponding thyroid cornu, 

 which thus articulates directly with the cricoid cartUage. Each of the upper and lateral 

 angles of the cricoid cartilage is, moreover, furnished with a deeply concave articular 

 surface, -wdth which the posterior extremity of the corresponding arytenoid cartilage 

 directly articulates. 



The Arytenoid Cartilages. — Each of these cartilages (PI. XVIII. fig. 2) is of an elon- 

 gated oval form, the anterior extremity of which is narrow and pointed, while the poste- 

 rior is rounded and convex. Its outer surface is hollowed to accommodate the apertor 



