564 ORGANS OF RESPIRATION. 



downwards, and supporting the arytaenoid cartilages above, 

 attached to each side of its free projecting margin. It 

 presents a middle, and two lateral centres of ossification, 

 sometimes separated from each other by vertical fissures, 

 and is often partially attached to the basilar ring of the 

 thyroid, and to the lateral quadrangular pieces of the 

 thyroid, which has induced many to consider the whole 

 of these pieces as forming together the cricoid cartilage 

 of birds. The long, narrow, triangular, arytsenoid car- 

 tilages, tapering forwards, broader, and united together 

 behind, generally ossified, bound the rima of the glottis with 

 their straight inner margin, and are attached to the thyroid 

 by their outer edge. Though generally ossified, like the 

 other laryngeal and tracheal parts of birds, the cricoid and 

 other laryngeal cartilages remain soft, flexible, and cartilagi- 

 nous in the strutheous birds, as in mammalia. The larynx 

 and trachea are raised chiefly by the hyothyrioideus and the 

 thyriotrachealis muscles ; the laryngeal opening is widened 

 by the thyrioarytcenoideus posticus muscle, and is com- 

 pressed by the thyrioarytanoideus lateralis, as in the higher 

 reptiles. The muscles of the upper larynx of birds appear to 

 be supplied chiefly by the superior laryngeal nerve, which pro- 

 tects the passage of the glottis, and the muscles of the lower 

 larynx, or organ of voice, by the inferior laryngeal, or vocal 

 nerve. 



In the embryo of birds, the lungs appear as two small sacs 

 or follicles, developed from the oesophagus or pharynx; their 

 apertures coalesce and elongate, to form a tracheal duct, on 

 the median plain; the interior parietes of these primitive 

 pulmonary sacs, develope peripheral septa or follicles, to form 

 the rudiments of their internal cells, which are arranged with 

 great regularity, though subdivided and ramified, around the 

 bronchial canals ; and the membranous tracheal duct, or 

 ductus pneumaticus, becomes cartilaginous, segmented, and 

 divided into a distinct superior and inferior larynx, with a 

 more simple intermediate, lengthened, annulated trachea. 

 Before the branchial openings have formed on the sides of 

 the neck, the integuments appear entire on both sides, and 

 when the embryo is more advanced, three lateral passages 

 are discovered on each side, leading into the cavity of the 

 pharynx. They are seen in the chick on the third day of in- 



