SEC. IV ANATOMY OF BIRDS 301 



dilatation which renders the corresponding structure — the " Adam's 

 apple," — so conspicuous in the human throat. Below, it communi- 

 cates directly with the pipe : above, it opens into the mouth by the 

 gloitidean fissure, or rima glottidis f , c), a median lengthwise chink, 

 which opens and shuts as its sides diverge or close together, and 

 which is further defended in front by a folding of the mucous 

 membrane of the mouth, constituting a rudiment of that curious 

 trap-door arrangement which, when fully developed, is called the 

 epiglottis (^, d, e). Exclusive of two broken upper rings of the 

 trachea (*, g), the cartilages (or oftener bones, — for they generally 

 ossify) of the larynx are five. One is a large single median and 

 inferior piece, the thyroid, or shield-piece (*, ®, ', a), forming the 

 most substantial part of the structure. It is somewhat triangular 

 or oblong, running to an obtuse end in front ; and with sides and 

 posterior angles which curl upward behind. To its lateral posterior 

 corner is attached on each side the small "horn" or corniculum 

 laryngis (^, ®, '^, h). There is a small median upper posterior piece, 

 supposed to represent all there is of the cricoid (^, ^, c), which in 

 man makes a ring around the larynx below the thyroid. To the 

 cricoid, as to a base, are attached a pair of straight slender arytenoids 

 (^, ', d), projecting forward along the upper surface of the larynx : 

 these form the rima glottidis, — the fissure of the glottis being 

 between them. The arytenoids are attached in front by slender 

 hgaments to the end of the thyroid (^ the little slips between d and 

 «), and they are supplemented by cartilaginous edges (*, /, /) ; but 

 there are no true vocal chords. Besides the extrinsic thyrohyoid 

 muscles, which pass from the larynx to the tongue -bone, the 

 laryngeal parts are operated by intrinsic muscles, the sum of the 

 motion given by which is the opening and shutting of the glottis 

 by drawing apart or pulling together the arytenoids. Four pairs 

 of such muscles are described for some birds. As named and 

 figured by Macgillivray for the rook, there are : the thyroarytenoids, 

 which are the openers of the glottis (^, ^•^) ; the oblique arytenoids 

 (1", 3'^) ; the thyrocricoids Q^, *'*) ; and the posterior thyroaicoids, 

 (11 and 12, 6,6). 



The Syrinx (Gr. a-ipiy^, surigx, a pipe) or Lower Larynx is the 

 voice- organ of birds ; in most respects a more complicated structure 

 than the larynx proper, and one so difierently constructed in 

 different birds that it affords characters of great significance in 

 classification. The highest group of Tasseres, for example, is 

 signalised by the elaboration of this musical organ, the marvellously 

 adroit fingering of the keys of which by the little muscular 

 performers sends through the tracheal sounding-pipe the tuneful 

 messages of bird's highest estate. A few degraded or disgraced 

 birds, as the ostrich and the American vultures, have no bucolic 



