18/6.] OF PASSERINE BIRDS. 515 



spot) posterior to its anterior extremity ; the third is narrower, and 

 terminates behind by a short descending hook. 



The syringal muscles are three in number on each side at their 

 insertion, although at their origin only two can be distinguished. 

 Tbese are an anterior and posterior longitudinal, which, from a 

 lateral point situated opposite the tracheal ring 19 above the last 

 one, diverge forwards and backwards to the tips of the bronchial 

 semirings. In Plate LI I. figs. 1, 2, & 3 the front, back, and side 

 views of the syrinx of Menura are figured. 



The anterior longitudinal muscle, whose diameter is about four 

 times that of the depressor trachea-, is of uniform size throughout, 

 being constituted of parallel fibres. It is inserted into the lower 

 margin of the expansion at the anterior extremity of the second 

 bronchial semiring, at a short distance behind its apex. 



The posterior longitudinal muscle, from being single above, 

 divides into two below. 



Before proceeding further it will be necessary to explain the way 

 in which these muscles arise. There is a large air-cell, the anterior 

 thoracic *, in which the syrinx and base of the heart are situated. 

 The visceral walls of this cell are so thin that the trachea may, to 

 all intents and purposes, be said to perforate it. Where it does so, 

 the membrane blends with its fascial sheath most intimately ; and 

 it is from the thus formed ring of junction that the long fibres of 

 the syringal muscles spring. This ring is not a simple horizontal 

 circle of fibrous tissue surrounding the trachea. In its anterior 

 half it is so ; but behind it descends for some distance on each side 

 to a median spot situated below its general level, at a distance be- 

 neath it equal to the diameter of the tube itself, to blend at the 

 angle thus formed with a strong fibro-cartilaginous ribbon, which 

 expands below the level of the bronchial bifurcation, to terminate as 

 a membranous covering to the front of the oesophagus. 



From the postero-lateral portion of the horizontal moiety of the 

 ring just described, and from its descending limb, the posterior 

 longitudinal muscle of the syrinx arises, on each side, powerful, 

 and in a single mass, of which the longer postero-external fibres, 

 as it descends, differentiate themselves off to form an independent 

 fasciculus, which is inserted into the posterior hooked extremity of 

 the third bronchial semiring. The other much larger internal 

 portion, composed mostly of much shorter and oblique fibres, is in- 

 serted into the posterior triangular surface of the tracheal three- 

 way piece (last tracheal ring), and into the posterior extremity of 

 the first bronchial semiring, a few of its tendinous fibres of termina- 

 tion apparently running on to the back of the membrane between 

 the first and second semiring, and perhaps slightly to the back of 

 the second semiring itself, although this last seems to be independent 

 in this respect. 



The comparatively slender musculus sterno-trachealis springs from 

 the lateral surfaces of the four or five tracheal rings above the last 

 two, emerging between the anterior and posterior intrinsic muscles. 

 * Vide Owen's 'Anatomy of Vertebrates,' vol. ii. p. 211. 



