| OCR Text |
Show 1876.] 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. These 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 LII. 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 beneath 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 inserted 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 termination 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. |
