938 SVRINX 
supplied by a long branch of the Hypoglossal, or 12th pair of 
cranial nerves (xii. 2 of diagr. p. 624). A branch descends on 
either side of the Trachea, being often accompanied along its whole 
length by thin muscles which extend from the Upper Larynx and 
the Hyoid apparatus (p. 452) to the Syrinx, and the Syringeal 
muscles proper are in fact the distal portion of such a long lateral 
mass as in the majority of Birds is now restricted to the lower 
third part of the Trachea, and is there separated into a variable 
number of pairs; but there are also others which, though belonging 
to the same category, only act upon the Syrinx indirectly. Of 
these there is in all Birds one pair, but in Anseres, including 
Palamedex, two pairs, of slender muscles, which arise at about the 
beginning of the lower third of the Trachea and are inserted upon 
the arms of the Furcula or upon the lateral processes of the 
STERNUM (p. 909), or again, but rarely however, on neighbouring 
soft parts and not upon bones. ‘These are the tracheo-clavicular 
and sterno-tracheal muscles. 
The proper vocal muscles, being those which are inserted upon 
the lower end of the Trachea or upon the Bronchi, shew an extra- 
ordinary amount of modification. Their number varies from one 
pair to seven, and they are either inserted upon the middle or lateral 
portion of the bronchial semirings (MESOMYODI, p. 546), or attached 
to the end of those semirings where they pass into the inner tym- 
paniform membrane (AcRoMyYODI, p. 1). The former is morphologic- 
ally the more primitive condition, and is found in an overwhelming 
majority of Birds; while the latter, with few exceptions, is restricted 
to the OSCINES (p. 659). But there are also other conditions— 
“ Anacromyodian,” “Catacromyodian” and “ Diacromyodian ” — 
according as these muscles are inserted on the dorsal, the ventral 
or on both ends of the semirings. Hence the distinction between 
OLIGOMYOD& (p. 654) and PoLyMyop& (p. 737), depending on the 
presence of few or many song-muscles, even if applied to Passeres 
only, cannot be maintained, for that group includes forms with any 
number of pairs from 1 to 7. Nor is the distinction between 
Mesomyodi and Acromyodi always safe. The Yyrannidxe for in- 
stance are anacromyodian, while the Pipridw and Cotingidx are 
catacromyodian, and these modifications can be shewn to have 
been derived (comparatively recently) from the weak mesomyodian 
and oligomyodian condition which prevails in the majority of the 
so-called Oligomyodx. On the other hand, the diacromyodian type 
can only have been developed from a strong muscular basis which 
could split into a dorsal and a ventral mass. Moreover, there 
are no Pusseres known to be intermediate between those that are 
diacromyodian and those that are not. We have therefore to 
distinguish between 
(1) Passeres diacromyodi, in which some of the syringeal muscles 
