576 
plete, thus forming a set of two antagonistic muscles, constant through- 
out the class. In the lower forms the ring lies flat, i. e., at right 
angles with the axis of the body; but in the Salamandrida its plane 
becomes oblique, owing, no doubt, to the curving of the trachea, in 
such a way that the ventral half of the ring lies somewhat anterior 
and the dorsal half posterior, a relation which persists through the 
Anoura. For convenience the two half rings may be treated sepa- 
rately and distinguished as follows: 
1. Musculus periarytaenoideus dorsalis. 
This muscle in the lower Urodeles sewes to rotate the curved 
Arytaenoids and thus dilate the lumen of the air passages‘). In the 
Salamandrida, owing to the partial rotation of the plane of the ring 
about its lateral axis, it lies posterior to the other, and, being thus 
behind the Arytaenoids, is not easily seen in a dissection from the 
ventral side. It has thus been generally overlooked by investigators. 
In the Anourans it suffers many deviations from the type. In Bufo 
it arises from the posterior cornua of the hyoid. In Rana it diffe- 
rentiates into two slips. With regard to the function of this muscle, 
I am not ready to speak definitely as yet, but it appears to act asa 
dilator in all cases. 
2. Musculus periarytaenoideus ventralis. 
This muscle is a constrictor. In the lower Urodeles this action 
is effected by an outward rotation of the Arytaenoids. From the 
Salamandrida on, it lies anterior to the previous muscle. It appears 
most commonly in the Anourans as a curved cylindrical muscle, arising 
from the cartilaginous epiphyses of the posterior hyoid horns. The 
muscles from the two sides run around the points of the Arytaenoids and 
become united to each other in the median line by means of a short 
tendon. They thus form a curved muscular band, embracing the 
Arytaenoids, serving to contract them, as in the Urodeles, but by a 
different way mechanically. 
c) The extrinsic system (Transverse system). 
This muscular system shows such irregularity in its origin, in- 
sertion, and position, that it seems impossible to give names to its 
elements which are capable of universal application. In general one 
may say that there exist among the lower Urodeles two transverse 
muscles, one of which disappears among the Ichthyodea; the other 
1) Wiper, Contribution to the Anat. of Siren lacertina. Zool. Jahr- 
bücher, Vol. III, p. 679. 
