THE MYOLOGY OF POLYODON lee 
immediately behind the posterior opening of the facial canal. 
The posterior fibers arise more and more dorsally and medially 
as shown in the figure 4. The anterior fibers are shortest, the 
posterior longest. Somewhat beyond its origin the muscle mass 
becomes indistinctly separated into its four parts, the anterior 
of which tends to overlap the next posterior. These are inserted 
into the dorsal margins of the four epibranchial cartilages. Some 
of the fibers also may have a cutaneous insertion in the tough 
skin at the dorsal angles of the gill slits. 
Innervation. Each muscle is supplied by the appropriate ramus 
posttrematicus which crosses its anterior surface a little above 
the insertion. In the case of the first gill this is a branch of the 
glossopharyngeal nerve, all the others are from the vagus. The 
next posterior ramus praetrematicus also crosses each of these 
muscles, passing over its median side, but I could find no fibers 
being given off from it to the muscle. There is also still another 
branch of the vagus which passes up under the origin of the 
muscle to anastomose with the ramus oticus trigemini. No mus- 
cular branches of this nerve, however, were detected. 
Blood supply. Blood is supplied by the pharyngeal branch 
(aph.) of the second efferent artery, which sends twigs into the 
medial side of the muscle, and from smaller arteries arising from 
the efferent branchials within each gill. 
Action. 'These muscles serve to raise the gill arches and draw 
them slightly toward the median line. 
In the nature of their origin, insertion, innervation, and to 
a certain degree their blood supply these muscles compare very 
closely with the retractor hyomandibularis. Their function is 
also similar. The fact that they do not appear on the side of 
the head is due simply to the pressure of an overhanging oper- 
cular apparatus. They are none the less superficial muscles, and 
quite homologous with the foregoing. The tendency for the first 
to overlap the one behind is parallel to the tendency on the part 
of the retractor hyomandibularis to overlap the muscles posterior 
to it. The confluence of these muscles at their origin is possibly 
not a primitive condition. From a consideration of Polyodon it 
is not easy to understand Vetter’s uncertainty regarding the 
apparently homologous muscles of Acipenser. 
