124 Cc. H. DANFORTH 
longer possible by ordinary methods to determine the real source 
of subsequent branches. From the thus-formed nerve or plexus 
there are two branches which pass down the posterior side of 
the cartilage and enter the muscle. It is possible that in some 
eases there are more than two of these branches. Sewertzoff (’11) 
has recently described a new nerve which, in some elasmobranchs 
and ganoids, enters the gill internal to the branches heretofore 
recognized. ‘This nerve, which he designates as Ramus praetre- 
maticus internus, I find to be present also in Polyodon (fig. 5, 
n.int.). It enters the gill posteriorly and follows the margin 
next to the mouth cavity well around toward the median line 
below. In crossing from the epi- to the cerato-branchial region 
it traverses a part of the adductor muscle and sends fibers over 
its surface and down into it. Whether or not these are actually 
motor in their nature I cannot state. All of these internal pre- 
trematic nerves to the gills are branches of the vagus. 
Blood supply. The intra-branchial branches of the correspond- 
ing efferent arteries supply these muscles. Usually there is one 
branch (a@.nu.), arising toward the dorso-medial end of the gill 
and accompanying one of the nerves that is especially well devel- 
oped. A large vein (v.ad.) on the posterior side crosses the 
cartilage lengthwise to reach the jugular. 
Action. These muscles approximate the roof and floor of the 
mouth and act as opponents to the dorsal and ventral muscu- 
lature. Speaking in general terms the adductors within the 
gills represent a group of disappearing organs. They are present 
and apparently uniformly developed in the elasmobranchs. In 
Chimaera also there are five of them (Vetter) but not strongly 
developed. In Acipenser their number is reduced to three. In 
Amia (Allis) there are two and in Ameiurus only one. Quite 
often they are altogether lacking in teleosts. Polyodon is thus 
shown to be exceptional in retaining a full complement (as many 
as there are gills) all of which are strongly developed. The 
matter of origin and insertion appears at first sight to present a 
problem, tor each muscle arises on the posterior side of an arch 
and is inserted on its anterior side, whereas in other fish the 
origin and insertion both appear from published descriptions to 
