THE MYOLOGY OF POLYODON 127 
Blood supply. They receive arteries from the recurrent bran- 
chials of their own gills. Their veins are tributaries of the infe- 
rior jugular. 
Action. Working in conjunction with the levator muscles they 
slightly increase the cavity of the pharynx. They also appar- 
ently tend to separate the gills from each other, especially in 
the case of the first and the fourth. 
MM. transversus dorsalis et transversus ventralis: figsures 6 and 7, 
m.tr.d., m.tr.v. 
There remain to be considered in this group two muscles, the 
transversi, of whose homologies I do not feel entirely certain. 
One is dorsal, the other ventral. Although entirely distinct from 
each other anteriorly, they become confluent posteriorly at their 
margins and form the proximal musculature of the oesophagus. 
Possibly we see here the phylogenetic origin of the upper striated 
muscle in the oesophagus of higher vertebrates. 
The transversus dorsalis (fig. 6, m.tr.d.), which is apparently 
unrepresented in Acipenser, is a thin flat muscle whose fibers 
extend transversely between the fourth epibranchial cartilages of 
the two sides. The muscle is attached to the inner side of each 
cartilage forward nearly to its anterior end. Posteriorly, where 
the line of their insertion is interrupted by the presence of the 
adductor IV, they are inserted in a strong membrane covering 
the latter. 
The individual fibers of the ventralis (fig. 7, m.tr.v.) do not 
cross the middle line but arise from a strong median aponeurosis 
and are inserted on the fifth ceratobranchial cartilages toward 
their posterior (distal) ends. The line of insertion does not 
extend forward beyond the posterior third of the cartilage. 
Innervation. Both muscles are supplied by small branches of 
the vagus. 
Blood supply. The dorsal muscle is supplied by the pharyngeal 
arteries, the ventral by the A. coraco-cardica. 
Action. They are decidedly constrictor in function, approxi- 
mating the cartilages with which they are connected and narrow- 
ing the cavity of the posterior part of the pharynx. 
