118 Cc. H. DANFORTH 
spiracular canal in front. With the preceding muscle it fills out 
the side of the face. It is inserted on the anterior aspect of the 
hyomandibular, from the lateral margin of the spiracular canal 
throughout the middle third of the cartilage. Toward their 
insertion the fiber bundles tend to become grouped and tendinous. 
Innervation. In a dissection of an adult the rather large nerve 
of supply clearly comes from the inferior maxillary division of 
the trigeminus. Serial sections seem to show it arising from the 
undivided main stem as Vetter suggests it may do in Acipenser. 
Perhaps its exact point of origin is subject to some variation. 
Blood supply. It is supplied by small twigs from the hyo- 
opercular artery, which pass over the hyomandibular cartilage 
and from branches of the external carotid anterior to the facial 
canal. The venous supply apparently is by branches of ‘the 
jugular. 
Action. Contraction of this muscle tends to rotate the hyo- 
mandibular on its median articulation, swinging its distal end 
and the attached operculum forward and outward. 
The partial division of this muscle is of some interest, since 
the homologies of the levator arcuus palatini and dilator operculi 
in teleosts are rather uncertain. This question has been taken 
up from several points of view by Vetter (’78), MeMurrich (’85) 
and Allis (97). It does not seem profitable to discuss it here 
beyond calling attention to this one point. Apparently the hyo- 
mandibularis of Acipenser, which Vetter homologizes with the 
above-mentioned teleostean muscles, is a simple muscle, and it 
appears from his account that the trigeminal nerve passes through 
it. In Polyodon the parts on either side of the exit of the nerve 
are separated at their origin. If this separation should extend 
to the insertion there would result two muscles, one deep, the 
other superficial, and the independent action of each would be 
somewhat different from their combined action in the form 
of a protractor hyomandibularis. This observation merely 
indicates a possibility, or it may be, shows a tendency in the 
phylogeny of this muscle. 
