322 PROCEEDINGS OF THE CANADIAN INSTITUTE. 
3. Hyoprsctoratis, (No. 1 Cuv. ; Retractor hyoidet, Ow.; Sterno- 
hyoid, Stan. et Vetter.) 
This muscle is exposed by removing the anterior portion of the 
hyohyoideus and the inner part of its posterior portion. It arises 
from the upper (dorsal) surface of the clavicle and from the strong 
ridge separating this muscle from the erector of the pectoral fin. It 
passes forwards, lying anteriorly on the upper surface of the urohyal, 
and being separated from its fellow by the median crest on that bone. 
It is inserted into the anterior portion of the urohyal below its 
small upper plate. (Fig. 4, Hy. P.) 
Innervation.— Branch from the united first and second spinal 
nerves. 
Action.—By its contraction it draws the anterior extremities of 
the hyoid arches downwards, and so enlarges the cavity of the mouth. 
In respiration the branchiostegal membrane closing the gill-slit, the 
action of this muscle will cause the flow of water into the mouth. 
This being then closed by the powerful add. mand., the hyo-pector- 
alis and hyohyoideus relaxes, and the geniohyoid then acting, draws 
the hyoid arches upwards and forces the water out by the gill-slit. 
I have ventured to indicate this muscle by a new name. That 
used by Vetter is not appropriate owing to the absence of any struc- 
ture which can be termed a sternum. Owen’s name also is faulty, 
gince the action is not so much to retract the hyoids as to depress 
their anterior extremities. The name applied above is analogous 
with those of the other muscles of the hyoid arch indicating its m- 
sertion and origin. 
V.—MUSCLES OF THE BRANCHIAL ARCHES. 
A.—VENTRAL MUSCLES. 
1. Muscutus Hyoprancuiatis, (No. 35 Cuv. ; Pharyngo-hyordeus, 
Pharyngo-arcualis and Interac. obl. vent. in part, Vetter). 
After having exposed the hyopectoralis, it should be cut through 
and reflected, and the fascia covering the under surface of the 
branchial arches then removed. The hyobranchialis (Fig. 4, HBr.) 
will then be seen as a stout muscle lying to the side of the 
median line on either side on the under surface of the branchial 
arches. It arises from the posterior surface of the hypohyoid by a 
round tendon, which is continued some distance backwards on the 
