320 PROCEEDINGS OF THE CANADIAN INSTITUTE. 



IV.— MUSCLES OF THE HYOID ARCH. 

 1. Geniohyoideus, (No. 27, Cuv.) (Fig. 3, GH). 



This muscle which runs along the inner side of the ramus of 

 the mandible, may be exposed by removing the integument from 

 the lower surface of the skull and turning back the intermandibu- 

 laris which covers its insertion. It arises from the posterior 

 portion of the lower (ventral) and outer surfaces of the ceratohyal, 

 and also from the epihyal at the bases of the upper branch iostegal 

 rays. It passes forwards as a thick muscle, inclining slightly in- 

 wards towards its fellow of the opposite side, the inner fibres being 

 inserted into a median aponeurosis between the two, no interdigita- 

 tion occurring. The greater bulk of the muscle inclines outwards, 

 and is inserted into the posterior surface of the anterior part of the 

 ramus of the mandible, being partly covered by the intermandibu- 

 laris. Crossing the anterior portion of the muscle obliquely are two 

 tendinous bands, (Fig. 3, ti, ti 1 ), to which are attached the cartilagin- 

 ous supports of the tentacles of the under surface. 



Innervation. — R. hyoideo-mandibularis facialis. 



Action. — According as the hyoid or mandibular arches are fixed 

 this muscle acts in different ways. If the hyoid is fixed by the 

 hyoclavicularis it acts on the mandible, depressing it. This is its 

 usual action. If, however, the mandible is fixed by the powerful 

 add. mand., it raises the hyoid arch and through it the operculum, 

 thus aiding the lev. and dil. operc. Through the tendons which 

 pass across it, it is the means by which the tentacles resting on these 

 tendons move, but the range of motion thus imparted is very small. 



The simplicity of this muscle contrasts somewhat with what occurs 

 in Esox, and agrees more closely with the arrangement in Barbus. 

 In Cyprinus the origin is similar, and in Barbus the muscles of either 

 side do not interdigitate as they appear to do in other fishes. In 

 Esox and Cyprinus a median enlargement of the muscle occurs. The 

 tendinous bands are of course peculiar to the Siluroids. 



2. Hyohyoideus, (Nos. 28 and 29, Cuv. ; Lev. and Dep. branchios- 



tegarum, Ow.) 



This is exposed by the dissection required for the preceding with 



the removal of the integument from the branchiostegal rays. It 



may be considered as being composed of two portions, of which the 



