49 



The depression of the lower jaw is effected by the paired geniohyoid muscles, each 

 of which arises from the external surface of the hypohyal and ceratohyal, and is 

 inserted into the inner surface of the symphysis of the lower jaw, that is to say, of the 

 junction between the two mandibles of opposite sides. 



From the lateral surface of the parasphenoid passes on each side a broad band of 

 muscle outwards to the hyomandibular and pterygoid bones : these are the palatal 

 muscles which constrict the cavity of the mouth in swallowing. 



Behind the last there are other transverse muscles passing from the side of the skull 

 to the inner surface of the hyomandibular and operculum. 



A strong broad transverse muscle passes from the inner surface of one mandible 

 anteriorly to the inner surface of the other ; this is the musculus transversus mandibulce. 

 It lies immediately beneath the skin of the floor of the mouth, and assists considerably 

 in the biting action of the mandible of the left side. 



Most of these muscles, although chiefly concerned in seizing and swallowin? food, are 

 also used in inhaling water through the mouth for respiration. The foUowino- muscles 

 connected with the branchial arches are almost entirely respirator}'. 



The most superficial of them is the levator operculi, which rises from the ridge and 

 process of the pterotic and is inserted into the head and upper edge of the operculum : 

 it raises the gill-eover. 



The gill-arches are raised in inspiration by the levatores arcuum hranchialium, which 

 arise from the inner side of the articulation of the hyomandibular bone with the prootic 

 and opisthotie, and are inserted into the outer surfaces of the bony branchial arches 

 into the epi -branchial bones. 



Internal to the last are other muscles which pass from the inferior conical process of 

 the basi-occipital to the pharyngobranchials of the 2nd, 3rd, and 4th branchial 

 arches, that is, to the superior pharyngeal bones. The first pharyugobranchial bone 

 which is small and slender, lies along the side of the posterior end of the parasphenoid, 

 to which it is connected only by connective tissue. 



A powerful muscle posterior to the last passes from the lower surface of the anterior 

 vertebraj forwards to the dorsal surface of the superior pharyngeal bones : this muscle 

 retracts those bones, and is therefore inspiratory. 



On each side two muscles arise from the anterior surface of the clavicle and pass 

 upwards and forwards to the posterior surface of the inferior pharyngeal bone. As 

 this latter bone is destitute of gills, and all the gill clefts lie in front of it, its retraction 

 assists to open these clefts : these muscles are therefore inspiratory. 



Expiration is effected by all the muscles which constrict the cavity of the mouth and 

 throat, the jaws being shut first. In addition to some of those already mentioned the 

 following, which bring the branchial arches into contact with one another, are expiratory. 



Transverse muscles between the two inferior pharyngeal bones, and between the 

 cerato-branchials of the 4th branchial arch. 



Transverse muscles between the superior pharvnoeal bones. 



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