COMPARATIVE ANATOMY OF THE MUSCULAR SYSTEM 133 



connective tissue partition (raphe). Slit up the raphe and find beneath it two 

 elongated parallel muscles, the geniohyoids, arising from the tip of the mandible 

 and inserted on the fascia of the sternohyoid. The sternohyoid parts on either 

 side of this place and passes around and dorsal to the posterior ends of the genio- 

 hyoids to be inserted on the hyoid arch. On each side of the geniohyoid a large 

 muscle, the external ceratohyoid, originates from the hyoid arch and passes pos- 

 teriorly around the angle of the jaw. The posterior end of the ceratohyoid is 

 covered by the posterior part of the mylohyoid, which is inserted on the surface 

 fascia of the ceratohyoid and on the gill region, on which the ceratohyoid is also 

 inserted. At the posterior end of the ceratohyoid is a fan-shaped muscle which 

 is inserted on the gill arches. This muscle, the levator branchiarum, serves to 

 lift the gills, while the ceratohyoid draws them forward. 



On the dorsal side of the head on either side of the median line is the temporal 

 muscle. Lateral to this is a large mass, the masseter muscle. Both are inserted 

 on the lower jaw at the angle of the jaw (the insertion of the temporal being con- 

 cealed by the masseter) and serve as elevators of the lower jaw. Separate the 

 masseter and ceratohyoid widely and find between and beneath them a smaller 

 muscle, the digastric, which extends from the skull to the angle of the lower jaw. 

 It is the depressor or opener of the lower jaw. Behind the digastric under the 

 posterior end of the ceratohyoid is a muscle slightly smaller than the digastric, 

 the levator arcuum, which passes to the gill arches and serves to lift them. 



Draw dorsal and ventral views of the head, shoulder girdle and gill arch 

 muscles. This region of urodeles is illustrated in K, page 137, Figure 146, and 

 W, page 247, Figure 58. 



D. THE MUSCLES OF THE CAT AND RABBIT 



In mammals those processes of change in the muscular system which we saw 

 beginning in the dogfish and progressing in Necturus reach a maximum. Owing 

 to the increase in size and strength of the limbs and the adoption of the habit of 

 elevating the body above the ground, the muscles associated with the limbs be- 

 come correspondingly increased in size and importance. We have already learned 

 that the girdle and limb muscles originate as small buds from the myotomes. 

 In mammals they have so increased in size and so spread from their original 

 locations that the original myotomes are scarcely recognizable in the adult, 

 although present in the embryo in the typical primitive arrangement. Further, 

 owing to the transformation of the gill arches the visceral muscles have under- 

 gone great changes. We shall attempt to point out to some extent the homology 

 between the muscles of the mammals and those of the lower forms. Muscles 

 become altered from the primitive segmented condition by processes of splitting, 

 fusion, and extension. 



For more complete details concerning the muscles than are given below see, 

 for the cat, R and J, and for the rabbit, B. The directions which follow are 



