NECTURUS 3 



Muscles of the Head and Visceral Arches. Examine the lower 

 side of the head, making out the following muscles and sketching 

 each as you proceed. 



(1) The mylohyoid muscle extends across the ventral surface 

 of the lower jaw, its fibres running transversely. Can you dis- 

 tinguish any differences between its anterior and posterior 

 portions, in their mode of attachment to the halves (rami) of the 

 jaw? Cut the mylohyoid across in the median line, turn the 

 halves back and see beneath 



(2) The geniohyoid muscle, attached in front to the sym- 

 physis (tip) of the lower jaw and behind to the ventral fascia 

 (see below) of the region of the neck, its fibres running at right 

 angles to those of the mylohyoid. 



Attached at about the middle of the geniohyoid on either 

 side is (3) an external ceratohyoid muscle which runs obliquely 

 outwards and backwards. Follow it towards the dorsal side: 

 how far does it extend? and where is its insertion? To answer 

 this last question it will be necessary to remove a fan-shaped 

 muscle, (4) the levator branchiarum, from the side of the neck, 

 which is attached at its smaller end to the region of the gills. 

 The external ceratohyoid pulls the gills forward, the levator 

 lifts their bases. 



On the dorsal surface of the head are a pair of (5) temporalis 

 muscles, the line separating them continuing back the length of 

 the body. In what direction do the temporalis muscles run? 



Lateral to each temporalis is (6) a large masseter muscle, 

 which has its origin from the posterior and lateral regions of the 

 skull. Its fibres run parallel to the external ceratohyoid, to be 

 inserted on the lower jaw at the angle of the mouth. 



Separate the masseter and the external ceratohyoid with the 

 handle of the scalpel and cut across the levator branchiarum, 

 exposing the small underlying muscle, (7) the levator arcuum 

 branchiarum, which arises from the posterior part of the cranium 

 and is inserted on the branchial cartilages at the base of the 

 external gills. 



. Lateral and anterior to the levator arcuum is (8) the depressor 

 mandibulse, or digastric muscle, of about equal size. It is at- 

 tached at one end to the cranium, and its fibres, running parallel 

 to those of the external ceratohyoid, are inserted upon the angle 

 of the lower jaw. 



