100 



THE ELASMOBRANCH FISHES 



The third or hyoidean somite is simpler than the rest, and from its lower part 

 the remainder of the external or posterior rectus muscle (p.r.) arises. The 

 relations of the muscles to the somites and nerve supply are tabulated by Neal 

 as follows : 



Myotome Muscle Innervated by 



( Rectus superior Oculomotor nerve III 



2q . J 



) Rectus internus Oculomotor nerve III 



(Rectus inferior Oculomotor nerve III 



Iv J 



)Obliquus inferior Oculomotor nerve III 



2d Obliquus superior Trochlearis nerve IV 



2v Rectus externus) ,_ ^ 



„ -r. X X ^ Abducens nerve v I 



3v Eectus externusl 



The position occupied by the inferior oblique muscle of the adult varies 

 somewhat in different forms. In Heptanchns the inferior oblique is attached to 

 the orbit practically against the superior oblique, while in Acanthias the two 

 maj^ be slightly apart. In the saw shark, Pristiophorus, an interesting condi- 



Fig. 102. Diagram showing the head somites in relation to the body somites in Squahis 

 acanthias. (From Neal.) The eye muscles are derived from the first three somites. 



hyp., hypobranchial (hypoglossus) musculature; m., mouth; ot., otic capsule; III and VI, 

 third and sixth head somites. 



tion obtains in which a second strip of the inferior oblique is attached along 

 the infraorbital plate. This is evidently a forerunner of the condition present 

 in the rays, in which the whole of the muscle has its attachment as a broad 

 band ventral to the orbit {Ehinohatis, Raja). In this respect, then. Prist io- 

 phorus is a transitional form between the sharks and the rays. 



Muscles op Visceral Arches 



The levator lahii siiperioris muscle in a type like Acanthias {Us., fig. 104) 

 consists of a single muscle on each side, which has its origin on the interorbital 

 space ventral to the cranium. The muscle passes l)ackward and outward along 

 the upper jaw and over the dorsal labial cartilages. At the angles of the mouth 

 it becomes tendinous and is inserted on the mandible. In all probability, this 

 muscle is comparable to Us. of Heptanchns. In Torpedo (fig. 105a) (see 

 Tiesing, 1896), the muscle is divided into two parts, a median (Us?) and a 



