114 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



pelvis to the breast-bone, a sterno-hyoid from the sternum to 

 the hyoid region, and a genio-hyoid from the hyoid to the 

 extremity of the lower jaw. Similarly the oblique muscles may 

 be subdivided into three or more layers (internal and external 

 oblique, transverse, etc.); intercostals, bet ween the ribs; scalenes, 

 from the anterior ribs to the side of the neck, and sterno- and 

 cleido-mastoid from the breast-bone and clavicle to the skull. 

 In the dorsal half of the trunk also a large number of separate 

 muscles may be distinguished, spinales, between the spinal 

 processes of the vertebrae ; inter-transversales, between the 

 transverse processes ; longissimus dorsi, arising from the ribs 

 and transverse processes, and extending along the back (con- 

 tinued in the cranial region as the trachelo-mastoid) ; recti capi- 

 tis, etc. The muscles of the diaphragm are indirectly derived 

 from the ventral portion of the myotome. 



In the gill region of the branchiate vertebrates special mus- 

 cles are developed from the corresponding myotomes to open 

 (levator and depressor arcuum) and to close (constrictors) the 

 gill slits. With the loss of the gills these muscles change in 

 their functions, and become connected 

 with the hyoid or disappear. The jaws 

 are opened by a digastric muscle arising 

 from the base of the skull, and inserted 

 on the angle of the jaw, while closure 

 of the mouth is effected by adductors, 

 called masseter, temporalis, or pterygoid, 

 according to their origin from different 

 regions of the skull. 



The muscles which move the ball of 

 the eye are, in all vertebrates, six in 

 number, and are derived from the three 

 anterior head somites of van Wijhe (p. 

 in). The most anterior of these devel- 

 ops into three rectus muscles, supe- 

 rior, internal, and inferior, and into the 

 inferior oblique; the second furnishes 

 the superior oblique, and the third the external rectus. It is 

 interesting to note that the nerve supply of these muscles corre- 



FIG. 124. Eye muscles 

 and nerves in shark. a, 

 abducens; om, oculomotor; 

 t, trochlearis nerves ; oi, 0s, 

 inferior and superior oblique 

 muscles ; re, ri, rif, rs, ex- 

 ternal, internal, inferior, and 

 superior rectus muscles. 



