THE ELASMOBRANCH FISHES 



99 



lamellae, is of immense size. In side view the Vs would appear very long and 

 acute, many of them being cut in transverse section. 



We may next consider the specialized or long muscles of the adult which 

 are present in tlie hccul ami in the ])liaryngeal region. 



Muscles of the Eye 



The muscles of the eye originate from head somites (fig. 102) which in a way 

 are like the body segments or somites previously described. The first or pre- 

 mandibular somite, the second or mandibular, and the third or hyoidean 

 somite, all take part in tlie formation of these nuiscles. It will be observed 



Fig. 100 Fig. 101 



Fig. 100. Trunk musculature, Zygaena side view. (From Maurer.) 

 Fig. 101. Transverse section, muscle bundles of tail, Lamna. (From Ewart.) 



d.b., dorsal bundles; dl., dorsolateral bundle; dm., dorsomedian bundles; I.b., lateral 

 bundle; II., lateral line; r.a., rectus abdominis; v.b., ventral bundle; vL, ventrolateral 

 bundle; vm., ventromedian bundle; x, part of ventral bundle removed to show overlapping 

 of lateral bundle. 



from the figure that somites four, five, and six, which are in the region near 

 the enlarging ear capsules, degenerate and consequently take no part in the 

 formation of eye muscles. 



The most anterior of these head somites, the premandibular,^ gives rise to 

 four of the six muscles of the eye. The first muscle to bud off from this somite, 

 arising ventrally as shown by the ruled lines in figure 103 (Lamb, 1902) is 

 the inferior oblique (i.o.). Following this the inferior rectus (i.r.) arises also 

 from the ventral part of the somite. From the dorsal side the internal or an- 

 terior rectus (a.r.) and the superior rectus (s.r.) are budded off. The second 

 or mandibular somite divides into two parts, a small upper part and a larger 

 lower part. From the upper part the superior oblique muscle (s.o.) arises. 

 The cells which form this muscle first take a longitudinal direction; later, as 

 they become muscle fibers, they lie more in a dorsoventral direction. From the 

 ventral part of the second somite a part of the rectus externus arises (Neal). 



1 In front of the premandibular has been described (Piatt, 1891) still another somite 

 (Acanthias) . 



