THE SHOULDER CURDLE AND PECTORAL FIN OF FISHES. 
555 
series. There are, then, thirty-one bundles, of which the first takes origin from the 
superior face of the propterygium and is inserted into the propterygial radial. 
(2) Abductor Musculature. 
The abductor musculature is in the form of a fan-shaped mass which radiates from 
a considerable area of the coracoid, and, to a slight extent, from the coraco-scapular 
region, to the outer surface of the fin. The muscle mass is exceedingly dense ventrally ; 
the insertion here extends from the anterior border of the coracoid right round to the 
posterior border, where it lies parallel with the ventralmost bundle of the adductor 
series. The result of this formation is that the postaxialmost adductor fibres (fig. 10) 
actually act as abductors ; so that, when the muscle as a whole contracts, the lower 
(postaxial) border of the fin is curled inwards, while the upper and middle portions 
are drawn outwards. Moreover, the obliquity of the muscle, owing to the relative 
lowness of its origin, tends to draw the whole fin somewhat downwards. From the 
foregoing description of the adductor musculature it is apparent that the action of the 
latter is to rotate the fin back into its position of rest at the side of the body ; this 
upward and backward sweep of the fin must be largely assisted by the strongly 
developed second adductor bundle. 
(i) Abductor superficialis (abd.s.). 
The muscle takes origin dorsally from the antero-lateral face of the girdle in the 
coraco-scapular region, rather below the upper level of the glenoid border ; as it is 
traced ventrally the origin increases in depth, and finally spreads round the lower 
edge of the glenoid border and upon the posterior face of the coracoid (see fig. 10). 
The first two (preaxial) bundles are not divided into layers (see fig. 11), and are 
inserted into the propterygium and propterygial radial. The succeeding two bundles 
give their deepest fibres to the metapterygium, the more superficial fibres terminat- 
ing on the propterygial radial and in the corresponding bundles of the profundus 
system. The bundles thenceforward become less and less distinct. 
(ii) Abductor profundus (abd.p.). 
This muscle takes origin from the metapterygium and metapterygial radials, and 
is inserted proximally into the palisade-like radials and distally into the actinotrichia. 
The muscle bundles are very distinct, and, unlike those of the adductor system, run 
parallel with the palisade cartilages. 
TRANS. ROY. SOC. EDIN., VOL. LII, PART III (NO. 21). 
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