210 ANATOMY 01? VERTEBRATES. 



The lower portions of both muscles are shown in fig. 1.35, U, 15. 

 Of the inner pair of muscles, a portion of the deeper layer, dis- 

 posed so as to raise as well as addvict the pectoral fin, is shown at 

 iG.ficr. 1-37. Each muscle is inserted into the bases of the fin-ravs, 

 resolving itself into fasciculi and short tendons corresponding in 

 number with those rays : by diflFerent comljinations of action these 

 fasciculi di-sarioate or ajiproximatc tlie rays. 



The ischial basis of the ventral fins in abdominal fishes may be 

 moved a little forward or backward liy the action of the ' infra- 

 carinales ' according as they lie in front or behind the pelvis. 

 The latter, ' retractor ischii," fig. 131, lo, pass backward to the 

 vent, inclose it, and are continued to the base of the anal fin. The 

 protractor ischii, fig. 135, IS, passes forward to be attached to the 

 lower end of tlie coracoid. The protractors are short in thoracic 

 fishes, e. g., the Perch, and less distinct from the lower parts of 

 the myocommas than in ventral fishes, e. g., the Salmon. In 

 fishes, e. g., the Lophius, where the ischia are wide apart, there 

 is a transverse muscle to draw them together, and antagonise 

 the portions of the side muscles that tend to draw them further 

 apart. The muscles which act upon the ^•entral rays, like those 

 of the pectoral ones, form a pair, or two layers of slightly decus- 

 sating fibres, on both the outer and inner sides of the base of the 

 fin. The outer or inferior muscles, fig. 135, IG, 17, depress or 

 extend the ventral fins ; the opposite muscles raise or flex them. 

 Tlie ]iortion of the deeper depressor shown at 17, fig. 135, serves 

 to exjiand or dilate the ventrals. 



The movements of the rays of the median fins are effected 

 by three or four pairs of small muscles attached to each ray. 

 The superficial ones, fig. 131, .r, arising from the skin, are 

 inserted into the sides of the base of the dermoneural or dermo- 

 ha3mal spine. The deep ones, ib. y, arise from the interncural 

 or interhffimal spine, and are inserted into the base of the 

 dermoneural or dermoluxjmal spine : the anterior of these, fie 

 137, 3, erects the spine ; the posterior, ib. 4, depresses it. The 

 myocommas answering to the neural and hwmal spines of the 

 coalesced or suppressed centres of the terminal caudal vertebra\ 

 change their direction like those spines, slightly divero-ing from 

 the axis of the trunk to l)e inserted into them : these modified ter- 

 minal segments, liy their connection with the interlocked myo- 

 commas of the great lateral masses, concentrate the chief force 

 of those muscles upon the (\audal fin. The rays of this im- 

 portant fill are mo\'ed by three series ol' nniscles,' the one super- 

 ficial, the second deep-seated, tlie third interspinous. The 



