MUSCULAR SYSTEM OF FISHES. 165 



become insulated from the middle ones, and metamorpliosed into a 

 continuous longitudinal muscle i^fig. 45. a) ; the change being essen- 

 tially the same as that which the bony segments themselves un- 

 44a dergo, when by anchylosis the sacral oi* cranial vertebra3 



are blended into a continuous longitudinal piece.* In 

 the Mackerel Professor Miiller found the middle fibres of 

 the caudal myocommata disposed in two entire cones : 

 fig. 44. a, is a transverse section of the tail to show the 



\i ' y' two concentric series of cut segments of the sheathed 



Caudal mus_ cones, on each side the spine, f 



'rei. In ordinary osseous Fishes the myocommata of one side 



are separated from those of the opposite side of the body by the 

 vertebra3, by the interneural and interhaemal aponeuroses, and by 

 the abdominal cavity and its proper walls (44, h. p). The ventral 

 portions recede from each other to give passage to the ventral fins 

 (v), and the ventral and lateral tracts sepai'ate to give passage to 

 the pectoral fins (p). 



From this part forwards, portions of the myocommata undergo 

 that change, analogous to anchylosis, which justifies their being 

 regarded as distinct longitudinal muscles : here the separated ventral 

 tract (subcoracoideus, d, f) derives a firmer origin from the ossified, 

 though slender hajmapophysis of the atlas (epicoracoid), when it 

 exists ; and, in consequence of the peculiar forward curve of the 

 strong hajmapophysis of the occiput (coracoid), it is not only ex- 

 panded but unusually elongated, in order to be inserted there. But 

 the serial homology of this fasciculus with the more normal ventral 

 portions of the succeeding myocommata, the ha^mapophysial attach- 

 ments of which have not risen above the aponeurotic state, is unmis- 

 takeable. The lateral portion of the anterior myocomma is attaclied 

 to the upper end of the coracoid and to the scapula ; the dorsal por- 

 tion to the supra-scapula, par-occipital and supra-occipital. We 

 recognise the dorsal portion of the posterior cranial myocomma in 

 the fasciculus called ' protractor scapulas ' («), which extends from 

 the supra-scapula forwards to the parietal and frontal cristoe ; and 

 the middle portion in that which is exposed by the removal of the 

 operculum, and which extends from the scapula to the mastoid in the 



* The continuatois of Cuvier group the portions of the myocommata above the 

 lateral line into three longitudinal muscles, compared respectively to the ' spinalis 

 dorsi,' 'longissimus dorsi,' and ' sacro-lumbalis ;' and the portions below the line 

 into two muscles, viz. 'oblitjuus abdominis,' and 'rectus abdominis.' (xiii. i. 

 pp. 305- 327.) : but Professor Miiller has well shown that the homologues of tho 

 obliqui abdominis do not exist in Fishes, (xxi. p. 223.) 



I Sec XXI. tab. ix. fig. IJ. 



M 3 



