164 LECTURE VII. 



ments or flakes are then easily separated, as we jSnd in carving a cod 

 or salmon at table. The vegetative similarity of the myocommata of 

 the trunk has led to their being described, by an abuse of synthesis, 

 as parts of one individual ' great side-muscle ' *, extending from the 

 occiput and scapular arch to the bases of the caudal fin-rays. The 

 modifications of the cranial vertebrae impress corresponding changes 

 on their muscular segments, and the essential individuality of these 

 segments has, on the other hand, been lost sight of, through the 

 opposite excess of analytic separation : special names are, however, 

 conveniently applied to their constituent, and in fact often separated 

 and independently acting, fasciculi. 



The fibres of each myocomma of the trunk run straight and nearly 

 horizontally from one septum to the next ; but they are peculiarly 

 grouped, so as usually to form semi-conical masses, of which the 

 upper («), and lower (b), have their apices turned backwards ; whilst 

 a middle cone (c), formed by the contiguous parts of the preceding, 

 has its apex directed forwards ; this fits into the interspace between 

 the antecedent upper and lower cones, the apices of which recipro- 

 cally enter the depressions in the succeeding segment, and thus all 

 the segments are firmly locked together, their general direction 

 being from without obliquely inwards and backwards, and their peri- 

 pheral borders describing the zig-zag course represented in Jiff. 44, 

 in which one myocomma is represented partly detached, and others 

 quite removed from the side of the abdomen. Thus, guided by the 

 fundamental segmental type of the vertebrate structure, we come 

 to recognise the ' grand muscle lateral ' of Cuvier, as a group of 

 essentially distinct vertical masses or segments. A superficial vicAV 

 of these segments, or an artificial analysis, has led to their being 

 regarded as forming a series of horizontal muscles extending length- 

 wise from the head to the tail : the upper portions (a) of the myo- 

 commata being grouped together, and described as a dorsal longi- 

 tudinal muscle with tendinous intersections directed downwards and 

 backwards; the lower portions (Z>) as a ventral longitudinal muscle, with 

 tendinous intersections directed downwards and forwards, whilst the 

 margins of the middle portions of the myocommata (c) being curved, 

 and usually bisected by the lateral mucous line, have been taken as 

 indications of two intermediate longitudinal muscles. In the Sharks, 

 indeed, instead of a curve the margins of the middle portions of the 

 myocommata form an angle with the apex turned forwards ; and in 

 the Rays the dorsal segments of the myocommata have actually 



* " Des grands muscles lateruu.r du tronc. II n'y en a essentiellement qu'un de 

 chaque c6t6." (xxiii. i, p. 287.) 



