32 



ANIMAL LOCOMOTION. 



oblique fibres are also present, and the tendons of the longi- 

 tudinal muscles in some instances cross obliquely towards the 

 tail, the fish has also the power of tilting or twisting its 

 trunk (particularly the lower half) as well as the caudal fin. 

 In a mackerel which I examined, the oblique muscles were 

 represented by the four lateral masses occurring between the 

 dorsal, ventral, and lateral longitudinal muscles — two of 

 these being found on either side of the fish, and corresponding 

 to the myocommas or " grand rmtscle laUmV^ of Cuvier. The 

 muscular system of the fish would therefore seem to be ar- 

 ranged on a fourfold plan, — there being four sets of longi- 

 tudinal muscles, and a corresponding number of slightly 

 oblique and oblique muscles, the oblique muscles being spiral 

 in their nature and tending to cross or intersect at various 

 angles, an arrest of the intersection, as it appears to me, 

 giving rise to the myocommas and to that concentric arrange- 

 ment of their constituent parts so evident on transverse 

 section. This tendency of the muscular fibres to cross 

 each other at various degrees of obliquity may also be traced 

 in several parts of the human body, as, for instance, in the 

 deltoid muscle of the arm and the deep muscles of the leg. 

 Numerous other examples of penniform muscles might be 

 adduced. Although the fibres of the myocommas have a 

 more or less longitudinal direction, the myocommas them- 

 selves pursue an oblique spiral course from before backwards 

 and from within outwards, i.e. from the spine towards the 

 periphery, where they receive slightly oblique fibres from the 

 longitudinal dorsal, ventral, and lateral muscles. As the 

 spiral oblique myocommas and the oblique fibres from the 

 longitudinal muscles act directly and indirectly upon the 

 spines of the vertebrae, and the vertebrae themselves to which 

 they are specially adapted, and as both sets of oblique fibres 

 are geared by interdigitation to the fourfold set of longitu- 

 dinal muscles, the lateral, sinuous, and rotatory movements of 

 the body and tail of the fish are readily accounted for. 

 The spinal column of the fish facilitates the lateral sinuous 

 twisting movements of the tail and trunk, from the fact that 

 the vertebrae composing it are united to each other by a series 

 of modified universal joints — the vertebrae supplying the cup- 



