DR. PETT1GREW ON THE MECHANISM OF FLIGHT. 201 



themselves, and their articular surfaces present various degrees of spir.ility to meet tin 

 requirements of the muscular system. Between the straight muscles, therefore, arranged 

 in dorsal, ventral, and right and left lateral sets, and those which run in a more or less 

 transverse direction, and between the simple joint whose motion is confined to one plain 

 and the ball-and-socket joint Avhose movements arc universal, every de nv of obliquity 

 is found in the direction of the muscles, and every possible modification in the disposition 

 of the articular surfaces. In the fish the muscles are for the most part arranged in 

 dorsal, ventral, and lateral sets, which run longitudinally; ami, as a result, the move- 

 ments of the trunk, particularly towards the tail, are from side to side and -inuous. As, 

 however, oblique fibres are also pivsent, and the tendons of the longitudinal on s in some 

 instances cross obliquely towards the tail, the fish ha> also tin: power of tilting or twist- 

 ing its trunk (particularly the lower half) as well as the caudal tin*. The spinal column 

 facilitates the lateral sinuous twisting movements of the tail and trunk, from the fact 

 of the vertebrae composing it being united to each other by a series of modified universal 

 joints— the vertebra supplying the cup-shaped depressions or sockets, the intervertebral 



substance the prominence or ball. 



The same may be said of the general arrangement of the muschs in the trunk and tail 

 of the Cetacea, the principal muscles in this case being distributed, not on the sides, but 

 on the dorsal and ventral aspects. The lashing of the tail in the Whales is consequently 

 from above downwards or vertically, instead of from side to side. The spinal column is 

 jointed as in the fish, with this difference, that the vertebrae (especially towards the tail) 

 form the rounded prominences or ball, the meniscus or cup-shaped intervertebral plates 



the receptacles or socket. 



When limbs are present, the spine may be regarded as being ideally divided, the 

 spiral movements, under these circumstances, being thrown upon the extremities by typical 

 ball-and-socket joints occurring at the shoulder and pelvis. This is peculiarly the case in 

 the Seal, where the spirally sinuous movements of the spine are transferred directly to the 

 posterior extremities f. 



Mackerel 



the dorsal, ventral, and lateral longitudinal muscles-two of the. bemg found on Cher srde of ,he hsh and corre- 

 sponding to the myocommas or •■ grand m «*c,e lateral » of Cnvier. The muscular system of the fish would therefore 

 seem to be arranged on a fonrfold plan,_there being fonr se,< of longitudinal muscles, and a correspondmg number o 

 s.ightly oblique and oblique muscles, the oblique tnuseles being spiral in thor nature and tendmg ,0 cross or mtcrsect 



„ ,, • l t - „= ;t a^nPirs to me srmne rise to the mv< >commas and to that con- 

 at various angles, an arrest of the intersection, as it appears to me, 8 mng 



centric arrangement of their constant parts so evident on transverse section. This exposition of the mnscn ar 

 fibres to cros! each other at varions degrees of obliquity may also be traced in several parts of the Unman body, as or 

 instance, in the deltoid mnscle of the arm and the deep muscles of the e : , Nnmerous other examples of penintorm 

 mnseles might be adduced. Although the fibres of the myocommas have a more or less longitudinal direc ion, he 



° , . aan n M; nnP snir.l course from before backwards and from within outwards, i. e. Irom the 



mvocommas themselves pursue an oblique spiral course irum u 



spL towards the periphery, where they receive slightly oblique fibres front the longUudma. dorsal, ventral and atera 

 muscles As the spiral oblique mvocommas and the oblique fibres from ,he lo„g,,odm«l muscles ac dn-ectly and 

 indirectly upon the spines of the vertehr*, and the vertebra, themselves, to which they are speca ly adapted, and as both 

 sets of oblique fibres are geared by i uterdigitation to the fourfold set of longrtndmal muscles, the lateral, smuous. and 



rotatory movements of the fish referred to in the text arc readily accounted for. 

 t That the movements of the extremities primarily emanate from the sp.nc ,s rendered probable by the remarkable 



