LATERAL MUSCLE IN TELEOSTET. 339 
Fourthly, that at the outermost extremity of myomere B6 (at 
the point where it is cut by the index line of B7) we find, very 
nearly *, the apex of one of the external backwardly-directed 
V-shaped myocommata. It has been pointed out that an 
imaginary line joining such apices forms the division between 
the mesio-dorsal and latero-dorsal portions of the surface area. 
lt is now apparent that any deep continuation of this superficial 
division must also be guided by expediency; for, internally to 
our superficial guide-point, we find a <-shaped area (the section 
of a hollow pyramid), which may be regarded in its entirety as 
part of a dorsal, posteriorly-directed pyramid, and so of the mesio- 
dorsal portion of the muscle; or, with equal justice, its lower 
arm may be regarded as part of the latero-dorsal portion, from 
the fact that its fibres follow the same direction as those in that 
portion. 
The question to be decided is this: granted that such a division 
of the lateral muscle is in either case an arbitrary one, is it 
expedient to frame it in terms of pyramidal masses or in terms 
of direction of fibres ? 
Before answering this question let us examine the fissure 
which divides the mesio-dorsal and the latero-dorsal portions 
near their anterior origins on the skull. It is found here that 
all the fibres above the cleft throughout its depth run in the 
same direction, namely, from above anteriorly to below posteriorly, 
and that all those below it take the opposite direction. That is 
to say, when they split apart, the two portions are not divided 
according to their pyramidal structure, but according to the 
direction of their fibres ; and this fact indicates that the division 
of the dorsal moiety of the lateral muscle into a mesio-dorsal 
longitudinal mass and a latero-dorsal longitudinal mass can most 
naturally be framed in terms of the direction of its component 
fibres. 
In text-fig. 3, B it has been pointed out that the index-line to 
the 7th myomere cuts the skin approximately at a point in the 
superficial line of division of the mesio-dorsal and latero-dorsal 
portions. From this point inwards a zigzag line has been drawn 
to indicate the arbitrary internal division- plane between the 
mesio-dorsal and latero-dorsal portions; above this plane the 
fibres are inclined upwards and forwards, below it they are 
inclined downwards and forwards. In A the superficial division 
can be found by bisecting the line of skin between the two 
portions of the myocomma which encloses the 6th myomere ; 
from this point the internal division-plane can be traced as 
min. 1B 
It will now be realised that by somewhat devious paths we 
have reached Gegenbaur’s position with regard to the structure 
of the dorsal moiety of the lateral muscle (of. p. 320). Our 
arbitrary division leaves us with a series of incomplete cones, or 
* Tn the section next in front of this the apex of the sixth myomere is actually m 
the surface. 
