320 MR. E. W. SHANN ON THE 
grant in aid of research provided me with an image-erecting 
binocular microscope, invaluable in tracing the intricate courses 
of muscle-fibres, giving comprehensive stereoscopic views of 
the muscle such as are unobtainable by simpler forms of optical 
apparatus. 
HIsToRICAL. 
Owen (8) emphasizes the fact that the lateral muscle is essen- 
tially an aggregate structure formed of a series of transverse 
muscles (myomeres). The divisions of the lateral muscle which 
he cites are based on the directions of the myocommata (‘ ten- 
dinous insertions”); but he does not regard these as having any 
true morphological value. The carinales, however, are regarded 
as entities. I can find nothing in Owen’s writing to show that 
he regarded the lateral musele | as divisible into a layers. 
Humphr y (4) recognises a true division of the lateral muscle, 
by a septum passing inwards beneath the lateral line, into a 
dorsal and a ventral moiety. His subsequent divisions of these 
moieties are based on the directions of the myocommata; at the 
same time he states that when these divisions are traced forwards 
to their insertions they become completely severed one from the 
other, The red fibres occur superficially on either side of the 
lateral line. The latero-ventral portion of the muscle (see table) 
is divided by the direction of its component fibres into a super- 
ficial layer (obiquus externus) and a deep layer (ob. internus), 
but there is no septum between these two layers. In this portion 
a third layer was noted in certain Teleosts (Bream, Dace), which, 
from its position beneath the ribs, was regarded as a possible 
homologue of the transversalis layer of Amphibia. Similar 
changes in the direction of the fibres enabled Humphry to dis- 
tinguish three areas of the mesio-ventral portion, two superficial 
and one deep, but these again were not defined by fascie. 
Gegenbaur (8) also recognises a horizontal plane of division at 
the lateral line whereby a dorsal is separated from a ventral 
moiety. He proceeds to divide these moieties in terms of cones, 
complete and incomplete, which are revealed in sections of fishes. 
It is desirable here to draw attention to a discrepancy which is. 
displayed in the illustration fig. 276, A, which represents the 
time-honoured caudal section of a Mackerel, originally drawn 
by J. Miller, of which Owen (amongst others) has made use. 
Gegenbaur’s "description of this section is perfectly lucid, and 
more precise than Owen’s; but, unfortunately, he has added 
a diagram of the superficial arrangement of the myocommata 
(fig. 576, B), wherein, if we may judge by the lettering, he 
represents the upper cones (a) as identical with the cones seen 
in section in fig. 276, Aa. In reality the superficial cones are 
directed posteriorly and their apices represent the angles which 
the uppermost superficial fibres of the true cones make with 
fibres of the incomplete cones (which are directed downwards 
and backwards), whereas the true cones are directed anteriorly 
