162 Macteay Memoriat Vouume. 
distinguished as the enveloping muscles, since they enclose certain other bands 
presently to be deseribed. Hach may be described as being continued into a meszal 
or zxner limb, which is in contact with its fellow of the opposite side, and a Zatera/ or 
outer limb, which immediately underlies the skin of the pleural region. The inner is 
considerably wider than the outer limb, so that in a sagittal section of the abdomen 
each enveloping muscle is seen to extend to a considerable distance into the next 
following segment. 
Immediately anterior to the enveloping muscle of the first five abdominal 
segments are two bundles of fibres, the cut ends of which are exposed by the removal 
of the terga, into which they are therefore inserted. The insertion of the dorsal of 
these bundles is above the inter-segmental hinge, and the muscle must therefore act 
as an extensor and not, like the greater part of the ventral muscles, as a flexor: it 
may be called the odfzgue extensor. As will be seen hereafter, it arises from what 
Milne-Edwards calls the wzascle central. The ventralmost of the two bundles is 
inserted, for the most part, below the inter-segmental articulation, and must therefore 
act as a flexor : it may be called the od/z7gue flexor. 
The enveloping muscles are, as already stated, paired, those of the right and left 
sides being distinet, but at about the middle of each segment there is a strong 
transverse band, covered by a shining aponeurotic sheath, passing between the 
enveloping muscles of the right and left sides. This is the ¢vansverse muscle of 
Milne-Edwards. In the fifth and sixth segments the aponeurotie band is of great 
width, extending, beyond the transverse muscle, from the middle of the fourth to the 
middle of the sixth segment. 
From each side of the transverse muscle of the first three segments there springs 
a narrow cylindrical band which passes outwards and slightly forwards and is inserted, 
along with the oblique extensor, into the tergum : we propose to eall it the accessory 
oblique extensor. 
Where the main ventral muscles are exposed from beneath by removal of the 
sterna and inter-sternal membranes and of the superficial flexors, there is seen on 
each side of the middle line of the second to the sixth segments a flat, slightly 
divergent band, inserted into the sternum near the middle line, and obviously acting 
as the main flexor. It is the muscle fléchisseur principal of Milne-Edwards, and may 
conveniently be distinguished as the longztudinal flexor. It is rather small and 
indistinct in the sixth segment: its representative in the first will be described 
hereafter. 
Immediately external to each longitudinal flexor is an oblique band of muscle, 
the fibres of which take a somewhat wavy course : this is the ventral portion of the 
