164 Macuray Menmoriat Vouume. 
and that of the preceding segment, forms the oblique extensor already referred to 
(p. 162). 
The central muscles are all connected together by some of the fibres of one 
passing into the next, and the first, as already mentioned, arises from the thoracic 
slip No. 7. Thus the whole series of central muscles forms a continuous wavy 
longitudinal band extending from the thorax to the end of the abdomen, and sending 
off slips, acting as extensors, to the six anterior abdominal segments. 
After being cut and reflected the enveloping muscle is easily traced. In such a 
typical segment as the third its outer limb bends round the pleural region, reaches 
the ventral aspect, and is connected by an aponeurosis with the longitudinal flexor of 
the segment next in front, forming the external flexor already noticed: before passing 
into the aponeurosis it gives origin to the diverging bundle of fibres already noticed 
as the oblique flexor. The inner limb turns downwards, parallel with its fellow of the 
opposite side, and then sweeps backwards and becomes the longitudinal flexor of the 
next following segment but one. 
The enveloping muscle of the thorax takes a different course : its mer limb is 
normal, becoming the longitudinal flexor of the second abdominal segment; but its 
outer limb divides into two bundles, one of which passes into the transverse muscle 
of the thorax and so becomes continuous with its fellow of the opposite side, while 
the other sweeps backwards and inwards, round the central muscle, and joins the 
transverse muscle of the first abdominal segment. 
Two other enveloping muscles present peculiarities: the outer limb of that of 
the first abdominal segment goes to the sternum of its own segment instead of that 
next in front ; and the inner limb of that of the fifth segment goes to the sternum of 
the sixth. 
The transverse muscles of the abdominal segments are for the most part made up 
of three strands, best seen in the third and fourth segments. One strand is antero- 
dorsal in position and is formed by the union of fibres from the right and left central 
muscles of the segment next in front : another is antero-ventral and springs from the 
inner limbs of the enveloping muscles of the segment next in front: the third is 
posterior and somewhat dorsal in position and is derived from the central muscles of 
the segment itself. 
In the first segment the transverse muscle consists of three portions, two of 
which have been referred to separately. The dorsal portion is normal, springing from 
the central muscle ; the middle portion is derived from the outer limb of the thoracic 
enveloping muscle; the ventral portion springs from slip No. 7 of the thoracico- 
