Bauhaus-Universität Weimar

Titel:
The Cyclopaedia of Anatomy and Physiology, vol. 2: Dia-Ins
Person:
Todd, Robert Bentley
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit25760/946/
938 
INSECTA. 
rectus. These oblique muscles are the anta¬ 
gonists of the recti, and when acting alone 
draw the posterior part of each segment back¬ 
wards and to the median line ; consequently, 
when the layers of only one side are in action, 
the anterior part of the body is flexed laterally 
in the form of a curve, but when those on both 
sides are in action the anterior part of the body 
is carried directly backwards. Beneath these 
oblique muscles there is another, which is 
formed of only two broad fibres. It arises 
from the anterior of the three ridges of attach¬ 
ment in the median line, and passing diago¬ 
nally forwards and outwards parallel to the third 
oblique, beneath its inner margin, is attached 
to the third ridge of insertion. This may be 
called the fourth, oblique (11). Beneath the 
posterior extremity of this muscle lies the 
third rectus (12), which is formed of three 
fibres, somewhat broader than those of the 
second or smaller rectus, but running longitu¬ 
dinally in exactly the same direction, and hav¬ 
ing the same origin and insertion. On re¬ 
moving the third rectus we expose the eighth 
layer of muscular fibres. This arises from the 
anterior ridge, and is formed of three broad 
fibres which are partially crossed at their origin 
by the third rectus. It passes diagonally out¬ 
wards, and is attached to the third ridge, extend¬ 
ing as far as the margin of the rectus and third 
oblique, by which it is concealed. This is the 
fifth oblique ( 13). When this layer is removed, 
the triangular and transverse m uscles are exposed. 
The triangularis (14) is composed of nine dis¬ 
tinct fibres, which originate in a longitudinal 
series that extends about half-way across the 
segment. The fibres pass from their origin 
diagonally backwards and outwards, with vary¬ 
ing degrees of obliquity, and are inserted by 
strong tendons into the anterior of the three 
transverse ridges (16). They arise also by 
distinct tendons, which indigitate with a set 
of short transverse fibres, ten in number, and 
which occupy the median line beneath the ner¬ 
vous cord, and form the transversus médius (15). 
This muscle contracts the diameter of the mid¬ 
dle of the under surface of a segment. The 
triangularis, when acting with its fellow of the 
opposite side, shortens the posterior half of 
the ventral surface of the segment ; but when 
acting singly, or in conjunction with the third 
oblique, shortens that side of the segment, and 
assists to bend the body laterally. It is a very 
powerful muscle in locomotion, and probably 
is of great use in shortening and contracting 
the segments during the transformations. The 
transversi abdominales (17) are six short broad 
and thick fibres, that form two sets, and 
originate at some distance from the median 
line, posteriorly to and on the outer side of 
the tendons of the third oblique, and passing 
transversely outwards are inserted directly into 
the tegument, about half-way across the segment. 
Like the transversus médius they contract the 
diameter of the ventral surface of the segment, 
and bring the sides towards the median line. 
Anteriorly to these muscles, but further from 
the median line, is another set of six short 
fibres, the abdominales anteriores (18), which 
arise at some distance from the median line, 
and passing transversely outwards are inserted 
into the lateral part of the segment. The 
abdominales laterales (19) are situated in the 
posterior half of the segment. They are in¬ 
serted by three great fasciculi of narrow ten¬ 
dons into the inner and inferior part of the 
segment, and consist of eight muscular fibres 
connected in the first tendon, four in the se¬ 
cond, and seven in the third. They form very 
powerful muscles, which interlace with each 
other, and originate directly from the tegu¬ 
ment of the sides of the segment, at different 
distances posteriorly to the spiracle. Some of 
them (20) are much longer than others, and 
the whole of them are connected with the false 
feet of the abdomen. On removing these 
muscles we expose the attachment of the ob- 
liquus posterior (21), which is composed of 
nine small fibres that pass diagonally outwards 
from their origin, the anterior ridge, to their 
insertion in the tegument at different distances 
beneath the lateral abdominal muscles. Ano¬ 
ther set of nine distinct fibres, the postéro¬ 
latérales ob liqui (22), which originate from the 
same ridge at the lateral pail of the segment, 
cross over the last lateral abdominal muscle, 
and are inserted between it and the one im¬ 
mediately before it. Besides these layers of 
fibres there are four other sets which seem to 
be particularly concerned in the function of 
respiration. Of these the ti'ansversus lateralis 
(28) arises tendinous from beneath the lateral 
part of the great rectus, and passing upwards, 
internal to the great longitudinal trachea (£), 
which it crosses, is inserted beneath the ex¬ 
ternal margin of the dorsal rectus (A). The 
second transversus lateralis (24) arises pos¬ 
teriorly to the first, crosses the trachea, and 
continuing its course upwards is inserted into 
the tegument of the back, about half-way 
across the dorsal rectus. These muscles ap¬ 
pear to be directly concerned in contracting 
the segments during expiration. Besides these 
muscles there are also the retractor spiraculi 
and the retractor valvulce, which belong also to 
the ventral and lateral surface of each segment. 
The retractor spiraculi (25) is attached by a 
long tendon (26) to the third ridge of insertion. 
It is a long, fleshy, and somewhat conical mus¬ 
cle, which passes upwards and obliquely back¬ 
wards to the spiracle, unto the lower margin 
of which it is attached, and is blended with 
the circular fibres that constitute an orbicular 
muscle to that orifice. It appears to be di¬ 
rectly concerned in forcible expiration, and 
draws the spiracle inwards and downwards, 
and when the orbicular muscle acts in con¬ 
junction with it assists in closing the spiracle. 
The remaining muscle, the retractor valvulæ 
(27), is the direct antagonist of the last. It 
is composed of five distinct fibres (fig. 401, c), 
which arise from the posterior margin of the 
spiracle, and partly also from the attachment 
of the retractor spiraculi (e), and then, con¬ 
verging, end in a tendon that passes diagonally 
upwards and backwards, and is inserted into 
a little elevation in the tegument. It is the 
proper retractor or levator muscle of the spira-
        

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