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lying between the two muscles. For these several reasons we cannot 
consider also the M. pyramidalis as a portion of the M. obliquus 
internus, as the M. marsupialis is. From this it follows again that 
the M. pyramidalis and the M. marsupialis cannot be homologous. 
What then is the significance of the M pyramidalis of the Mono- 
delphys? An answer to this question will be found in the following 
record of an investigation into the mode of insertion of the M rectus 
abdominis in some mammalia. In Tarsius spectrum the Os pubis 
presents only a small place for insertion, which is narrower than the 
breadth of the muscle. The insertion is nevertheless effected over 
the whole breadth of the muscle because the inferior part of the 
M. rectus bends round anteriorly and laterally at the linea alba, 
so that a small triangular portion of the muscle is disposed in front 
of the larger unbent portion of the M rectus. This portion bears 
a close resemblance to a M. pyramidalis; however, it has not an 
insertion of its own into the linea alba, but at the linea alba its 
fibres merge directly into those of the M. rectus. We see then that 
in Tarsius the M. rectus has a U-shaped insertion. However in case, 
through some cause or other, such a triangular piece of muscle 
should become independent and obtain an insertion of its own into 
the linea alba, a M. pyramidalis is originated. In Insectivores such 
a case is encountered in the crossing of the two Mm recti, as des- 
eribed by Lrcun'). In the embryo the crossing commences in the 
most caudal part. The Mm recti draw near to the middle line, the 
right M. rectus splits up in two in order to allow the left M. rectus 
to pass (at Talpa europea) or otherwise both Mm. recti split and 
in this way a more complicate network is formed. The right M. 
rectus then inserts itself into the left Os pubis and the left M. rectus 
into the right Os pubis. As the embryo develops, the process 
continues proximally, until the full-grown condition is reached. In 
an investigation of embryos of Talpa I detected likewise a bending 
of the medial border of the M. reetus anteriorly and laterally ; now 
when the unbent parts of the rectus cross, the triangular anterior 
layer must of necessity be disconnected from the rest of the M. 
rectus and gets an insertion of its own, in other words it becomes 
a M. pyramendalis. The impossibility of a connection between bent 
and unbent pieces of the M. rectus is dictated here by the circum- 
stance that the medial border of the M. rectus, along which the 
bending took place anteriorly afd laterally, is shifted, after the 
crossing, towards the opposite side. In embryos, in which the 
_ 1) Lecue. Zur Anatomie der Beckenregion der Insectivera. Kon. Svenska Vet. 
Akad. Handl. Bd 20, NO. 4, 1883. 
