MUSCULAR SYSTEM 609 
muscles extending from the pro-acetabular ilium to the knee. When 
typically developed, it arises with a short tendon from the outer face 
and the apex of the pectineal process or ilio-pubic spine and runs as 
along tendon between the insertion of the m. sartorius and the 
patella over the outer surface of the knee-joint, where it is covered 
by the origins of the m. flezor perforans and perforatus dig. tii., and 
the m. perforatus dig. vi., then perforating the lateral head of the m. 
peroneus superficialis, it lastly forms one of the heads of m. flex. 
perforat. 1, or vi. Its nerve-supply comes from the last branch but 
one of the middle crural plexus. This muscle is subject to many 
modifications, and upon their extremes were founded the two 
groups ANOMALOGONATE and HoOMALOGONATZ (see also ANATOMY, 
page 16). One of the functions of this peculiar muscle, which is 
similarly developed in Crocodiles, but absent or not differentiated 
from the ilio-tibial and ilio-femoral mass in other Vertebrates, is 
that its contraction closes the second and third toes.! 
M. caud-ilio-femoralis, when fully developed, consists of two 
parts, inserted by a single strong and ribbon-like tendon near the 
end of the first third of the hind face of the femur. The caudal 
part is longer than the other, and arises from the transverse processes 
of one or more caudal vertebre passing externally over the distal 
half of the ischium and pubis. The iliac part, which is the ‘‘accessory 
femoro-caudal ” of some writers, is more or less triangular and arises 
in most cases from the outer face of the distal half or mid-third of 
the pro-acetabular ilium. This double-headed condition is the most 
primitive and obtains in most NipIFuG#&, but in many of them as 
well as in many of the Nrp1coL® either the caudal or the iliac head 
is absent, though in very few is the whole wanting. The absence of 
the caudal head may possibly be correlated with the strength of the 
1 Owen, in 1835, described (Cyclop. Anat, Physiol. i. p. 296) the disposition 
of this muscle, which he called the gracilis, as passing ‘‘ first, over the con- 
vexity of the knee-joint, and afterwards over the projection of the heel, [so] that 
from its connection with a flexor of the toes, these must necessarily be bent 
simultaneously with every inflection of the joints of the knee and ankle. As 
these inflections naturally take place when the lower extremities yield to the 
superincumbent weight of the body, birds are thus enabled to grasp the twigs on 
which they rest whilst sleeping, without making any muscular exertion.” This 
ingenious explanation of the perching and roosting of Birds was apparently first 
given by Borelli (De motu animalium, Rome: 1680-82), and has been copied 
and made much of by many subsequent writers, though Sundevall in 1851 drew 
attention to the faultiness of the idea, since the ambiens muscle is absent in such 
typical perching Birds as the Coccygomorphx and Passercs, while it is present in 
the Anseres. Elsewhere I have pointed out (Zhier-reich, Vogel, p. 148) that Birds 
possessing it can spread and stretch their toes freely while the leg is also 
extended because then only it is not interfered with, and accordingly it is fully 
developed in running, wading, swimming and rapacious Birds, but absent in 
those which hop and climb. 
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