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 

 a long 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. flexor perforans and perforatus dig. in., and 

 the m. perfwahis dig. ii., then perforating the lateral head of the m. 

 peroneus superficialis, it lastly forms one of the heads of m. flex. 

 perforat. ii. or Hi. 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 Anomalogonat^ and Homalogonat^ (see also Anatomy, 

 page 16). One of the functions of this peculiar muscle, which is 

 similarly developed in Crocodiles, but absent or not diflferentiated 

 from the ilio-tibial and ilio-femoral mass in other Vertebrates, is 

 that its contraction closes the second and third toes.^ 



M. caud-ilio-feinoralis, 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 vertebrae 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 NiDiruG.^, but in many of them as 

 well as in many of the NlDiCOL^ 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, Romee : 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 avibicns muscle is absent in such 

 typical perching Birds as the Coccygomorphs. and Fasseres, while it is present in 

 the Ansercs. Elsewhere I have pointed out {Thier-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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