1873.] CERTAIN MUSCLES OF BIRDS. 629 



back of the leg superficially. A small part of the main muscle, the 

 inner, goes straight forward to end at the inner side of the upper 

 extremity of the tibia by a flat tendon ; but most of it joins the ac- 

 cessorius to be continued down the leg. Some birds, as the Eagles 

 and Owls, have no semitendinosus at all ; some, as the Anserine 

 birds and Penguins, have no accessory simitendinosus, in which case 

 all the fibres go straight to the tibia-head ; whilst in most the above- 

 described condition maintains. 



Semimembranosus. — This ribbon muscle runs parallel to, deep of, 

 and next to the semitendinosus. It arises from the outer border 

 of the anterior margin of the ischium for about a third of an inch, 

 at the place where it is in contact with the pubis, the origin extending 

 down to the lower end of the slight prominence at the point where 

 the ischium slightly overlaps the pubis. It is inserted along with 

 the tibial end of the semitendinosus into the inner side of the head 

 of the tibia by a broad flat thin tendon. This muscle is very con- 

 stant in birds : in the Grebes it is extremely thin, and may sometimes 

 be absent, as stated by Sundevall ; but I have seen it in some fresh 

 specimens of Podiceps minor, though but very slightly developed. 



Ambiens. — This peculiar, small, but very long muscle is triangular 

 or fusiform in shape. It arises from the tip of the short anteriorly 

 directed spine which is situated just above the anterior border of the 

 acetabulum, and runs along the inner side of the thigh to the inner 

 side of the knee, where it is covered by the sartorius, which is above 

 it in the former part of its course. Its thin tendon then crosses the 

 knee, running in the substance of the fascial extensor tendon, just 

 in front of the patella, to the outer side, where it joins the fibres of 

 origin of the flexor perforatus digitorum. 



Femoro- caudal. — This long ribbon muscle is covered superficially 

 by the tensor fasciae and biceps above, as well as by the semiten- 

 dinosus lower down. The sciatic artery and nerve cross it super- 

 ficially at right angles close to its insertion as they course from the 

 sciatic foramen, parallel to the femur, to the popliteal region. The 

 femoral vein separates this muscle from the adductor muscles at their 

 insertions, except in Daceh, where it crosses the femoro-caudal su- 

 perficially*. It arises from the (anterior) transverse processes of the 

 two last coccygeal vertebrae, and is inserted into the linea aspera of 

 the femur, at about one third its length from the trochanter. 



An accessory head, arising from the upper three fourths of the post- 

 acetabular ridge, and from the ridge which forms the lower margin 

 of the origin of the obturator externus, joins the tendon of insertion of 

 this muscle, and is also partly inserted into the linea aspera, between 

 it and the head of the femur. It is thin, muscular, and broad, 

 covering the obturator externus superficially, and is partially inter- 

 sected by a fibrous sheet where it crosses its anterior border. The 

 sciatic artery and nerve cross it superficially ; and the nerve to the 

 semimembranosus is deep of it, whilst that to the semitendinosus is 

 superficial in some cases ; the biceps completely covers it. 



Of the above-described muscles, five of them (the ambiens, the 



* In Centropusphasianus the main artery of the leg is also the femoral, and not 

 the sciatic, as in other birds ; it therefore runs with the femoral vein in that bird. 



