Page 629. 
199 ON CERTAIN MUSCLES OF BIRDS. 
prominence on the outer side of the fibula, about halfway down the leg. 
In the loop above this tendon, and consequently quite out of the way 
of compressing forces, one of the nerves to the leg and fout is con- 
tinued. In one or two birds, as Phaéthon, the biceps does not pass 
through any loop, but is inserted directly by a broad flat tendon into 
the upper part of the fibula. 
Semitendinosus.—This flat ribbon muscle runs amen parallel with 
the lower fibres of the biceps, just below it. Its origin is mostly 
from the tip of the transverse process of the first free coccygeal 
vertebra, and from the fibrous membrane between it and the inferior 
border of the ilium. Near its origin it, being superficial, curves over 
the posterior inferior angle of the ilium, and covers the inferior fibres 
of origin of the tensor fascie, running upwards and forwards towards 
the inner side of the head of the tibia, and so getting covered an- 
teriorly by the inserted end of the biceps. A rhomboidal sheet of 
muscle, arising from the anterior end of the linea aspera, descends 
to form an accessory head to this muscle, joining it anteriorly, on its 
outer side, by an oblique tendinous raphe, which continues down the 
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 acces- 
sorius 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 semitendinosus, 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 constant 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 
