Charles R. Bardeen 339 
Biceps, caput longum.—The long head of the biceps is differentiated 
from a special anlage which, near the ischial tuberosity, is closely fused 
with. that of the semitendinosus. This anlage is well marked in 
an embryo of 14 mm. (Plate VIII, Fig. 1) and the muscle is differen- 
tiated in one of 20 mm. (Plate VIII, Fig. 3). To the anlage in the -14 
mm. embryo two nerves are given each of which is associated at its origin 
with corresponding nerves to the semitendinosus. In the 20 mm. embryo 
two nerves are likewise given to the muscle, but in this instance the 
nerves arise nearly in conjunction with one another from the tibial 
portion of the sciatic nerve. 
In the adult two nerves are commonly distributed to the muscle. One 
of these enters the proximal portion of the muscle, the other in the distal 
third. ‘The terminal branches of these nerves are distributed across the 
muscle bundles of the biceps about midway between their tendons of 
origin and insertion, but nearer the proximal than the distal tendon. The 
more distal nerve sends back recurrent branches across the muscle bundles 
when the more proximal nerve is absent or ill developed. 
Biceps, caput breve. See p. 332. 
The long head of the biceps or lateral crural flexor is probably repre- 
sented in the urodeles by the ischio-flexorius and in reptiles by the flexor 
tibialis externus. In the mammals it usually arises from the tuber ischii and 
is inserted into the tibia or into the fascia of the leg, often as far as the 
foot. In marsupials it arises from the tuber ischii and the caudal vertebre. 
In the carnivora and some of the other mammals it has occasionally a double 
origin. As in the case of the semitendinosus the caudal origin of this muscle 
is looked upon, however, by many investigators as a caudo-femoral muscle in- 
‘serted into the lateral flexor rather than as a true head of the muscle. Ac- 
cording to Testut the long head of the biceps represents a muscle which 
primitively arose from the ilium and the coccyx. The sacrotuberosal liga- 
ment represents a transformation of that portion of the muscle which origi- 
nally extended between the ilio-sacro-caudal region and the present origin of 
the muscle from the tuber so that the ligament may be looked upon as the 
tendon of insertion of the muscle. In the human embryo the ligament de- 
velops after the anlages of the ischial tuberosity and the long head of the 
biceps: have appeared. It apparently is differentiated from the tuber ischii 
toward the ilium, sacrum, and coccyx. In the human adult fasciculi from 
the coccyx, sacrum, or sacrotuberosal ligament to the head of the biceps are 
frequent. 
The distal insertion of the muscle in most of the lower mammals takes 
place further down the leg than in man. In most of the lower mammals, 
according to Parsons, 04, aS mentioned above, extensions from the tendons 
of the semitendinosus, gracilis, and biceps into the crural fascia serve to 
form a sheath for the tendon of Achilles. According to A. Forster, 03, in 
foetuses the insertion of the biceps takes place into the sural fascia and even 
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