Charles R. Bardeen Bon 
2. Individual Muscles. 
Adductor magnus.—See p. 313. That portion of this muscle which 
is attached to the distal end of the femur represents the praesemimem- 
branosus of the lower mammals and belongs primitively to the hamstring 
group. 
Semimembranosus.—This muscle arises from a special anlage in close 
association with that of the sciatic portion of the adductor magnus (Plate 
II, Fig. 3; Plate VIII, Fig. 1). The belly of the muscle becomes dis- 
tinct before the tendons. In an embryo of 20 mm. (Plate VIII, Fig. 3) 
there is a flat tendon of origin which is closely applied to the adductor 
magnus and which arises from the ischium. The tendon of insertion 
fuses with the tibial blastema near the back of the knee joint. The nerve 
enters near the center of the muscle belly. In the adult the nerve enters 
by several branches into the substance of the muscle about midway be- 
tween the tendinous attachments of the muscle bundles composing it. 
The superior branches curve upwards either on the surface or within 
the substance of the muscle. There is much individual variation in the 
exact mode of distribution of the branches of the nerve to this muscle. 
The semimembranosus is probably represented in urodeles by a part of the 
(caudali)-pubi-ischio-tibialis and in reptiles by a portion of the flexor tibialis 
internus. In most mammals it arises from the ischium or pubis, runs paral- 
lel with, and may be incompletely differentiated from the presemimembra- 
nosus, mentioned above in connection with the adductor magnus, and is 
inserted into the _ tibia. It may be fused with the semitendinosus. 
A. Forster, 03, has shown that although in the lower mammals the semi- 
membranosus is a flexor and may send a tendinous expansion to the plantar 
aponeurosis, in apes and monieys it is chiefly an internal rotator of the leg. 
In many mammals it is associated with a caudo-femoral (W. Leche) muscle 
which extends from the caudal vertebre to the distal end of the femur. 
In man it may be longitudinally doubled, may be partially fused with the 
adductor magnus or the semitendinosus and may arise from the ischial spine 
or the sacro-tuberosal ligament as well as from the tuber ischii. 
Semitendinosus.—This muscle is formed from two anlages, one of 
which is differentiated in close conjunction with the anlage of the ischial 
tuberosity, the other more distally. These anlages correspond with the 
two parts of the muscle found in the adult and to each a separate nerve 
is given (Plate VIII, Fig. 1). The anlages are visible in an embryo of 
14 mm. and the muscle is well differentiated in one of 20 mm. (Plate 
VIII, Fig. 3). In the latter the tendinous inscription which subdivides 
the muscle is as distinctly marked as in later life. The tendon of in- 
sertion is inserted relatively more distally in the 20 mm. embryo than in 
the adult. 
