The Nerves and Muscles of the Leg 
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In the embryo as in the adult a special nerve is given to each portion 
of the muscle. The nerve to the more proximal part arises from a more 
distal set of spinal nerves than that to the more distal part. It gives rise 
to branches which enter between the bundles of the proximal portion of 
the muscle about midway between the tendon of origin and transverse 
tendinous inscription. The more distal nerve enters the distal portion of 
the muscle by branches which have a similar distribution with respect to 
that portion. 
The semitendinosus is probably represented in urodeles in the (caudali)- 
pubi-ischio-tibialis and in reptiles by a portion of the flexor tibialis internus. 
In monotremes it arises with the semimembranosus from the tuber ischii, 
is inserted into the tibia, and is supplied both by the obturator and sciatic 
nerves. In the higher forms it is either single as in man, double as in 
several insectivores, or has two heads of origin, one of which usually springs 
from the tuber ischii, the other from the caudal vertebre. This last, accord- 
ing to W. Leche, is probably the most primitive condition. The tendinous 
inscription of the semitendinosus marks the region where the two heads join 
the common belly in this type of muscle. Humphrey believed the tendinous 
inscription to mark the place where in the lower vertebrates the caudo- 
crural joins somewhat perpendicularly the flexor musculature of the thigh. 
In most of the lower mammals and in all the apes the tendon of insertion 
sends fibrous expansions far down in the crural fascia and together with 
similar expansions from the biceps and gracilis helps to form a sheath for 
the tendon of Achilles (Parsons, 04). 
In man the semitendinosus and long head of the biceps sometimes arise 
independently from the ischium, a variation which is supposed by Le Double, 
97, to be a reversion to a primitive condition in which the two muscles were 
quite independent. Klaatsch, 02, on the other hand, states that in the 
lowest mammals the muscles are more closely united than in the higher. In 
the human embryo the two muscles are closely united from their earliest 
differentiation and the union extends relatively more distal than in the adult. 
The semitendinosus may be more or less fused in the adult with the semi- 
membranosus or connected by fasciculi with the long head of the biceps. 
The semitendinosus in the embryo extends more distally in the crus than 
is normal in the adult. The fascial extension of the tendon in the adult is, 
however, frequently well marked and may be muscular (Gruber, 86). 
Proximally the semitendinosus in man may be reinforced by fasciculi from 
the pelvis or coccyx. These fasciculi may join the muscle at its tendinous 
inscription (Le Double). In the normal development I have found nothing 
that seems to represent a “latent” caudal head of the muscle. It is note- 
worthy that the proximal segment of the semitendinosus is innervated by 
a more distal set of spinal nerves than the distal segment (see above). The 
proximal end of the biceps is likewise innervated by a more distal set of 
spinal nerves than the distal end of that muscle. The proximal ends of 
these two muscles may therefore represent a caudo-femoral anlage shifted 
distally into the thigh. 
