74 H. D. SENIOR 
interossea lies upon the lateral side of the tendon of the m. 
tibialis posterior (i.e., between tendon and malleolus) and then 
passes across the plantar end of the sinus tarsi to join the plantar 
rete. 
The a. peronaea posterior superficialis is placed anteriorly and 
laterally to the n. tibialis and les upon the posterior surface of 
the m. flexor hallucis longus. At the distal border of the muscle 
the artery lies near the a. interossea and interosseous membrane. 
In this situation it gives off a very large r. calcaneus lateralis 
which runs upon the lateral side of the tuber calcanei. This 
branch eventually forms the termination of the a. peronaea. 
The a. tibialis posterior superficialis follows the posterior 
surface of the n. tibialis until the latter bifurcates in the neigh- 
borhood of the medial malleolus. Here the artery is placed 
between the n. tibialis and the tendon of the flexor hallucis 
longus. The a. tibialis anterior, pars distalis, has relations 
identical with those of the adult. 
The relations existing between the various structures of the 
leg were ascertained by making a reconstruction which, from 
the fact that it yields but little information when entirely built 
up, is unsuitable for reproduction. The data gained from an 
examination of its separate parts are embodied in two diagrams 
(fig 7, A and B) which may serve as a rough guide to the inter- 
pretation of anomalies. These diagrams show, approximately, 
the course taken by the embryonic aa. poplitea profunda, 
interossea, and peronaea posterior superficialis. 
The order in which the various structures passing from the 
leg to foot are arranged in the hollow between the medial mal- 
leolus and tuber calcanei is as follows: m. tibialis anterior and 
a. interosseus together; m. flexor digitorum longus; a. peronaea 
posterior superficialis; n. tibialis, a. tibialis posterior super- 
ficialis, and m. flexor hallucis longus. 
Embryos of about this age are instructive mainly by reason 
of the fortunate circumstance that the individual muscles are 
differentiated before the continuity of the axial artery has 
been broken. Only one important change has occurred at this 
stage of development; this consists in the appearance of a com- 
