Charles R. Bardeen 301 
sufficiently clearly the position of the various nerves and muscles. The 
muscles are all attached to the skeletal apparatus by distinct tendons. 
The main nerve trunks run in the connective tissue which serves to 
separate the various muscles from one another and to divide each muscle 
into its constituent parts. 
2. Individual Muscles. 
Ihopsoas muscle (Plate VI, Figs. 1 and 2). The iliopsoas muscle 
arises from a mass of tissue which embraces the femoral nerve as it 
passes into the limb-bud. In subsequent development the iliacus muscle 
spreads out over the ilium, the psoas major muscle extends up along 
the course of the roots of the femoral nerve to form its attachments to 
the vertebral column, and in close union the two muscles extend distally 
to be attached to the lesser trochanter. The psoas minor seems to be 
differentiated from the psoas major muscle anlage, but this is uncertain. 
To the psoas major as it is developed toward the vertebral column 
branches are given from the femoral nerve or its roots of origin as far 
anterior as the 22d spinal (2d lumbar) and occasionally as far as the 
21st spinal nerve. These branches extend in between the developing 
bundles of the muscle and have a complex, extensive distribution. 
The nerve to the psoas minor muscle frequently arises from the trunk 
of the genito-femoral or from the lumbo-inguinal branch of this nerve. 
To the ilacus muscle as it spreads out over the surface of the ilium 
several branches, often united in a plexiform manner, are given. These 
branches pass across or near the superficial surface of the muscle about 
midway between the crest of the ilium and the combined iliopsoas ten- 
don. Special nerve branches are likewise usually distributed from the 
‘main trunk of the femoral nerve to the fleshy portion of the muscle as 
it passes over the acetabulum and the head of the femur. 
There is considerable variation in the exact mode of distribution of 
the nerves mentioned. Frequently a special layer of the iliacus covers 
the nerves distributed to this muscle. The trunk of the femoral nerve 
may be divided by one or more bundles of the iliopsoas muscle. The 
variations in the distribution of the nerves to the iliopsoas muscle do not 
readily lend themselves to statistical treatment and hence this is here 
omitted. 
The iliopsoas is probably represented in the urodela and reptiles by the 
posterior portion of the pubi-ischio-femoralis internus and the anterior margin 
of the ilio-femoralis. The psoas muscle, which is phylogenetically younger 
than the iliacus, is by many (see Pardi, 92) considered to be a prevertebral 
muscle belonging primitively to the trunk musculature. Its ontogenetic de- 
