354 MALL. [Vou. XIV. 
One of the most complicating elements in the study of the 
development of a muscle is the formation of nerve plexuses. In 
those instances in which there is but little cross shifting in the 
movement of a muscle, its nerve remains single or nearly so. 
But when there is considerable cross shifting very early in the 
development we have the formation of a plexus. Furbringer 1 
has shown from comparative anatomical studies that there is a 
plexus formation in the nerves supplying the limbs which shift 
in development, and that the plexus formation indicates that 
the limb is wandering. In the development of the extremities 
in man it has been clearly pointed out by the method of recon- 
struction that the arm bud makes a rapid excursion from the 
head backward in the early embryo.? In so doing it appears to 
drag the nerves together, thus forming a plexus. The upper 
intercostal nerves do not shift and, therefore, do not form a 
plexus, while the lower intercostal nerves passing to the anterior 
abdominal wall shift in their development, and thus favor the 
plexus formation. ; 
In this shifting and mixing of nerves the portions of the 
muscle plates which they supply are also hopelessly mixed, and 
this I think accounts, in part at least, for the fact that the 
muscle buds are no longer defined in the arm and leg by buds. 
But after the muscles are fairly well outlined they move on to 
their destination without any more mixing of nerve fibers 
further than additions which may grow along the existing 
trunks from the spinal cord. The differentiation of the muscles 
in the extremities takes place after the plexus nearest the 
spinal cord is formed, thus permitting of a second group of 
nerve anastomoses in the extremity. This process in the fore- 
arm, for instance, accounts for the double nerve supply of some 
of the muscles in it. 
Physiologists have studied the distribution of nerves in 
muscles much more carefully than anatomists, and we have to 
thank Mays? for very careful description of nerve plexuses in 
1 Furbringer, Morph. Jahrb., Bd. v, p. 324. 
2 His, Abhandl. d. k. s. Ges. d. Wiss., Bd. xiv, p. 341, Taf. II; and Mall, Journ. 
of Morph., vol. v, p. 459, Pl. XXX. 
8 Mays, Zeit. f. Biol., vol. xx, p. 449. 
