No. 2.] VENTRAL ABDOMINAL WALLS IN MAN. 355 
muscles. In a segmental muscle like the frog’s rectus abdomi- 
nis, irritation of the nerve supplying one segment not only 
makes this contract but the rest of the muscle also, showing 
that there must be an anastomosis between the nerves supply- 
ing the different segments.! Therefore, the irritation of the 
nerve passing to a single segment sets the whole muscle to 
contracting. The heart is affected in a similar way, for irri- 
tation of the minutest sympathetic twig has the same effect 
upon the heart beat as the irritation of its main trunk.2, And 
the same is true regarding the influence of the splanchnic 
nerve upon the muscle walls of the vena portae.? 
In addition to the physiological verification of these nerve 
plexuses we have ample confirmation of them by Nussbaum, 
and very recently again by von Bardeleben and Frohse# for a 
number of muscles in man. So every bundle of nerves which 
leaves the spinal cord to pass to a muscle has a chance to mix 
its fibers once, twice, or three times, as the case may be, 
before it reaches the muscle fibers, and a nerve stimulus may 
pass to a given muscle ina number of ways and from a number 
of sources. 
The plexus within the muscle will, no doubt, prove to be a 
valuable object for investigation, but it does not concern us much 
when we consider the origin of a given muscle. I have given 
it for the sake of completion and have discussed the subject 
somewhat extensively in order to show a number of difficulties 
in my way, as well as my standpoint in the present problem. 
Development of the Rectus. — In the earliest human embryos 
which have been studied it is observed that the allantoic stalk 
and the umbilical vesicle are located much nearer the head than 
in the adult. The relation of these organs to the body of the 
embryo is shown in the accompanying figure, which is taken 
from an embryo not over two weeks old.® It’ will be noticed 
1 Mays, Ueber die Nervatur d. Musculus rectus abdominis d. Frosches, Heidel- 
berg, 1886. 2 Johansson, Archiv f. Physiologie, 1897, p. 103. 
8 Mall, Archiv f. Physiologie, 1892, p. 423; and Johns Hopkins Hospital 
Reports, vol. i, p. 148. 
4 von Bardeleben and Frohse, Verhandl. d. Anat. Ges., 1897, p. 38. 
5 The anatomy of this embryo is fully described by me in the Journ. of Morph., 
vol. xii, p. 417, and in His’s Archiv, Supplement Band, 1897. 
