THE NOTOCHORD AND ALIMENTARY CANAL 449 
with the supply of motor nerves to the alimentary canal; they form 
essentially an efferent gut-system in contradistinction to the sympa- 
thetic or skin-system. 
A marked distinction exists between these cranial and sacral 
nerves. The vagus never supplies the large intestine, the sacral 
nerves never supply the small intestine. Associated with the large 
intestine is the bladder, the whole system arising from the original 
cloacal region; the vagus never supplies the bladder, its motor 
nerves belong to the sacral outflow. The motor nerves to the 
ureters, to the urethra, and to the trigonal portion of the bladder 
between the ureters and the urethra, do not arise from the sacral 
outflow, but from the thoracico-lumbar. These muscles belong really 
to the muscles in connection with the Miillerian and Wolffian ducts 
and skin, not to the cloacal region. 
The motor innervation then of the alimentary canal reveals this 
striking and suggestive state of affairs. The motor innervation of 
the whole of the small intestine arises from the cranial region, and 
is immediately followed by an innervation from the sacral region for 
the whole of the muscles of the cloaca. It thus indicates a head- 
region and a tail-region in close contiguity, the whole of the spinal 
cord region between these two extremes being apparently unrepre- 
sented. Not, however, quite unrepresented, for Elliott has shown 
recently that the ileo-colic valve at the junction of the small and 
large intestine is in reality an ileo-colic sphincter muscle, and that 
this muscle receives its motor nerves neither from the vagus nor 
from the sacral nerves, but from the thoracico-lumbar outflow or 
sympathetic system. This may mean one of two things, either that 
a band of fibres belonging to the skin-system has been added to the 
gut-musculature, for the purpose of forming a sphincter at this spot, 
or that the region between the vagus territory and the cloaca is repre- 
sented by this small band of muscle. The second explanation seems 
to me the more probable of the two. Between the mesosomatic 
region represented by the vagus, and the cloacal region, there existed 
a small metasomatic region, represented by the pronephros, with its 
segmental duct, as already discussed in Chapter XII. That part of 
the new alimentary canal which belonged to this region is the short 
piece indicated by the ileo-colic sphincter, and innervated, therefore, 
from the same region as the organs derived from the segmental duct. 
Such innervation seems to me to suggest that originally the 
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