448 THE ORIGIN OF VERTEBRATES 
Tf now for some cause the one set of muscles either disappeared, 
or were so altered as no longer to present any appearance of 
antagonism, then there would be left a single set of muscles, the 
inhibitory and motor nerves of which would leave the central 
nervous system at different levels, and the older such systems might 
be, the greater would be the modification in the shape and arrange- 
ments of parts in the animal, so that the two sets of fibres might 
ultimately arise from very different levels. 
As mentioned in the introductory chapter, the whole of this 
investigation into the origin of vertebrates arose from my work on 
the system of efferent nerves which innervate the vascular and 
visceral systems. One of the main points of that investigation 
was the proof that such nerves did not leave the central nervous 
system uniformly along the whole length of it, but in three great 
outflows, cranial, thoracico-lumbar, and sacral; there being two 
marked gaps separating the three outflows, caused by the inter- 
polation of the plexuses for the innervation of the anterior and 
posterior limbs respectively. All these nerves are characterized by 
the presence of ganglion-cells in their course to the periphery, they 
are, therefore, distinguished from ordinary motor nerves to striated 
muscle in that their impulses pass through a ganglion-cell before 
they reach the muscle. 
The ganglia of the large middle thoracico-lumbar outflow 
constitute the ganglia of the sympathetic system. 
The functions of the nerves constituting these three outflows are 
very different, as I pointed out in my original papers. Since then a 
large amount of further information has been obtained by various 
observers, especially Langley and Anderson, which enable the 
following statements to be made :— 
All the nerves which cause contraction of the unstriped muscles 
of the skin, whether pilomotor or not, all the nerves which cause 
secretion of sweat glands wherever situated, all the nerves which 
cause contraction or augmentation of the action of muscles belonging 
to the vascular system, all the nerves which are motor to the muscles 
belonging to all organs derived from the Wolffian and Millerian 
ducts, eg. the uterus, ureters, urethra, arise from the thoracico- 
lumbar outflow, never from the cranial or sacral outflows. It is 
essentially an efferent skin-system. 
On the other hand, the latter two sets of nerves are concerned 
