448 THE ORIGIN OF VERTEBRATES 



If 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 Miillerian 

 ducts, e.g. 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 



