THE NERVOUS SYSTEM 44o 



the white rami communicantes to the vertebral ganglia and thence to 

 visceral receptors. Axones of splanchnic efferent spinal neurones 

 (preganglionic fibres) pass from the spinal cord via the ventral roots 

 and white rami communicantes to terminate in various sympathetic 

 ganglia. The axones of the cells of these ganglia (postganglionic 

 fibres) complete the path by passing to the splanchnic effectors (heart 

 muscle, smooth muscle or gland). Thus while the somatic efferent 

 peripheral path consists of only one neurone, whose body is in the 

 ventral gray of the cord or brain, and whose axone passes without 

 interruption to the striated voluntary muscle, the splanchnic efferent 

 peripheral path (except the branchiomotor) consists of two neurones. 

 The first neurone body is in the gray of the cord or brain, the second 

 neurone body is in a sympathetic ganglion. The preganglionic 

 fibres emerging in the seventh cervical to the third lumbar spinal 

 roots end either in the vertebral or prevertebral (cocliac, superior 

 mesenteric, inferior mesenteric) ganglia. The vertebral ganglia send 

 part of their axones to the peripheral nerves (gray rami communicantes) , 

 thence passing to superficial splanchnic eft'ectors (smooth muscle of 

 hairs and of superficial blood-vessels and glands of skin). Other 

 axones may pass to the head. The axones of the prevertebral 

 ganglia pass to the glands and smooth muscle of the viscera (Fig. 

 310). The other segments of the cord do not send out white rami 

 communicantes except the second, third and fourth sacral. These 

 fibres do not communicate with vertebral ganglia but terminate in 

 prevertebral (pelvic) ganglia, which in turn send axones to the lower 

 colon, rectum, bladder and genitals. In the head, preganglionic 

 fibres emerge with the third nerve and pass to the cihary ganglion, 

 thence as postganglionic fibres to the ciliary and sphincter iridis 

 muscles of the eye; others emerge with the seventh and pass to the 

 sphenopalatine and submaxillary ganglia; others with the ninth and 

 pass to the otic ganglion. Many of the axones of these ganglia pass 

 to the salivary gland, others may be vasomotor. The vagus nerve 

 sends preganglionic fibres to most of the viscera. These fibres prob- 

 ably terminate in the peripheral sympathetic gangha. There is 

 thus a cranial (III, VII, IX and X nerves), thoraco-lumbar and sacral 

 outflow of preganglionic fibres. Most visceral structures appear to 

 receive a double innervation, one from the thoraco-lumbar and one 

 from either the cranial or sacral system. By some writers the term 

 sympathetic has been restricted to the thoraco-lumbar, the two others 

 being termed autonomic. 



