PERIPHERAL AUTONOMIC MECHANISMS 



981 



may contain both pre- and postganglionic fibers and 

 Zuckerrnan (449) and Sheehan & Pick (341, 387) lia\e 

 shown that mix ed ra mi are not u ncommon. Further- 

 more, it has been shown by Wrete (445-447) and 

 other investigators (5, 341, 351, 392) that there are 

 intermediate ganglia^ locate d dis tally in the grey 

 £ami or in the proximal part of the spinal nerves 

 especially in the cervical and lumbar region (cf. 250). 

 Groups of ganglion cells are also distributed along the 

 internal carotid nerves and plexuses (36, 316). 



Each preganglionic fiber belonging to the vaso- 

 motor and pilomotor systems runs, as shown by 

 Langley (263), either upwards (middle and upper 

 thoracic region) or downwards (lower thoracic and 

 lumbar region) in the svmpathetic trunk. Usually 

 each descending fiber makes connections with nerve 

 cells in three (cat) or four (dog) consecutive seg- 

 mental ganglia, the ascending fibers supplying as a 

 rule a greater number of ganglia. The fibers running 

 to the superior cervical, the stellate and the coccygeal 

 ganglia may, on the other hand, send all their 

 branches to one ganglion. The preganglionic fibers 

 from an entire white ramus have more extensive con- 

 nections and may activate as many as eight^ganglia 

 (265). Pathways for topographically more restricted 

 sudomotor discharge ha\e been demonstrated in the 

 cat by Patton (336). 



It is generally as.sumed that a .single preganglionic 

 fiber is connected to postganglionic neurons belong- 

 ing to one efTector system only. Evidence for such an 

 organization has been gi\en for some fiber groups in 

 the cer\ical sympathetic (see the ne.xt section on fiber 

 types). Although the well-known observations of 

 dissociation of, for instance, vasomotor and sudo- 

 motor acti\ity in certain skin areas .suggest a func- 

 tional discreteness, on the whole more direct proofs 

 for this assumption do not seem to exist. On the con- 

 trary, it has been shown that preganglionic fibers giv- 

 ing collaterals to sudomotor neurons in the upper 

 sacral ganglia may descend to more caudally located 

 ganglia supplying other autonomic effectors (336). 



The postganglionic outflow \ia grey rami to the 

 spinal nerves follows these nerves to the periphery, 

 giving innervation to segmental areas. These sympa- 

 thetic dermatomes correspond on the whole to the 

 sensory 368 (42, 169, 357, 359, 360, 444), but in 

 many instances a great variability exists in overlap 

 and in the area of autonomic innervation of each 

 peripheral nerve (188, 358). Furthermore, the areas 

 of sudomotor and of pilomotor supply from a given 

 ganglionic outflow inav not correspond (350). 



Parasympathetic Division 



The nerve cells giving rise to the cranial parasym- 

 pathetic are located in the general visceral efferent 

 column in the brain stem, and the preganglionic 

 fibers emerge via the third, seventh, ninth and tenth 

 crani al ner ves. There is no general agreement about 

 the exact localization of the cells (167, 304, 319, 408, 

 409, 430, 432, 433). 



The general organization of the parasympathetic 

 components in the third, seventh and ninth cranial 

 nerves is relatively simple compared with the sympa- 

 thetic system, each component connecting with 

 neurons in only one ganglion from which the post- 

 ganglionic fibers are generally distributed to locally 

 restricted structures only. In contrast to this, the 

 autonomic efferents of the vagusjiave a wide distribu- 

 tion and their connections with the intramural ganglia 

 of the thoracic and abdominal viscera are not as yet 

 fully understood. 



Analysis of the origin and course of the vagal auto- 

 nomic efferents has met with several difficulties. One 

 severe obstacle has been that sensory fibers are the 

 dominating component of the vagus, a fact making a 

 quantitative determination of the number of the fine 

 preganglionic fibers difficult. Foley & DuBois (149) 

 showed that only 20 to 35 per cent of the total 

 number of fibers just distal to the jugular ganglion 

 are parasympathetic efferents in the cat. Evans & 

 Murray (136) found that the corresponding values 

 for the rabbit cervical vagus are 20 to 25 per cent and 

 that less than 10 per cent of the vagal fibers entering 

 the abdomen belong to the parasympathetic system. 

 Another oJKtacle has been that synipathetic fibers 

 join^t^ie^yagusto a variable degree (cf. 95, 319, 354). 

 In the rabbit, as shown by Evans & Murray, about 

 5 per cent of the total number (altogether about 

 26,000) of the fibers going to the abdominal viscera 

 remain intact after cervical vagotomy. They are 

 probably of sympathetic origin, but their destination 

 and function are as yet unknown. 



Heinbecker & O'Leary (197) have claimed that 

 there are efferent neurons in the nodose ganglion 

 with both centrally and peripherally directed proc- 

 esses and with motor function for the bronchi and the 

 duodenum. Recent investigations (94, 136) have, 

 however, gi\en both anatomical and physiological 

 evidence that makes the construction of Bishop and 

 O'Leary both unnecessary and impossible to accept. 



Although it may thus be regarded as firmly estab- 

 lished that the parasympathetic efferents in the vagus 

 nerve all emerge from the central nervous system, 



