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HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY II 



dog, monkey and man, as shown by Sheehan (385). 

 In the white communicating rami of cat, dog and 

 rhesus monkey, Rexed (356) found that the great 

 majority of the myelinated fibers had a size of i to 

 3.5 IX. The same values were reported by Ranson & 

 Biliingsley (353) for the cat. Unfortunately there are 

 no reliable quantitative data concerning the number 

 and size of the nonmedullated preganglionic fibers 

 in the ventral roots or white rami. According to 

 Duncan (112) up to 30 to 40 per cent of the fibers 

 in the thoracic \entral roots of the rat may be un- 

 myelinated whereas the number is considerably less 

 in the cat. 



Table 3 illustrates the high variability in myelina- 

 tion of the preganglionic fibers in the cervical sym- 

 pathetic trunk. Data concerning the number, size 

 and myelination of the vagal autonomic efTerents 

 have been given (94, 136, 149, 319). 



The postganglionic fibers of the grey communicat- 

 ing rami in the cat, dog, monkey and man are pre- 

 dominantly nonmedullated, but some small medul- 

 lated ones are found to a variable degree (261, 341, 

 356, 387). Many of the postganglionic fibers from 

 the superior cervical ganglion in the cat (23, 25, 352) 

 and rhesus monkey (25) are medullated; in the rabbit, 

 fewer are present (25). It may be stated that the post- 

 ganglionic fibers in mammals are for the most part 

 generally of the nonmedullated variets'. An interest- 

 ing exception has been found in the short ciliary- 

 nerves which in the cat are of the thin myelinated 

 type (86, 160). The myelin sheaths seem to make 

 their appearance at a considerably later stage of de- 

 velopment in autonomic than in somatic nerve fibers 

 (378). 



Caliber spectra of peripheral nerves show that the 

 fibers in a given nerve may be divided into separate 

 and often distinct size groups. Since the first experi- 

 ments by Erlanger and Gasser, evidence has ac- 

 cumulated showing that the groups obtained on the 

 basis of fiber size may correspond to fiber groups with 

 different functions and with different neurophysio- 

 logical properties. In many instances, however, a 

 given caliber group contains several functionally dif- 

 ferent fiber groups and fibers with the same function 

 may be distributed to more than one caliber group. 

 That the autonomic fibers can be divided into groups 

 each subserving a particular function has been shown 

 in some instances (see below). It must be stressed, 

 however, that there is as yet no anatomical basis for 

 a division either of the preganglionic or of the post- 

 ganglionic fibers into more than two groups accord- 

 ing to size and to other morphological characteristics. 



Although there may exist different, more or less 

 distinct size groups, as indicated by neurophysiologi- 

 cal studies, caliber spectra of the medullated fibers 

 have not revealed them, and no quantitative data 

 at all concerning the size of the nonmedullated fibers 

 have been given. 



An example may illustrate the view expressed 

 above. In a recent work (103) the medullated fibers 

 in the cervical sympathetic trunk of the cat have been 

 divided into three subdivisions: a) 6.5 to 5 m (about 

 200 fibers), h} 4.5 to 3 fi (about 2000) and c) 2.5 to 

 1.5 ju (about 2000). The groups obtained were as- 

 sumed to correspond to the Si, S2 and S4 fibers of 

 Eccles (see below). It is clear, however, that the pre- 

 ganglionic medullated fibers justifiably may equally 

 be separated into two, five or more subdivisions, a 

 fact which makes the construction unacceptable. 



There are other difficulties when an attempt is made 

 to correlate anatomical and functional groups in the 

 autonomic nervous system. Thus, for instance, it was 

 claimed by Langley (258) that many medullated 

 preganglionic fibers lose their myelin sheaths before 

 entering the ganglia. Another obstacle is the very 

 great variability in myelination between animals of 

 the same species, making it almost impossible, as 

 pointed out by Ranson (23) among others, to take 

 the presence or absence of mvelin as a criterion for 

 a differentiation of functional types of fibers. 



On the basis of neurophysiological properties, the 

 peripheral autonomic fibers may be divided into two 

 principal groups corresponding to the B and C types 

 in the classification of Erlanger and Gasser. It is 

 beyond doubt that in mammals the slow conducting 

 C component is represented by unmedullated fibers 

 only and there is good e\idence that the B fibers 

 are medullated ones of the variety found in the 

 autonomic nervous system (cf. 24, 108, 134, 183, 

 192, 193, 195). The autonomic nonmedullated fibers 

 have physiological properties distinctly different from 

 those of the C fibers arising from dorsal root ganglia 

 (161), but the significance of this is not known. 



The C component has not as yet been divided into 

 more distinct subdivisions, although it is clear that 

 this group must in many instances contain fibers 

 going to very different effectors. A composite C wave 

 may be observed in the postganglionic nerves from 

 tlie superior cervical and ciliary ganglion in the cat 

 (117, 440), but no clearly separable subgroups have 

 been recorded. This applies also to the postganglionic 

 nerves from the superior cervical ganglion of the 

 rabbit (25, 109), from the stellate ganglion of the 

 cat (45, 49, 123), and from the inferior mesenteric 



