476 PHYSIOLOGY 



a tonic state of activity which has its seat in a peripheral ganglion-cell connected with 

 the visceral muscle. 



This view, which was first put forward by Claude Bernard, has been specially defended 

 by Dastre and Morat. These observers point out that vaso-dilator action diminishes 

 or disappears as nerve-strands are stimulated more and more peripherally, and con- 

 clude that the vaso-dilator fibres run to the sympathetic ganglion and inhibit their 

 tonic action. The untenability of this view has been demonstrated by Langley. Thus 

 the chorda tympani fibres run to a local nerve-' centre ' in the hilum of the submaxillary 

 gland. On Bernard's theory stimulation of the fibres peripherally to the centre should 

 cause contraction of the arteries ; but it is found that, after paralysis of the ganglion 

 by nicotine, stimulation of the post-ganglionic fibres causes dilatation, so that the 

 nerve fibres given off from the local centre are not vaso-constrictor but vaso-dilator. 

 Moreover it can be shown that the sympathetic fibres which do cause constriction 

 make no connection with the cells in the hilum of the gland, but run on the walls of 

 the arteries to their distribution. These fibres are connected, not with cells of the 

 submaxillary ganglion (the local nerve-centre), but with cells in the superior cervical 

 ganglion. 



We must conclude that the inhibitory nerves in these visceral structures exert 

 their influence directly on the peripheral tissue, and not by a diminution of activity 

 of a tonically acting peripheral centre. 



AFFERENT FIBRES AND THE AUTONOMIC SYSTEM 



(REFERRED PAIN) 



The supply of afferent fibres to the viscera is very small in proportion 

 to the supply to the outer surfaces of the body. According to Langley, in a 

 visceral nerve, such as the hypogastric, only about one-tenth of the medul- 

 lated fibres are afferent, and the proportion of afferent fibres in the splanchnic 

 nerves is probably not very different from this. At the two ends of the 

 alimentary canal, i.e. at the mouth and anus, the afferent visceral fibres 

 become of more importance, since through their means co-operative somatic 

 reflexes have to be excited as well as the simple visceral reflexes. Thus in the 

 pelvic visceral nerve about one-third of the fibres are afferent, and the vagi 

 contain a large number of afferent fibres from the lungs as well as from the 

 other viscera innervated by it. 



According to Dogiel, afferent visceral fibres arise from sensory cells of the sympathetic 

 ganglia and, passing in to the posterior spinal ganglia, divide and form pericellular 

 endings round a special type of posterior root-cells, the axon from which divides again 

 into a number of branches which end in connection with typical unipolar cells. Thus, 

 according to this observer, a few afferent sympathetic fibres can stimulate a considerable 

 number of posterior root-cells. Langley, however, has not been able to obtain any 

 experimental confirmation of the origin of branching afferent fibres from cells of the 

 sympathetic system. 



It is probable that the afferent fibres of the visceral nerves arise, like 

 those of the somatic system, from ganglion-cells of the posterior spinal 

 ganglia. Every white ramus contains afferent fibres, stimulation of which 

 may evoke a rise of blood pressure as well as movements of the skeletal 

 muscles. In spite of the supply of afferent fibres to the viscera, most of these 

 organs are very insensitive to ordinary stimulation such as handling or cut- 

 ting. In operations on man for resection of the gut, if the abdominal cavity 

 be opened under local or general anaesthesia, cutting and suturing may be 



