532 PHYSIOLOGY 



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 cutting. 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 conducted without any anaesthetic and 



without causing any pain to the patient. On the other hand, impulses 



may arise in the afferent nerve-endings of the viscera, as a result of 



disease or certain forms of stimulation, e.g. stretching or compression, 



which may reach consciousness and give rise to the sensation of pain. 



This pain is not, however, localised in the viscera, but is referred to 



certain parts of the surface of the body. When the afferent autonomic 



fibres of a nerve are the seat of pain, the primary referred pain is in 



the area of the cutaneous somatic fibres of the nerve. It has been 



shown by Mackenzie and by Head that visceral disease may cause 



hyper-sensitivity of the corresponding areas of the skin, and a method 



has been elaborated by these observers for utilising this referred pain 



or skin tenderness as a means of localising the site of the disease. 



