1 62 VETERINARY PHYSIOLOGY 



probability fibres connected with the tactile sense are similarly 

 sorted out, and possibly even those connected with the conduc- 

 tion of thermal and of nocuous stimuli. 



As regards the course of the Ingoing Fibres, the most valuable 

 advance has been made by the study of the results of lesions 

 of the cord in the light of the observations of Head and his 

 co-workers on the course of these fibres in the peripheral 

 nerves (see p. 111). 



While section of a cutaneous nerve destroys epicritic and 

 protopathic sensibility, and leaves deep sensibility intact, 

 lesions of the spinal cord are apt to interrupt the passage of the 

 impulses concerned with different kinds of sensation. Thus, 

 section of a cutaneous nerve abolishes any sensation of pain 

 from the surface of the skin, but leaves the possibility of the 

 sensation of pain being produced by severe pressure on deep 

 structures, while a lesion of the cord is apt to interrupt the 

 passage of all stimuli producing pain, whether coming by 

 epicritic, protopathic or deep fibres. A redistribution has taken 

 place all the various fibres carrying the impulses derived from 

 nocuous stimulation have got shunted into one tract carrying 

 them up to the brain. Similarly the impulses connected with 

 thermal sensation coming by epicritic and by protopathic fibres, 

 and those connected with the sense of touch and pressure, each 

 get shunted into specific tracts (figs. 81 and 82). 



This must mean that these impulses get sorted out by 

 passing through synapses in the cord, and the position of these 

 synapses and the course of the axons running from them to 

 the brain have been deduced partly from histological investiga- 

 tion of the fibres of the posterior roots, partly by careful study 

 of the symptoms and of the degenerations which follow definite 

 lesions of the cord. 



By the developmental method, by the different characters of 

 the fibres, and by the degenerative changes, a posterior root has 

 been divided into four main bundles : 



1. A set of large fibres passing forward to form synapses with 

 the cells of the ventral horn at the same and other 

 levels of the cord. 



'2. A second passing up the dorsal columns of the same side, 

 some running right on to the top of the cord, but many 

 entering the grey matter as they course upwards. 



