THE NERVOUS SYSTEM 



439 



reason. All spinal nerves have their seat of nutrition either in 

 the spinal cord or in the ganglia just outside it. The nearer 

 to the spinal cord the point at which the section is made, 

 the greater the length of nerve which degenerates ; the further 

 away from the cord the point at which section is practised, the 

 shorter the length which degenerates. When the nerve degener- 

 ates, the fatty medullary sheath breaks up, forming globules 

 around the axis cylinder. The latter also degenerates, and ulti- 

 mately breaks up. The remarkable fact about these changes is 



No. i. — Degeneration of afferent 

 fibres caused by a section of superior 

 root above the ganglion. 



No. 2. — Degeneration of afferent 

 fibres following a section of superior 

 root below the ganglion. 



No. 3. — Degeneration of efferent and 

 afferent fibres following a section of 

 the entire nerve. 



No. 4. — Degeneration of efferent 

 fibres following a section of inferior 

 root. 



Fig. 131. 



-Diagrams to illustrate Wallerian Degeneration of Nerve 

 Roots (Waller). 



the rapidity with which they occur, especially in the dog. Four 

 days is sufficient to show their commencement. Small nerve- 

 fibres degenerate more quickly than large. 



The microscopical changes in degeneration are very charac- 

 teristic. In medullated nerves the axis cylinder swells and 

 breaks up, and the medullary sheath disintegrates into droplets 

 of myelin. Each detached piece of these undergoes a similar 

 change, until the nerve is represented by a bundle of connective 

 tissue without axis cylinder or medullary sheath, and the latter is 



