20 Anatomy of the Nervous System 



pathways of the excitations which constitute or underlie all 

 nervous functions. 



One of the most fruitful conceptions which have been 

 brought to bear upon the study of the structure of the central 

 nervous system is that of functional components, which has 

 made possible the analysis upon a functional basis of the 

 main patterns in the almost inconceivably complex system of 

 nerve centres and tracts which confronts the student of the 

 finer anatomy of the brain. According to this principle, all 

 peripheral nerve fibres may be divided into four groups 

 according, first, to whether they conduct impulses towards 

 (afferent) or away from (efferent) the central nervous system ; 

 and, second, to whether they connect at the periphery with 

 somatic or visceral structures. Moreover, while this suffices 

 for a complete classification of the fibres in the spinal nerves, 

 each of these groups as it occurs in the cranial nerves may be 

 subdivided into two components — general and special — 

 according to whether the structures innervated have changed 

 greatly from their primitive condition or not — e.g., the simple 

 nerve endings in the intestine belong to the general visceral 

 components while the gustatory fibres are special visceral 

 sensory. Thus the cranial nerve components are eight^ in 

 number: — general and special somatic afferent, general and 

 special visceral afferent, general and special somatic efferent, 

 and general and special visceral efferent. 



The importance of this analysis from the standpoint of 

 neuro-anatomy is due to the fact that these different com- 

 ponents, having been identified at the periphery, can be 

 traced into the brain, where they are found to connect with 

 distinct centres of gray matter, or nuclei. The fibres arising 

 in these nuclei may thus be recognized as secondary fibres 

 of the particular functional type being studied, and these 



^Actually only seven of these are found in the cranial nerves of 

 mammals, the general somatic efferent group being confined to the spinal 

 nerves. 



