104 HUMAN BIOLOGY 



in those organs where they occur. In the spinal cord of man 

 internuncial neurones make up fully two-thirds of the mass 

 of this organ. 



The conditions that obtain in the human cord afford a 

 basis for the understanding of those in man's brain. This 

 organ Hke the cord is provided with nerves but the cranial 

 nerves, twelve pairs in all, are very individual and not of the 

 same uniform character as the spinal nerves. Some of the 

 cranial nerves, such as the olfactory, are purely sensory 

 but most of them are mixed motor and sensory hke the 

 spinal nerves. The majority are easily reducible to the 

 plan of a spinal nerve, but with a predominance in either 

 sensory or motor elements. In one respect, however, they 

 are very unhke spinal nerves. They contribute to the forma- 

 tion of the central organ with which they are connected only 

 a relatively small amount of substance. In consequence the 

 mass of the brain is made up ahnost entirely of internuncial 

 neurones. In fact, entire sections of the brain are formed 

 exclusively of this type. Thus the whole of the cere- 

 bellum is internuncial in composition and the same is true 

 of the human hemispheres. Since these parts together 

 constitute 98 per cent of the weight of the brain and because 

 much of the remaining 2 per cent is also composed of inter- 

 nuncial material, it follows that the human brain, in contrast 

 with the cord, is formed almost exclusively of this type 

 of nerve cell. 



When the evolutionary history of the sensory, motor and 

 internuncial neurones is traced, an interesting sequence is 

 disclosed. In the simplest form of nervous system to be met 

 with such, for instance, as that seen in the tentacles of 

 sea-anemones, the only nervous element present is a sensory 

 neurone that extends directly from the surface of the animal 

 to the subjacent muscle. By means of such a nervous element 

 the muscle is set in action much as a trigger sets off a gun. 

 Since this type of nervous organization includes only one 

 form of neurone it may be designated mononeuronic. 

 The form of neurone here involved is most akin to the 

 sensory neurones of higher animals, which may therefore 

 be looked upon as approaching most nearly the primitive 

 ancestral type. 



