THE SPECIAL NEEVOUS MECHANISMS 113 



into the medulla. The greater number of the impulses passing 

 along these paths are of the nature of stimuli, which lead to 

 muscular responses only, and they are rearranged and co- 

 ordinated in the cerebellum and mid-brain, so that the actions 

 resulting from them are purposeful and appropriate. Some of 

 them, but apparently only a small fraction of the total, proceed 

 upwards to the cortex after passing through synapses in the 

 medulla and mid-brain, and give rise to the changes of conscious- 

 ness which we recognise as the sensations of pain, heat, cold, 

 touch, pressure, resistance, etc. 



General Sensibility in the Head. — The same kinds of general 

 receptors are found in the skin, the muscles, and the joiuts in 

 body, limbs, and head. But the impulses coming from the 

 region of the head and face enter the central nervous system 

 through the sensory fibres of the cranial nerves. Some of these 

 are nerves of special sensation, and we consider them separately. 

 Others (the 3rd, 4th, and 6th) are purely motor nerves, and 

 go to the muscles of the eyes. The remainder (that is, the 5th, 

 7th, 9th, and 10th) are mixed — that is, they contain both sensory 

 and motor fibres — and they may be compared with spinal nerves. 

 They, with the purely motor nerves, convey efferent impulses 

 from the medulla and mid-brain to the muscles of the eyes, face, 

 jaws, and neck, and they also carry afferent impulses, arising 

 as the result of the stimulation of the general receptors of those 

 regions, into nuclei in the medulla and lower parts of the mid- 

 brain. The internal paths in the brain are rather complicated, 

 and we cannot attempt to describe them here ; but there is the 

 same general scheme as in the case of the special nerves: the 

 afferent impulses coming from the face and head go up to the 

 mid-brain, and through this to the cortex, on the one hand, and 

 into the cerebellum on the other. 



The Sensation of Smell. — This stands quite apart from all the 

 other sensory mechanisms in that the connection of the oKactory 

 organ is with the cortex direct, and not with the medulla and 

 mid-brain. The organs of smell and vision and hearing have 

 also a different embryogenic origin from that of the other sense 

 organs in that they arise as parts of the brain, which become 

 pushed out, so to speak, and come into relation with other parts 

 arising from the embryonic integument. The auditory organs 

 originate in this way from the hind-brain vesicle, the visual 

 organs from the fore-brain vesicle, but the olfactory organs 



8 



