5i2 STRUCTURE OF VERTEBRATES 



or by the skeletomuscular system (the proprioceptive field). They 

 thus serve the generahsed senses of touch and temperature, the 

 special sense organs, including the ear and the acustico-lateralis 

 system generally, and the proprioceptors which tell us of the 

 position of our limbs, and probably also in man, help to determine 

 our general senses of well-being. 



Visceral sensory fibres convey impulses from the alimentary 

 canal and viscera generally, and the blood vessels (the intero- 

 ceptive field). The nerves of the chemical sense, including the 

 special senses of smell and taste, probably belong here. 



Somatic motor fibres convey impulses to the skeletal muscles, 

 and are distinguished in man as most of those which are, or appear 

 to be, under the control of the will. 



Visceral motor fibres convey impulses to glands and to smooth 

 muscle. They also supply the voluntary muscles of the gill arches 

 and jaws, developed in association with the alimentary canal. 



In mammals both sets of afferent fibres normally go in by the 

 dorsal root, and the efferent fibres emerge by the ventral root. The 

 more primitive arrangement, however, appears to have been 

 for the visceral efferent fibres to use the dorsal roots ; this appears 

 to be the case in lampreys, and in Amphibia and fishes they are 

 found in both roots. In amniotes efferent fibres are only occasion- 

 ally found in the dorsal roots of spinal nerves, but they are present 

 in the facial, glossopharyngeal and vagus, which are the dorsal 

 roots of cranial nerves. Here, as in the separation of the roots, the 

 cranial nerves of gnathostomes resemble the spinal nerves of 

 lampreys. 



The nature of the nervous impulse is basically the same in 

 all types of fibre, and it is the same no matter what the stimulus 

 from which it originates. A wave of change of electrical potential, 

 accompanied and perhaps caused by a wave of change in per- 

 meability, travels in both directions from the point of stimulus. 

 Normally, sensory nerves are only stimulated at their outer end, 

 motor nerves at the inner end, so that they conduct in one way 

 only, but experimentally stimuli can be applied along their 

 length, when it is seen that conduction takes place in both direc- 

 tions. The speed of conduction varies with the fibre and with the 

 temperature ; the fastest mammalian fibres conduct their 

 impulses at about loo metres per second. At the nerve-muscle 

 junction and at a synapse the nerve fibre liberates a chemical 

 substance which causes response or starts a new impulse in the 



