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PRINCIPLES OF GENERAL PHYSIOLOGY 



an end to the contractile process and the muscle cell relaxes. A nerve cell may 

 be in a state of excitation due to changes within itself, perhaps brought about 

 by chemical influence outside, but the effect upon it of stimulation of a certain 

 nerve fibre may be to quell the state of excitation or to make it inaccessible 

 to the action of the chemical stimulant. Suppose, however, that the state of 

 excitation is brought about by the reception of continuous stimuli brought in 

 by nerve fibres making synaptic junctions with the cell in question. It is clmr 

 that the excitatory state of the cell could be abolished by making the synaptic 









FIG. 109. PLKXCS OF NKRVES AROUND A SMALL ARTERY OF THE SVI.KKN. Oolgi's method. 



a, Artery, surrounded hy branched nerve fibres, which end freely. 

 y, Nerve from which the plexus arises. 



r, Pulp substance. The broad whit* space around the artery is the Malpi^hian substance. In places nerve fibres 

 are seen passing to the pulp. 



(Retziua, 1892, Taf. XXI., Fig. 1.) 



membrane, of which we spoke above, impermeable to the excitatory process 

 arriving by the nerve fibres ; in other words, a block might be produced. In the 

 case of the nerve cell, therefore, at least two different modes of inhibition are 

 possible : the influences exciting it may be cut off, or the cell itself made incapable 

 of responding to them, although they may be duly received. As another instance, 

 we might take the ventricular muscle of the heart. It is conceivable that its 

 natural beats might be stopped in several ways. The muscle itself might be made 

 temporarily inexcitable, the rhythmic stimuli coming from the auricle might be 

 prevented from reaching the ventricle by the production of a block on their 

 course, or the contractile mechanism alone might be put out of gear, leaving 

 the excitatory process intact, and so on. 



