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A TEXTBOOK OF PHYSIOLOGY 



cause the secretion of a gland, secretory. Similarly, it may be 

 musculo-inhibitory, vaso-inihibitory, secreto-inhibitory, etc. 



Most nerves of the body are mixed nerves, containing both afferent 

 and efferent fibres. 



The Reflex Arc. The sensory neuron, when stimulated through its 

 nerve-ending, transmits its impulse inwards, passes it on to the 

 efferent neuron, which transmits it outwards, and effects some action 

 or other. This chain sensory surface (receptor), afferent conductor 

 (the joining -synapse), the efferent conductor, the reacting organ, or 

 effector forms what is known as the reflex arc. In any reflex arc at 

 'least two neurons are essential. Generally, a third neuron connects 

 the afferent and efferent neurons, and in many reflex arcs several 

 connecting neurons are interposed in the reflex path. 



The receptors are classified according as 

 they respond to stimuli from (1) without 

 the body (extero-ceptive) ; (2) from the animal's 

 own tissues e.g., the muscles, tendons, joints, 

 or the labyrinth of the ear (proprio-ceptive) ; 

 (3) from the viscera (entero-ceptive). 



The Conductors consist of the axons of 

 afferent and efferent neurons, and the synapse 

 between these. The code of interaction 

 between the afferent and efferent conductors 

 of the spinal cord has been studied in what 

 is known as the " spinal animal " that is, an 

 animal in which a division of the cord has 

 been made in the lower cervical region. Such 

 an animal can be kept in health with careful 

 attention, and studied months after the 

 initial effects of the lesion have subsided 

 FIG. 314. EFFECTOR NERVE (see p. 674). The conduction of the nervous 

 ENDINGS IN MUSCLES OF i mpu i se in a reflex arc differs materially from 

 Rate?.') ( aWn conduction in the nerve-fibre only. This is 



mainly owing to the interposition of the 

 synapses. 



Conduction across a synapse is much slower than along a nerve- 

 fibre, and is easily fatigued. Want of oxygen particularly causes 

 failure of conduction at this point, and drugs such as nicotine painted 

 on ganglia paralyze the synapses in them. The synapse interposes a 

 valve-like action in the chain of conductors, permitting conduction of 

 the impulse in one direction only namely, from the receiving afferent 

 to the effecting efferent neurons. 



The Effector Organs form the connection between the efferent 

 nerve and the cells of the tissue affected. In some cases they have 

 a definite structure for example, the motor end-plate in muscle. 

 In other cases there is merely a "receptive substance," dependent 

 for its nutrition rather on the cells of the tissue than on the nerve- 

 fibre with which it is in connection. It therefore does not degenerate 



