870 THE CENTRAL NERVOUS SYSTEM 



absence of general consciousness, but in the absence of consciousness 

 of the particular act itself. But the term is now more correctly 

 used so as to embrace all kinds of actions which are not directly 

 voluntary, whether the individual is conscious of them or not. For 

 example, when the sole of rhe foot is tickled, the leg is irresistibly 

 and involuntarily drawn up by reflex contraction of its muscles; yet 

 the person is perfectly cognizant both of the movement and of the 

 sensation which accompanies the afferent impulse. Many reflex 

 actions usually associated with sensations proceed normally when 

 consciousness is entirely in abeyance; during sleep most of the 

 ordinary reflexes can be elicited. 



Anatomical Basis of Reflex Action. Since the essence of reflex action 

 is that the arrival of afferent impulses in the spinal cord or brain causes 

 the discharge of efferent impulses, there must be some connection 

 between the incoming and the outgoing nerve-fibres. In unicellular 

 animals, such as amceba, there is no differentiation of any special 

 nervous or conducting path. A stimulus applied at one point may 

 cause contraction anywhere. Even in the lowest multicellular animals 

 or metazoa e.g., in the sponges there is no special nervous tissue. 

 In some species of hydra, however, many of the surface or ectodermic 

 cells (p. 6) possess deeply-placed contractile or muscular processes, 

 and stimulation of the surface cells is followed by contraction of these 

 processes. We may imagine that the first beginnings of an actual 

 nervous system may have arisen by a further differentiation of such 

 an ectodermic cell into a receptive portion at the surface, a deeper con- 

 tractile portion, and an intermediate strand of protoplasm connecting 

 the two, and capable of conducting the excitation from surface to 

 muscular process. In such a reflex arc the nervous link would consist 

 only of the conducting strand analogous to the nerve fibre joining the 

 receptive or sensory element to the contractile element, and the dis- 

 tinction between afferent and efferent fibre would not exist. When 

 development has gone a step further, and the neuro-muscular process 

 is interrupted by a second epithelial cell transformed into a nerve-cell, 

 the afferent fibre enters one pole and the efferent fibre leaves the other 

 pole of the same cell. It is this condition which we actually find when 

 the nervous system first emerges in the animal scale as an unmistakably 

 differentiated structure namely, in the Ccelenterates in such forms as 

 the jellyfishes. Here the three types of cell; receptive or sensory cell, 

 reactive or central cell, and motor or contractile cell, are connected 

 together by conducting paths or nerve- fib res. In a simple reflex action 

 three events can be distinguished : stimulation of a receiving mechanism, 

 conduction of the excitation, and the consequent reaction or end-effect. 

 The receiving mechanism or receptor may consist of ordinary sensory 

 nerve-endings in the skin, or of special sense-endings, as in the retina 

 or internal ear. The conducting mechanism or conductor in all except 

 the very simplest nervous systems is made up of at least two neurons, 

 one the afferent portion of the reflex arc connected with the receptor, 

 the other the efferent portion of the arc, connected with the organ, 

 sometimes termed the effector organ a muscle, e.g., or a gland which 

 accomplishes the end-effect. The transference of the excitation from 

 the afferent to the efferent neuron takes place across the intervening 

 synapse. The simple isolated reflex arc, as thus described, although 

 a convenient abstraction, corresponds but little to anything which 

 actually exists in one of the higher animals. With increasing com- 



