Integrative Systems 



141 



111. 



I 



EPINEURIUM 



NODE OF RANVIER 



P= 



111 



NEURILEMMA 



MYELIN SHEATH 



NEURITE(AXON) 



-ENDONEURIUM 



NUCLEUS OF NEURILEMMAS 



A LONGIT SECTION 



B.CROSS SECTION 



Fig. 132. A portion of (A) longitudinal and (B) cross section of a nerve prepared 

 by the vom Rath method. A nerve is a bundle of axons (neurites) covered by an 

 epineurium. Each axon is surrounded by an inner myelin (fatty) sheath and an 

 outer cellular neurilemma sheath. (Courtesy, Neal and Rand: "Chordate Anat- 

 omy," Philadelphia, The Blakiston Company.) 



nervous organ is designated as afferent or sensory; one which trans- 

 mits away from a central organ and toward a reacting organ is efferent 

 or motor. The reacting organ, usually a muscle or gland, is called an 

 effector. In the case of a gland, the efferent fibers may be related to 

 the secreting cells themselves, controlling their secretory activity, or 

 to muscle-fibers in the wall of the gland, controlling the discharge from 

 the gland. 



A nervous reaction of the simple type known as a reflex is initiated 

 by stimulation of a receptor. A nervous impulse produced by effects 

 of the stimulation is transmitted from the receptor along a conductor, 

 which may be a nerve-fiber belonging to the receptor cell (as in the 

 olfactory organ), or a fiber of another neuron (as in the ear). From 

 this conductor, the impulse is relayed over to another neuron which 

 is related to the effector. But few reactions are as simple as this. Most 

 reflexes are complicated by the fact that additional neurons are inter- 

 polated between the two conductors of the simple "reflex arc" just 

 described (Fig. 133). By this means, a simple and sharply localized 

 stimulus may bring into action a complex system of effectors, as when 

 a pin-prick causes a jerking of the entire arm. It is these interpolated 

 neurons — association and correlation neurons — which make up 

 a large part of the bulk of the central nervous organs. 



The physical nature of the relationship between two adjacent 

 neurons in a nervous circuit is doubtful. In some cases there is appar- 

 ently continuity of the delicate protoplasmic fibrils of the two neurons. 

 There is, however, much evidence that, at least in higher animals, the 

 related fibrils are not physically continuous but merely closely approxi- 



