THE NEURON AND ITS CONDUCTING PATHS 111 



which travels over the afferent conductor to the motor cell, and from 

 here over the efferent path into the terminals of the axon which are 

 modified to form a motor end-organ in close alliance with the tissue 

 effecting the reaction. Thus, it will be seen that a reflex circuit 

 consists of a receptor, an afferent path, a center, an efferent path and 

 an effector. In accordance with the different kinds of responses, the 

 receptors and effectors present different structural and chemical 

 peculiarities. For example, an afferent impulse may arise in the tactile 

 corpuscles of the skin and eventually give rise to motion, the effector 

 being formed in this particular case by the skeletal musculature. 

 But the impulse may also be generated in the retina of the eye or in 

 the organ of Corti and nevertheless lead to motion. This list might 

 be extended almost indefinitely, because besides the ordinary responses 

 of skeletal muscle, a large number of reactions are also brought about 

 with the help of smooth muscle. In the latter group are to be placed 

 the vasomotor and pilomotor actions, as well as the movements re- 

 sulting in the domain of the stomach, intestine, ureter and bladder. 

 Another group of very important sensory impulses produce secretory 

 effects. But quite irrespective of the character of the reaction it 

 should be kept in mind that any response executed in consequence of 

 a sensory impression without the intervention of the will, constitutes 

 a reflex. 



The Structure of Nerves. — Each neuron is to be regarded as an 

 elongated conductor, but naturally, neurons are generally combined 

 into groups and do not appear singly. In the central nervous system 

 an aggregation of the cell-bodies of several neurons is known as a 

 nucleus and, in the peripheral system, as a ganglion. Furthermore, if 

 a group of cells of this kind regulates a certain function, it is designated 

 as a center. The former term, therefore, refers to an anatomical 

 entity and the latter to a functional entity. The nerve-fibers passing 

 away from these cell-bodies are generally bound together into bundles 

 which are known as nerves. A nerve, therefore, represents a collection 

 of nerve-fibers outside the central nervous system. It is formed in 

 the following manner: the axon passes away from the cone-shaped pro- 

 jection of the cell-body, and soon becomes enveloped in a tubular 

 membrane which constitutes the medullary or myelin sheath. In 

 many cases, a second investment is found externally to this one 

 which is known as the primitive sheath or neurolemma. Having 

 acquired these sheaths, the axon, or, as it is now called, the axis-cyl- 

 inder, becomes the nerve-fiber. Many of these are bound together 

 to form a bundle, and many bundles to form a nerve. The individual 

 . fibers are supported by a fine stroma or endoneurium. The connective 

 tissue investing the individual bundles of fibers, is known as the peri- 

 neurium, and that surrounding the nerve as a whole, as epineurium. 

 When a nerve divides, one or more of its bundles of fibers separate 

 from its main trunk in the form of a branch. It frequently happens, 

 however, that these branches do not pursue an independent course 



