564 PHYSIOLOGY OF THE NERVE CELL AND THE SPINAL CORD 



Again a nerve cell with its processes, etc., may be intercalated as a new ele- 

 ment between two primary paths, as shown in the schema (Fig. 255) based on 

 the neuron theory. The terminal fibers of an afferent nerve fiber (JB) or its 

 collaterals (0) unite with a nerve cell (column cell) somewhere in the central 

 nervous system (D). This cell sends forth an axis-cylinder process which has 

 several collaterals (E} and these in turn serve to bring it into contact with 

 an anterior horn cell (A). By this arrangement it is evident that an impulse 

 coming via a single afferent fiber is transmitted to a large number of efferent 

 fibers. If now we imagine one or more such cells interposed between the cell 

 (D) and the motor cell (A) the connection is made still more extensive. 



Thus by means of collaterals and intercalated cells, and possibly through 

 the agency of the pericellular network, every provision is made for an afferent 

 impulse to be carried over at almost every point of its course, either directly or 

 indirectly, to the motor cells of efferent fibers, and here we have the anatomical 

 basis for facts, long since established by physiological observations, that a given 

 afferent impulse may give rise to a great variety of efferent effects. Just what 

 arrangement obtains by which different paths have been accommodated to cer- 

 tain special functions is a question which for the present can scarcely be an- 

 swered. Only in a general way may we state that between certain parts of the 

 nervous system a connection be it purely anatomical or only functional is 

 more easily established than between other parts. This, however, is but hedging 

 the real question. 



3. KINDS OF NERVES 



A. CLASSIFICATION ACCORDING TO FUNCTIONS 



Physiologically nerves may be divided into two large classes: afferent and 

 efferent. The former bring messages from all parts of the body to the central 

 nervous system, the latter convey impulses from the central system to periph- 

 eral organs in all parts of the body. 



The number of fibers in the posterior root is somewhat larger than the num- 

 ber in the anterior. In two frogs, weighing 23 and 63 g. respectively, Birge 

 found in the posterior roots 3,781 and 5,335 fibers, and in the anterior roots 

 3,528 and 4,283 respectively. According to counts made by Dale the posterior 

 roots of the coccygeal nerves of the cat always contain more fibers than the 

 anterior. Stilling previously had obtained the same result in man. 



1. To the efferent fibers belong : 



(a) The Motor Nerves i. e., all nerves whose stimulation produces contrac- 

 tion of muscles, whether skeletal muscles, vascular muscles, muscles of the intes- 

 tine, glandular ducts, bronchioles, etc. 



(Z>) Secretory Nerves, page 257 (salivary glands), page 263 (gastric 

 mucosa), page 269 (pancreas), page 396 (sweat glands). 



(c) Inhibitory Nerves i. e., nerves which check or stop any dissimilatory 

 process e.g., the cardio inhibitory in the vagus (page 188), the vasodilator 

 nerves (page 234), the inhibitory nerves of the intestine (page 288). 



Little is known with regard to the inner processes which result from stimu- 

 lation of inhibitory nerves in the different organs. We have some observations 

 tending to show that when the vagus is stimulated changes are set up in the 

 heart which antagonize the processes taking place during contraction, and from 

 these observations the conclusion has been drawn that the vagus exercises a 

 nutritive control over the heart. Other observations which have been reported 

 at page 190 show, however, that under favorable circumstances an animal with 



