148 PRINCIPLES OF ANIMAL BIOLOGY 



Each organ governed by the autonomic system is innervated twice, 

 one nerve coming to it from the thoracolumbar system, one from the 

 craniosacral. One of these nerves is an activator, the other a depressor. 

 Each organ is thus accelerated by one of the major divisions of the 

 autonomic system, inhibited by the other; but neither division is exclu- 

 sively excitatory or wholly inhibitory, each division exciting some 

 organs, depressing others. The thoracolumbar system accelerates the 

 heart but inhibits movement of stomach and intestine. The iris of the 

 eye is constricted by the craniosacral, dilated by the thoracolumbar. 



The excitation or inhibition is apparently accomplished by producing 

 a chemical substance, and the organ responds to this substance. Accord- 

 ing to current theory, all the nerves belonging to the craniosacral system 

 produce the same substance, which is probably acetylcholine. In like 

 manner, the thoracolumbar nerves produce one substance which has 

 been called sympathin. Acetylcholine inhibits the heart, increases 

 stomach movement and secretion, contracts the rectum and urinary 

 bladder, dilates the vessels of the salivary glands, and constricts the iris 

 of the eye. Sympathin produces the opposite reaction in each of these 

 organs. 



Nerve Impulse. — The impulse which is carried along a neuron like 

 that in Fig. 118 travels at a speed of about 120 meters per second in mam- 

 mals, about one-fourth of that velocity in a frog. The rate is in some way 

 related to the presence or absence of a sheath around the branches of the 

 cell, and to the structure of that sheath if one is present. The axon 

 of the cell in Fig. 118 is surrounded by a white layer of noncellular fatty 

 substance known as the myelin (medullary) sheath, which is divided into 

 segments by irregularly placed nodes. Not all neurons possess such a 

 sheath. Those of the autonomic system do not, and in them the impulse 

 travels much more slowly — only 10 or 12 meters per second. Among 

 myelinated nerve fibers, those with the longer segments of myelin between 

 nodes conduct, in general, more rapidly than those with short segments of 

 the sheath. There is some reason from experiment to believe that the 

 impulse jumps from node to node; the longer the segments between nodes, 

 therefore, the faster the impulse travels. 



According to present view, the nerve impulse is a surface phenomenon. 

 The membrane of a nerve fiber — not the cellular covering or neurilemma 

 and not the myelin sheath, but the outer film of the nerve cell itself — is 

 charged positively on the outside, negatively on the inside. The charges 

 are really borne by ions, which are located on opposite sides of the some- 

 what impermeable membrane. This membrane keeps them apart and so 

 prevents them from neutralizing one another (Fig. 123). The impermea- 

 bility prevents neutralizing, and the separation of the ions in turn is 

 supposed to hel]) keep up the impermeability. If, now, something (a 



