176 STUDIES IN ADVANCED PHYSIOLOGY. 



The word 1 1 automatic ' ' has been used a number of 

 times, and a definition of the meaning of this term is im- 

 perative in order to understand what is meant by saying 

 that both the ganglia of the heart as well as the muscle 

 itself are automatic. Such a definition of this term is un- 

 fortunately not yet forthcoming. Physiologists are so far 

 entirely unable to explain what it is that causes the ganglia 

 to originate their nervous impulses, or the muscle to con- 

 tract at definite periods. There is probably something 

 either in the structure of the heart or the manner in which 

 it works that excites the ganglia or the muscle substance, 

 but so far it has been impossible to locate that. Thus the 

 term automatic is used to designate a physiological con- 

 dition of things, the meaning of which we do not yet have. 



2. Nerves reaching the heart from the outside. In 

 addition to the intrinsic nerves of the heart three kinds of 

 nerves reach it from the outside. Two of these nerves 

 reach the heart from the vagus or pneumogastric nerve 

 coming from the brain. The third kind reaches it from 

 the sympathetic system. 



The fibres of the pneumogastric which run to the heart 

 are really not fibres of the pneumogastric at all ; they are 

 fibres of the eleventh pair of cranial nerves called the spinal 

 accessory. A branch of the spinal accessory nerve unites 

 itself with the pneumogastric and follows the course of the 

 pneumogastric through the skull and the neck. If this 

 cardiac branch of the pneumogastric be examined histo- 

 logically it is found to consist of two sets of fibres, a set of 

 small fibres and a set of larger ones. Both, however, are 

 medullated. On reaching the heart the large fibres can be 

 traced into the heart, leading probably to the intrinsic 

 ganglia, while the set of small fibres seems to run along the 

 surface of the heart and loses itself in the walls of the 

 ventricles mainly. These smaller nerves are sensory and 

 carry to the brain the sensations of the heart. 



