112 HUMAN PHYSIOLOGY 



sweeping in all directions, but there are some submerged 

 clumps of gray matter in both these divisions. A series 

 of small, communicating cavities can be followed through 

 the brain-stem and into the hemispheres. These spaces 

 contain a clear fluid identical with that held between the 

 layers of the meninges. 



The Medulla. To look at the medulla one would 

 think it merely an extension of the spinal cord. But 

 this short segment of the nervous axis has unique powers. 

 The function which we must first recognize is the con- 

 trol of breathing. We say that the medulla contains 

 the respiratory center. This was inferred about a hundred 

 years ago when it was shown that cutting the cord below 

 the skull permanently stops the breathing and so, in 

 the case of any of the higher animals, causes immediate 

 death. Cutting through the brain-stem just above the 

 medulla does not stop the breathing and is, therefore, 

 not instantly fatal. Comparing the results of these 

 two experiments, physiologists have been led to believe 

 that there is in the medulla an important station from 

 which breathing is directed. Its position has been 

 more narrowly defined by slicing across the medulla 

 repeatedly, beginning at the top, and noting at what 

 level the progressive destruction has abolished the 

 breathing movements. 



The muscles used in breathing are, with minor ex- 

 ceptions, supplied with motor nerve fibers originating 

 in the spinal cord. Hence, the cells in the medulla 

 must not be thought of as directly connected with these 

 muscles but with the lower order of nerve cells in the 

 cord. Here for the first time we have an example of 

 what is common enough in many other mechanisms, 

 a higher center presiding over a number of centers of 

 lower rank, as a colonel commands the captains in his 

 regiment. It may be added that in the respiratory 

 system the "captains" exercise no discretionary power 

 when their superior ceases to hold them to their work. 



The respiratory center is much subject to reflexes. 



