110 HUMAN PHYSIOLOGY 



isolated collections. Some of these are distinctly 

 tributary to certain cranial nerves and are described 

 as the nuclei of these nerves. The cerebrum and the 

 cerebellum have gray matter spread in a thin layer 

 over their surface, a development called the cortex. 

 We have every reason to believe that it, is upon the 

 organization of the cortex, especially of the cerebrum, 

 that the rank of an animal depends. The human 

 cortex is thus of extraordinary interest to us. The 

 convolutions seem to be devices to increase the extent 

 of cortex. The interiors of the cerebrum and the cere- 

 bellum are taken up chiefly with white matter, fibers 

 sweeping in all directions, but there are some' submerged 

 clumps of gray matter in both ihese 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- 



