194 ADVANCED PHYSIOLOGY 



scribed on page 124. From the lung region the blood hurries to 

 the innumerable cells of the body each of which is needing 

 its share of food and oxygen. From the capillaries these pass 

 into the lymph which surrounds the cells, and thence through 

 the cell walls into the protoplasm. This final step is the most 

 important of the entire respiratory process, and is naturally 

 called internal, or tissue respiration. 



Up to this point the respiratory process has been described 

 as though serving only to carry oxygen to the cells ; but this is 

 only half the story, for in the cell protoplasm numerous ele- 

 ments combine with oxygen, and among other things c?^bon 

 dioxid is thus formed. This passes out of the cells, into te 

 lymph, thence into the plasma of the blood (not into the hae- 

 moglobin save in very limited degree) and is carried to the lungs ; 

 here it dialyzes through into the air sacs, and then is expelled 

 during expiration. 



NERVOUS CONTROL OF RESPIRATORY MOVEMENTS 



That breathing is under the guidance of the nervous system 

 is evident. One can breathe fast or slowly at will; one breathes 

 faster when he is running than when he is still, and faster 

 when he is excited than when he is calm. These changes in 

 the rate of breathing generally take place without any thought 

 whatever; but they are, nevertheless, brought about under 

 the direction of the brain. Unlike the heart, the breathing 

 muscles are not automatic, and unless stimuli come to them 

 from the central nervous system they will not act. The 

 source of these stimuli is in the lower part of the brain, 

 in the medulla, at the place known as the respiratory 

 center. 



No nerves go directly from the respiratory center to the 

 intercostal muscles or diaphragm; they first go down in the 

 spinal cord for some distance and then pass out in nerves 

 which leave between the vertebrae. One set of nerve fibres 

 an each side f the body leaves the spinal cord in the neek 



