In the brain the medulla oblongata contains a respiratory center, which controls the breathing mechanisim. The respiratory center consists of a specialized inspiratory center and an expiratory center. The centre is bilateral and its two halves which are connected together by commiseural neurons. The sides of this centre are connected with motor respiratory neurons. The nerve cells of the centre are connected with the breathing apparatus forming a reflex arc. These nerve cells are sensitive to chemical composition of blood. There is another center present in the pons region of the brain called pneumotaxic center this functions in moderating the functions of the respiratory rhythm center. The axons from the nerve cells of these centers lead to the intercostals muscles through the intercostals nerves and to the diaphragm via the phrenic nerves. These nerve fibers transmit impulses to the external intercostals muscels and internal intercostals muscles alternately. The walls of the alveoli have sensory endings, which are stimulated by changes in the tension of the alveolar walls. The streaching of alveoli send stimuli to the expiratory center of the medulla through the vagus nerve, which inhibits further respiration. This sequence of events is called Herring – Breuer reflex.