l8o EQUINE PHYSIOLOGY. 



sleep and in unconsciousness from drugs, though it is also modi- 

 fied by the will. 



It is under the control of the cerebro-spinal nervous system, 

 though the sympathetic forms plexuses at the root of the lung. 



The respiratotj centre is in the floor of the fourth ventricle, 

 and a puncture at that point stops respiration immediately. If 

 the brain is removed above the medulla, the act still goes on. If 

 the carotid and vertebral arteries are ligated, the animal develops 

 dyspnoea at once, owing to a deficiency of oxygen at the centre. 

 The nerves of respiration are the two phrejiks, two pneiimogas- 

 trics, i7itercosiah to the muscles of the chest, the cervical and 

 lumbar to the muscles of the neck, back and abdomen. 



The Phrenics. — If the phrenics are both cut, the diaphragm 

 is paralyzed ; if a single one, that side only remains passive. If a 

 section is made of the cord above the fifth cervical nerve, we 

 have instant stopping of respiration, due to paralysis of the dia- 

 phragm and thoracic muscles, and immediate death. Life may 

 be continued a short time after both phrenics have been divided, 

 as the intercostal muscles still act. 



The Intercostals act very simply by supplying the ordinary 

 muscles of expiration and inspiration. If a single one is divided, 

 the remainder do its work. 



The Cervical and Lumbar nerves supply the muscles gen- 

 erally of forced inspiration and expiration. 



The PneumogastriCS. — These are distributed at the roots of 

 the lungs, and if one is cut, respiration becomes very slow. Sec- 

 tion of both causes marked decrease in respirations, and death in 

 four or five days. A weak galvanic current applied to the nerve 

 increases the number and force of the respiratory acts. Stim- 

 ulation of the central end of its divided superior laryngeal branch 

 causes slow respiration ; and if strong, paralyzes it in forced ex- 

 piration. 



ALIMENTATION. 



Alimentation is the process of receiving or taking nourishment ; 

 it may be divided into digestion and absorption. 



Digestion is the preparation of food for absorption. 



Absorption is the process by which the digested food enters 

 the circulation. 



Nourishment or food is necessary to repair waste and for the 

 growth of the animal. 



