272 THYSIOLOGY OF MUSCLES AND NERVES. 



such as are rhythmic, in wliicli contraction and relaxa- 

 tion of the muscles concerned take place in regular 

 alternation, as in respiration and in the movements of 

 the heart ; such as are tonic, in which the contractions 

 are more constantly enduring, even if the degree of 

 contraction varies, as in the contraction of the vascular 

 muscles, and of the rainbow membrane of the eye ; and 

 such as are irregular, i.e. the peristaltic movements of 

 the intestine. Our knowledge of automatic movements 

 is based principally on those connected with respira- 

 tion ; but the conceptions gained in this case may be 

 directly applied to the other cases. It will be suffi- 

 cient therefore to speak of respiratory motion only. 



Eespiration begins immediately after birth, and 

 its movements continue from that time throughout life. 

 In the higher animals (mammals and birds) they are 

 unconditionally necessary for the preservation of life, 

 • for only by their means is sufficient oxygen conveyed 

 to the blood to provide for all the vital processes. On 

 the other hand, when the organ from which the ex- 

 citement of the respiratory muscles proceeds is in 

 any way insufficiently nourished or is otherwise in- 

 jured in condition, resj)iratory action ceases and life is 

 threatened. This organ is a limited point in the 

 onedulla oblongata, formed of a mass of nerve-cells, in 

 which the excitements originate, and from which they 

 are conveyed by the nerves to the respiratory muscles. 

 This is called the respiratory centre {Lehenshnoten 

 of the Germans, noeud vital of the French), because 

 of its importance to life. It is the spot which the 

 matador in bull-fights must reach by a skilful blow 

 with his knife, to bring the enraged animal to the 

 ground ; it is the spot which, if crushed between the 



