RESPIRATION 133 



regulated by the carbon dioxide circulating through the 

 medulla. 



There is a surgical condition known as shock, common after 

 prolonged operations, especially abdominal. In such there is a 

 diminished amount of carbon dioxide in the blood, and in con- 

 sequence a dangerous fall in blood-pressure. To this state the 

 term acapnia has been applied. It is believed to result from 

 the stimulation of afferent nerves increasing the pulmonary 

 ventilation, but the exact condition is not yet decided. 



The Nervous Mechanism Governing Respiration. — A large 

 number of nerves is connected with the production of respira- 

 tory movements. The facial dilates the nostrils, the vagus 

 supplies the larynx, the phrenic the diaphragm, certain spinal 

 nerves supply all the muscles of the trunk engaged in respiration, 

 and besides these purely motor functions, there are an enormous 

 number of sensory nerves connected with respiration. Such 

 extensive ramifications require for the proper discharge of their 

 functions a central co-ordinating mechanism, and this is known 

 to exist in the medulla. The position of this centre has in certain 

 animals been very accurately denned, though it is not repre- 

 sented histologically by any special group of cells. In general 

 terms it may be spoken of as being situated close to the deep- 

 seated origin of the vagus, and in front of the vasomotor centre. 

 The fibres which pass from it down the cord end in motor nuclei 

 in the grey matter of different levels, corresponding to the out- 

 flow of the nerves connected with the muscles of respiration. 

 There is also evidence of decussation of the fibres, or at any rate 

 a connection between those on the right and left side of the cord, 

 as we shall see presently in dealing with the phrenic. 



The respiratory centre consists of two halves, each of which 

 is capable of working independently. By some it has also been 

 considered to consist of two parts — an expiratory and inspira- 

 tory. There is no definite indication of the former, but the 

 latter is well marked ; and of the two acts the inspiratory is the 

 most important, for, as we have already seen, expiration may, 

 under some circumstances, be a purely passive act. Neverthe- 

 less, there are special expiratory functions of central origin, such 

 as the act of straining, as in parturition, micturition, defalcation, 

 and coughing — events which are certainly not of a passive 

 character, and might be considered to originate in a special 

 expiratory centre. 



It is known that, besides the motor nerves previously described 

 as being connected with the process of respiration, the respira- 

 tory centre is connected extensively with sensory nerves — prob- 

 ably every sensory nerve in the body, for the centre may be 

 readily stimulated through any sensory branch. Further, the 



