Respiration. ( . | L 



second stage the inspiratory muscles are less active, the 

 expiratory still powerful, and the convulsions cease. In 

 the third stage the animal lies unconscious, occasionally 

 gasping, the mouth open (even in the horse), the pupils 

 dilated, the pulse barely perceptible or absent. During this 

 stage the blood pressure rapidly falls. Death occurs in 

 from five to six minutes from the commencement of the 

 first stage. 



The Nervous Mechanism governing respiration may tem- 

 porarily be under the control of the will, but under ordinary 

 circumstances is independent of it. The respiratory centre 

 lies in the medulla, close to the deep origin of the 

 pneumogastric nerves ; it is supposed to consist of an 

 inspiratory and expiratory portion, and there are good 

 reasons for believing that the pneumogastric contains fibres 

 which lead to expirator}'- efforts on the one hand, and 

 inspiratory on the other. From experimental inquiry, 

 it is certain that the vagus contains fibres which may 

 have the effect of controlling and augmenting respiration. 

 But the nervous mechanism is by no means so simple as 

 the above statement might suggest. The number of 

 muscles brought into play during the respiratory process, and 

 the exact order in which they must work, necessitate the 

 most careful co-ordinating nervous arrangement, and that 

 will be fully brought home to us when we consider that 

 the respiratory wave from the nostrils to the diaphragm, 

 and from the abdominal muscles to the nostrils, must be 

 evenly and regularly performed. 



Further, the nervous influences concerned in the process 

 are complicated by their connection with the spinal cord, 

 which under some conditions has been known to carry on 

 the respiratory process independent of the centre in the 

 medulla, though under ordinary circumstances destruction 

 of the centre in the medulla means instantaneous death. 



If the spinal cord be divided below the origin of the 

 phrenic nerves there is paralysis of the intercostal and 

 abdominal muscles, but the action of the diaphragm is 

 rendered stronger, while expiration is carried out by the 



