THE LUNGS AND RESPIRATION. 231 



bonate (Na H CO 3 ), and so dissolved and carried to the 

 lung, where it is liberated by the acid action of the 

 oxy haemoglobin. It will thus be seen that even the ap- 

 parently simple phenomena of the gaseous interchanges in 

 the blood, in lungs and capillaries are carried on in strict 

 obedience to known physical and chemical laws and 

 arranged with a nicety which is certainly striking. 



So far nothing has been said of the nitrogen of the 

 atmosphere. This gas seems to play no part whatever in 

 the respiration of the body. It is, of course, carried by the 

 blood, just as any other gas, but is not used up by the 

 tissues, and so serves only to help maintain the pressure of 

 internal liquids against the air. 



THE INNERVATION OF THE RESPIRATORY SYSTEM. 



The nerves which are immediately concerned in the 

 movements of respiration are the motor nerves, going to the 

 intercostal muscles and to the diaphragm, and for forced 

 expirations to the muscles of the abdominal wall. All these 

 motor nerves come from the spinal cord, but take their 

 origin further up in the medulla. Hence, cutting the 

 upper end of the spinal cord at once destroys the power to 

 breathe, because it cuts through the path of these motor 

 nerves. 



1. The Respiratory Center. These nerves, however, 

 are but the mere avenues along which the impulses to 

 breathe are carried. The impulses themselves originate in 

 the respiratory center, which lies in the medulla just at the 

 end of the calamus scriptorius in the fourth ventricle. 

 (See Brain.) It seems to be a paired center, for when the 

 medulla is cut through the middle line the breathing on 

 each side continues, but an injury to the center itself at 

 once stops breathing. As we usually think an animal is dead 

 as soon as it stops breathing, this point has been called the 

 "vital point." A rather horrible example of an injury to 

 this center is the execution by hanging. In this form of 

 taking life the odontoid process of the axis is pulled out of 



