630 MANUAL OF PHYSIOLOGY. 



peduncles. The antero-lateral columns are also distributed in 

 part through the pyramids and peduncles to the cerebrum, and 

 in part by the restiform bodies and peduncles to the cerebellum. 

 In the pyramids the decussation of the anterior columns takes 

 place, and it is believed that this is the point at which the direct 

 channels carrying voluntary motor impulses to the skeletal mus- 

 cles pass across from one side of the body to the other. 



It must always be remembered that the medulla is the only 

 route between the brain and the spinal cord, and in it some 

 inedullated channels cross and separate to pass to their cerebral 

 connections, and the gray part of the spinal marrow is spread out 

 on the floor of the fourth ventricle, and amplified by the addition 

 of several separate foci of gray matter. 



THE MEDULLA OBLONGATA AS A CENTRAL ORGAN. 

 A number of groups of ganglion cells with special and specific 

 duties are located in the medulla ; indeed, those acts which are 

 obviously most important for the due execution of the vegetative 

 functions are, for the most part, arranged and governed by the 

 nerve cells of the medulla. Some of these centres may be called 

 automatic, though they are variously affected by many impulses 

 arriving from distant points, and others are purely reflex in their 

 action. The former are the more immediately essential, and will 

 therefore be considered first. 



RESPIRATORY CENTRE. 



The centre which regulates the motions of breathing has been 

 known to be situated in the floor of the fourth ventricle, at the 

 upper and back part of the medulla, ever since Flourens showed 

 that injury of this spot the vital point was followed by almost 

 instant cessation of respiratory movements and death. 



This centre is a good example of a so-called automatic centre, 

 that is to say, the blood flowing through the medulla and nour- 

 ishing the cells suffices to supply them with the necessary energy 

 for their periodic activity, and we know that the quality of the 

 blood reaching this part modifies the activity of the cells; for 

 the less oxygen and the more carbonic acid contained in the 



