238 INTRODUCTION TO NEUROLOGY 



ments show that the spinal segments from which all of the motor 

 respiratory nerves arise cannot of themselves effect the coor- 

 dinations necessary in respiration. This is in marked contrast 

 with many other reactions (both visceral and somatic), whose 

 performance is still possible after the separation of the spinal 

 cord from the brain. 



If now, in a third animal, the medulla oblongata is cut across 

 at any point above the middle of its length, say at the lower bor- 

 der of the pons, the respiratory processes are in no way disturbed. 

 This shows that there is a respiratory correlation center in the 

 lower half of the medulla oblongata, that is, somewhere in 

 the region corresponding to the "visceral area" of the fish 

 brain. 



The air tubes of the lungs are provided with smooth muscle- 

 fibers by which their caliber may be contracted. These muscles 

 are innervated by the vagus, and the hyperexcitation of their 

 motor nerves may impede respiration, this being one of the fac- 

 tors which cause asthma. The cerebral center from which these 

 intrinsic muscles of the lungs are innervated has been shown to 

 lie in the middle part of the dorsal motor vagus nucleus (Fig. 73, 

 nuc dorsalis vagi). These are preganglionic neurons, the cor- 

 responding postganglionic neurons lying in sympathetic ganglia 

 distributed along the pulmonary branches of the vagus (Fig. 1 12). 



The apparatus described in the preceding paragraph is, how- 

 ever, not responsible for the maintenance of the regular rhythm 

 of breathing. Physiological experiments show that there is some- 

 where in the lower part of the medulla oblongata a respiratory 

 center which performs this function. This center may appa- 

 rently be excited to activity directly by variations in the compo- 

 sition of the blood which reaches it, especially either by a de- 

 ficiency in oxygen or by an excess of carbon dioxid. Its activity 

 may also be modified by nervous influences reaching it through 

 the peripheral afferent nerves, the vagus being the only nerve 

 which appears to be able to act directly on the respiratory center, 

 though the strong excitation of almost any sensory nerve of the 

 body may under some circumstances indirectly affect the res- 

 piratory rhythm. Coughing and sneezing are special cases of 

 this sort. The reflex mechanism of the cough is illustrated in 

 Fig. 113. 



