NERVOUS MECHANISM OF RESPIRATION. 291 



excitation, therefore, takes place, and a condition of apncea (apncea vera) 

 is produced. 



Whatever explanation we choose to adopt of the exact mode in which 

 the excitation is brought about, there can be no doubt that the absence 

 of oxygen, or rather the need of the centre for oxygen, is a very con- 

 siderable factor in determining its activity. "We have hitherto only 

 considered the cases in which this need was increased or diminished by 

 altering the amount of oxygen supplied to the centre. But we may also 

 alter it by increasing or diminishing the metabolic activity of the centre 

 itself, by heating or cooling the medulla oblongata. Thus Pick x and 

 Goldstein 2 surrounded the exposed carotid arteries of a dog with a 

 vessel with hollow walls, between which hot or cold water could be 

 passed, so changing the temperature of the blood flowing to the medulla 

 oblongata. They found that in each case the respiratory rhythm under- 

 went marked alterations. If the blood is warmed above its normal 

 temperature, so increasing the rapidity of the metabolic changes in the 

 centre, the respirations become much quicker and shallower (tachypnaea, 

 Gad) ; whereas, when the metabolic changes are depressed by lowering 

 the temperature of the blood, the movements become much slower. 

 The latter result may also be produced, as Fredericq 3 pointed out, by 

 direct application of pieces of ice to the exposed medulla oblongata, — a 

 further proof of the fact that the respiratory impulses are originated 

 there, and not in the cord, since we cannot imagine that the application 

 of cold should act as an excitant of inhibitory impulses. 



There are thus two kinds of stimuli which may act on the respiratory 

 centre in the medulla oblongata, and provoke its activity. The stimulus 

 derived from the blood is, under normal conditions of respiration, 

 constant. The stimulus derived from the afferent nerves is subject to 

 rhythmical variation, and, as we shall see later, this variation is of 

 considerable importance for the maintenance of the normal respiratory 

 rhythm. The centre can, however, go on discharging rhythmically in 

 the absence of any rhythmic afferent stimulation. We have therefore 

 to discuss how the centre is able to respond to a constant stimulus with 

 a rhythmic discharge. 



The simplest explanation of this process, if explanation it can be 

 called, is to say that it is a property of the nerve cells as of the muscle 

 cells of the heart to respond to a constant stimulus by a series of rhyth- 

 mic discharges. Many physiologists, especially Pfluger, have attempted 

 to form a conception of the intra-molecular processes which lead to this 

 transformation. Pfluger 4 considers that the molecular processes in a 

 living cell may be divided into two sets — those which tend to produce a 

 discharge of energy, and those which tend to prevent a discharge. In 

 more modern terms, we might speak of these two processes as katabolic 

 or dissimilative, and as anabolic or assimilative. In a condition of rest, 

 these processes balance one another exactly. The effect of applying a 

 stimulus to the ceil is to increase the metabolic processes. In order 

 that these excitatory processes may prevail against the inhibitory, it is 

 necessary that they should attain a certain superiority above the latter. 

 As soon as this condition is reached, the excitatory processes break 



1 Arch./, d. yes. Physiol., Bonn, 1872, B<1. v. S. 38. 



2 Verhandl. d. phys.-med. Gesellsch. in Wiirzhurg, 1872, N. F., Bd. ii. 



3 Arch. f. Physiol., Leipzig, 1883, Suppl., S. 51. 



4 " Untersuch. neber d. Physiol, des Elektrotonus," Berlin, 1879. 



