332 PHYSIOLOGY. 



motor centers there are two kinds of fibers, pressor and depressor. 

 (1) The pressor fibers cause a vascular narrowing, due to a reflex 

 stimulation of the vasoconstrictor center. (2) The depressor fibers 

 cause dilatation of the arterioles and a fall of arterial blood-pres- 

 sure, due to reflex inhibition of the vasoconstrictor center, as with the 

 nervous depressor. 



There are also reflex vasodilator fibers, which lower arterial blood- 

 pressure by a stimulation of the vasodilator center, as in the congestion 

 of erectile tissue and the afflux of blood to glands in activity. 



The vasomotor changes can be studied by means of instruments 

 which register the changing volume of a part at each systole of the 

 heart and the varying diameter of the arterioles. These instruments 

 are known by the names of plethysmograph and oncometer. 



Pathological Conditions of Circulation. In mitral regurgitation 

 the dilatation and hypertrophy of the left side of the heart are due 

 to the blood running back from the ventricle at each systole. This 

 state of affairs keeps the auricle overfilled, and the backing of the 

 blood causes congestion in the pulmonary capillaries, which results in 

 cough and dyspnoea. 



Sudden death can result from a thrombus of the coronary artery 

 or an obliterating arteritis of this vessel. 



The secretion of the urine is, to a great extent, under the varying 

 arterial tension due to vasomotor activity. In fever the vasomotor 

 system is concerned in the flushing of the face and body. 



Hutchison states that the missing of beats in the pulse may be 

 caused by excess of excitability in the auricles, so that they contract 

 before the ventricles have finished their systole, or to occasional want of 

 conduction from auricle to ventricle in the muscle ; or by an excess of 

 excitability in the ventricles, so that they contract before the impulse 

 arrives from the auricle, hence, when this impulse does arrive the 

 ventricle is refractory and does not respond. We can. separate the 

 causes in part by the fact that ventricular intermission, during which 

 the auricle continues to beat regularly, must always be twice as long 

 as the ordinary pulse interval. 



Clinical experience teaches that irregularity of the heart seems 

 especially to take place when the auricle is diseased. There is a form 

 of irregularity of the heart due to a want of synchronism in action 

 between the two auricles or two ventricles respectively or between the 

 two sides of the heart as a whole. 



Cardiac Dyspnoea. Dyspnoea of heart disease is due to insufficient 

 oxidation of a slowed blood-current and to effect of C0 2 on the center 



