VASCULAR NERVOUS SYSTEM. 8/ 



ventricle, the rhythmical action is resumed, provided that 

 the middle part of the base of the ventricle remains 

 intact. So soon as this part is cut off or destroyed, the 

 rhythmical contractions cease ; it is therefore believed to 

 contain the motor centre for the rhythmical motion of the 

 ventricle. After its removal, the ventricle responds to 

 each single or mechanical excitation by a single contrac- 

 tion, determined by the direct action of the excitant on 

 the muscular fibre. Similar motor centres are inferred to 

 exist in the sinus, auricles, and bulb. Application to 

 the beating heart of a trace of solution of the alkaloid 

 muscarine, stops it in diastole. The effect is promptly 

 counteracted by the application of solution of atropine in 

 similar manner and quantity. A heart so ' atropinized ' 

 cannot be stopped in diastole, either by mechanical or 

 electrical excitation of its inhibitory centre. 



Ganglia exist in the hearts of the higher animals, but 

 nothing is known of their functions. 



Influence of the Nervotis System on the blood vessels 

 (Vascular Nervoits System}. The principal vaso-con- 

 strictor centre is situated in the upper part of the floor 

 of the fourth ventricle ; subordinate centres exist in the 

 spinal cord, both in mammalia (Strieker) and in the frog. 

 The channels of the influence of these centres on the 

 arteries are contained in the lateral columns of the spinal 

 cord, from which they extend by the anterior roots and 

 rami communicantes to the ganglia and prsevertebral 

 plexuses of the sympathetic system, whence vaso con- 

 strictor nerves are distributed to the arteries. 



The constrictor centres are in constant action ; their 

 activity varies with the CO2 tension of the blood, and is 

 consequently augmented by arrest of the circulation in the 

 brain. The centres are also influenced by excitation of 

 sensory nerves, of which the ordinary effect is to increase 

 their activity. But in the case of the afferent fibres which 

 reach the vagus from the heart (Depressor fibres), the 



