kl . ;i I.A1ION OF TIIK CIRCULATION 



is not allowed to grt dry. It is the pro|>crty of the muscle of 

 the heart to go on contracting anil relaxing so long as it is 

 alive and not interfered with. 



The nerves going to the heart do carry impulses from the 

 brain to the heart, but these are only to regulate the strength 

 and frequency of the beat, not to originate it. When work 

 is being clone, the muscles require more blood to bring them 

 mori- oxygen, and to take away more waste products, and, in 

 order to meet this, the heart beats stronger and faster. This 

 the heart does in accordance with influences which are brought 

 to hear on it through the nerves going to it. Or again, a 

 man may faint from a " blow in the stomach," or from severe 

 mental or bodily pain. This is caused by a sudden weaken- 

 ing or even stopping of the heart, and is brought about by 

 impulses reaching the heart from the brain along one of the 

 nerves going to the heart. 



The lower part of the brain, where it becomes continuous 

 with the spinal cord, is called the spinal bulb or medulla 

 oblongata (see chapter on the Nervous System). This is the 

 region of the brain which regulates the heart. From the 

 spinal l.ulj) srvrr.il neives arc -ivcn off on each side, ;md 

 une of tlu-M- is (ailed the vatjus nerve. It nms down the 

 neck to the thorax and abdomen, giving branches to the 

 heart, lungs, and stomach. Impulses of sufficient strength 

 sent along the vagus nerves to the Heart cause it almost at 

 once to stop beating, its muscular fibres relax, and then, 

 instead of contracting again, remain relaxed, so that the heart 

 becomes dilated with blood, and remains still and motionless. 

 After a short time, which is generally measured onlyby seconds, 

 the heart commences to beat again, the beats being as strong as, 

 and, indeed, often at first stronger than before. If the impulses 

 reaching the heart are weak ones only, the effect is, not to stop 

 the heart, but to diminish the rate of beat, and usually at the 

 same time to make each beat weaker. It is by gentle impulses 

 pacing down the vagus nerves from the spinal bulb that the 

 heart's beat is chiefly regulated, and, indeed, in many 

 animals such impulses arc almost always passing, and so 

 restraining the heart. When such an animal requires a 

 more active circulation, as when it is running, these im- 

 pulses for the time cease, and in consequence the heart, with 



