122 ANIMAL PHYSIOLOGY 



are about 75 in the male, and 85 in the female ; and in old 

 age the average is lower. 



The heart's action is easily influenced in its regularity, 

 strength, and rapidity by the amount of blood in the body, 

 and by the nervous impressions conveyed from other parts. 

 Thus, it may be weak from want of blood, or, when the 

 deficiency is sudden or great, it may be fluttering or irregular ; 

 and, on the other hand, the rarer phenomenon is occasionally 

 observed of interference with the heart's action dependent 

 011 a superabundance of blood. Emotions also, and conditions 

 of the viscera, send impressions through the nerves which 

 readily disturb the heart. But it is important to observe 

 that the rhythmic action may continue when all connection 

 with other parts has been cut off. A turtle's or a frog's heart 

 will continue to beat when removed from the body, and the 

 successive contractions of its parts will continue to take 

 place in regular sequence, even though there is no longer 

 any blood to stimulate it. When it is divided vertically the 

 portions continue to beat, and when, divided transversely 

 the rhythm continues in the basal part, but is lost in the 

 apex. There are not only numerous nerves, but likewise 

 minute nerve-centres, the ganglia of fiemalc, scattered over 

 the heart ; and by these, kept in communication with one 

 another by the copious nerves in the auriculo-ventricuJar 

 groove, the action is immediately governed (p. 216). No doubt 

 it is difficult to understand how the nerves are stimulated to 

 produce rhythmic contraction; but it must not be forgotten 

 that the pulsation of the heart is only one of a great number 

 of instances of periodicity in nervous action, and that any 

 nervous action frequently repeated has a tendency to go oil 

 recurring. 



90. The Arteries, into which the blood is sent by the 

 heart, are a series of branching, elastic, and contractile 

 tubes. They have a smooth internal lining, and externally 

 have a tough felted coat of areolar tissue ;' but the main 

 thickness of their walls consists of a middle coat of elastic 

 and muscular fibres intermixed and arranged circularly, lying 

 among meshes of elastic membrane. In the larger arteries 

 the muscular fibres are exceedingly small, and the elastic 

 fibres abundant; but as the vessels get smaller there is a 



