704 PRINCIPLES OF GENERAL PHYSIOLOGY 



The muscular tissue of the vertebrate heart initiates -the heat, and is also 

 responsible for the transmission of excitation from one part of the heart to 

 another. In the mammal, a localised bundle, that of His, conveys the excitation 

 from auricles to ventricles. The rate of the automatic rhythm is greatest in the 

 sinus tissue, so that this acts as the pace-maker. In the mammal, a remnant of 

 sinus tissue, the " Keith-Flack " or " sino-auricular " node, is the initiator of the 

 beats, and is in direct connection with the nerves controlling the rate of the 

 heart beat. 



The heart requires an abundant supply of oxygen, which is provided by 

 the copious flow of blood through the coronary circulation. The arterioles of 

 this system are very sensitive to the dilating action of products of the muscular 

 metabolism. 



The inhibitory action of the vagus nerves may show itself in different ways, 

 on rate, strength, conducting power, or excitability of the muscle. These effects 

 appear to depend chiefly on the particular function of the tissue in which the 

 fibres end. The duration of the state of excitation is lessened by vagus stimula- 

 tion, a fact which explains the abolition of the T-wave of the electro-cardiogram 

 by the vagus, since its action is naturally more pronounced at the base. 



There are also excitatory nerves, the accelerators, supplied to the heart muscle ; 

 their action is directly opposed to that of the vagus nerves. 



Both kinds of nerves can be excited reflexly. 



The importance of the elastic nature of the walls of the arteries is pointed out. 

 It accommodates, temporarily, the blood driven out by the rhythmic beats of 

 the heart, converting the flow through the capillaries into a continuous one. 

 Incidentally, it gives rise to the pulse wave. This wave is due to the elastic 

 recoil of the arterial wall, and must not be confused with the actual mass 

 movement of the current of blood. 



The resistance to flow in the blood vessels is due to the internal friction of the 

 blood, so that changes in the viscosity of the blood change the resistance. 



The total volume of blood in an animal is a function of the outer surface of 

 the animal. 



A brief description is given of the methods used for the investigation of 

 changes in the heart and circulation. 



The arterioles are supplied with two kinds of vasomotor nerves, vaso- 

 constrictor or excitatory, and vaso-dilator or inhibitory, in respect of the normal 

 tonus of the arterial muscular wall. 



The constrictor fibres are all of sympathetic origin. That of the dilators is 

 more varied. In some organs it is peculiar, the vaso-dilator impulses being 

 conveyed by the ordinary sensory fibres in an " antidromic " direction. These 

 fibres to blood vessels are, apparently, lateral branches of the sensory fibres, and 

 can thus give rise to axone reflexes, as in inflammation. 



While stimulation of sensory nerves in general causes rise of blood pressure 

 by reflex arterial constriction, there is one set of nerve fibres, arising from the 

 aorta and the heart, which always produces reflex fall of blood pressure. These 

 are known as the fibres of the depressor nerve. 



In reflex rise of blood pressure, excitation of the vaso-constrictor centre is 

 combined with inhibition of the tone of the vaso-dilator centre. In -reflex fall, 

 excitation of the vaso-dilator centre is combined with inhibition of tone in the 

 vaso-constrictor centre. Reciprocal innervation holds, therefore, as in the case of 

 skeletal muscle. 



Stimulation of the central end of the afferent nerve from an organ causes 

 reflex dilatation in the organ itself, with constriction elsewhere, thus ensuring the 

 maximal supply of blood to the organ. 



Strychnine and chloroform show their usual actions of converting inhibition 



