278 PHYSIOLOGY. 



Bottazzi holds that the tonus is due to sarcoplasm. The curves 

 he believes should be considered spontaneous isometric curves of 

 sarcoplasm. Muscarin stimulates the changes in tonus; atropin and 

 nicotin completely paralyze them. The tone of a heart is not constant, 

 and it opposes dilatation of the heart from pressure on the inside of its 

 cavities during their relaxation. 



Lessening in tone is one of the most frequent occurrences in heart 

 disease, and when tone is decreased, conductivity and excitability are 

 increased; and this may explain some of the most frequent cases of 

 irregularity of the cardiac organ. 



Digitalin applied to the heart-muscle of the terrapin, shortens it, 

 raises the line of tonicity of the auricles and diminishes the rate and 

 the variations in tonicity. In man digitalis has the power to increase 

 the tone of cardiac muscle, and prevent dilatation of the heart. The 

 poisons of diphtheria and influenza lower heart-tone. 



THE ARTERIES. 



All vessels leaving the heart are arteries. From it proceed the 

 aorta and pulmonary artery, the former from the left, the latter from 

 the right ventricle. All of the branches of the arteries continue to 

 divide to form smaller arteries, these in turn become arterioles, which 

 are followed by capillaries (hair-like vessels) . To cause as little friction 

 as possible the branches are almost uniformly given off at an acute 

 angle. The total area of the cross-sections of the branches is usually 

 greater than the sectional area of the original trunk from which the 

 branches sprung. As the distance from the source is increased the 

 area supplied by the branches is increased also, giving the general 

 impression of a cone in its contour; its base is outlined by the 

 capillaries, its apex being represented by the point from which the 

 branch springs from the parent trunk. 



The pulmonary artery arises from the right ventricle in front 

 of the origin of the aorta under whose arch it very shortly passes, 

 then to divide into two main branches, one for each lung. Within 

 the lung-substance they divide and subdivide very rapidly to form 

 numerous capillaries, in order that the blood may become thoroughly 

 oxidized. The arteries, (unlike veins) are usually found empty and 

 dilated after death. 



Arterial Structure. The walls of the arteries are composed of 

 three coats: an internal one* of endothelial nature, the tunica intima; 

 a middle coat of muscular fibers, tunica media; and an external, 

 cellular coat, tunica adventitia. 



