3io THE VASCULAR SYSTEM 



the second, third and fourth cervical nerves (Fig. 69). It is 

 probable that in these cases the stimuli reach the upper part 

 of the spinal medulla by a downward spread from the lower 

 extremity of the sensory nucleus of the vagus. Under these 

 circumstances, the sterno-mastoid and the upper part of the 

 trapezius are usually found to be tender to the touch, for these 

 muscles, although receiving their motor supply from the 

 accessory nerve, receive sensory branches from the cervical 

 plexus (p. 128). 



THE CARDIAC BLOOD-VESSELS. The heart is supplied by 

 the right and left coronary arteries, which arise from the aortic 

 sinuses (p. 318). The orifices of these vessels are so situated 

 that the blood may have difficulty in entering them in 

 atheromatous changes in the wall of the aorta near its origin 

 or in the degenerative changes of the aortic valve which lead 

 to aortic stenosis. Under these conditions, the muscular wall 

 of the heart undergoes degeneration and the circulatory 

 disturbance becomes more pronounced on that account. 



The veins of the heart join the coronary sinus, which pours 

 its blood into the right atrium (auricle). 



The Musculature of the Heart. The muscular fibres of 

 the atria (auricles) pass uninterruptedly from the one atrium to 

 the other, but, with the exception of the atrio-ventricular 

 bundle (of His), they do not become continuous with the 

 muscular fibres of the ventricular walls. Both atrial and 

 ventricular fibres are attached to the fibrous rings which 

 bound the atrio-ventricular orifices and constitute what 

 Keith has termed the atrio-ventricular base of the heart. 



The primitive tubular heart of the embryo is so altered by 

 the flexures which it undergoes that the atrio-ventricular 

 orifices become placed side by side with the arterial orifices, 

 and not, as in the embryo, at opposite ends of the ventricle. 

 Keith believes that, as a result of these changes, the ventricular 

 muscle fibres are so arranged that, when the ventricle contracts, 

 the distance of the ventricular apex from the "aortic base" 

 (arterial orifices) is unaltered, whereas its distance from the 



