258 VETERINARY PHYSIOLOGY 



(b) The rate of the rhythmic movements of auricles and 

 ventricles is increased. 



(c) The force of contraction of auricles and ventricles is 

 increased. 



It is probable that the cardiac sympathetic also carries 

 ingoing fibres which enter the cord in the lower cervical 

 region. The pain experienced in the arm in heart disease 

 is generally thought to be due to the implication of these 

 fibres leading to the sensation which is referred to the cor- 

 responding somatic nerves (p. 151). 



The vagus is thus the protecting nerve of the heart, 

 reducing its work and diminishing the pressure in the 

 arteries. 



The sympathetic is the whip which forces the heart to 

 increased action in order to keep up the pressure in the 

 arteries. 



III. CIRCULATION IN THE BLOOD AND LYMPH VESSELS 



The general distribution of the various vessels arteries, 

 capillaries, veins, and lymphatics has been already con- 

 sidered (fig. 107, p. 225). 



Structure 



(The structure of the walls of each must be studied practically.) 

 The capillaries are minute tubes of about 12 micro- 

 millimetres in diameter, forming an anastomising network 

 throughout the tissues. Their wall consists of a single 

 layer of endothelium. On passing from the capillaries to 

 arteries on the one side, and to veins and lymphatics on 

 the other, non-striped muscle fibres make their appearance 

 encircling the tube. Between these fibres and the endothelium 

 a fine elastic membrane next appears, while outside the muscles 

 a sheath of fibrous tissue develops. Thus the three essential 

 coats of these vessels are produced : 



Tunica intima, consisting of endothelium set on the in- 

 ternal elastic membrane. 



Tunica media, consisting chiefly of the visceral muscular 

 fibres. 



Tunica adventilia, consisting of loose fibrous tissue. 



