52 



THE HEART 



part of the heart which has the feeblest contraction, and 

 is least in that part where contraction is strongest. Here 

 then is a partial differentiation of the two fundamental 

 properties of heart-muscle — rhythmicity and contractiUty.j 

 It now remains to us to show how far this explanation 

 of the nature of the heart-beat is apphcable to the more 



Fig. 8. — A generalised type of vertebrate heart (Keith), a, sinus 

 venosus ; b, sino-auricular canal ; c, auricle ; d, ventricle ; c, bulbus 

 cordis; f, aorta. 



comphcated heart of the mammal. A g'enerahsed form of 

 primitive vertebrate heart is shown in Fig. 8. It is 

 composed of four serial chambers : (a) the sinus, (b) the 

 auricular canal, part of which is invaginated into (d) the 

 ventricle, (e) the bulbus cordis. The auricle is a lateral 

 diverticulum of the auricular canal. The sinus and 

 auricular canal may be regarded as forming the rhythmic, 

 the auricle and ventricle the contractile parts. 



