66 THE HEART. 



THE HEART. 



The heart consists, in its simplest form, of a muscular 

 dilatation provided with a valve or valves at either open- 

 ing, and a venous antechamber or reservoir, which in 

 the lower vertebrates is more perfect than in man and 

 mammalia. In the osseous fishes another dilatation 

 (the bulbus arteriosus) exists between the ventricle and 

 the branchial arteries, the function of which is to store up 

 energy during the ventricular systole, as the arteries do 

 in mammals. In the cartilaginous fishes the bulb is a 

 muscular organ in which energy originates, and it is often 

 provided with valves. These complications in the struc- 

 ture of the central organ are rendered necessary by the 

 simplicity of the circulation. In the batrachians the bulb 

 is less required, for only part of the blood-stream passes 

 through the respiratory apparatus, but the auricles are 

 still provided with valves. In the mammalian heart, the 

 mechanism of respiration renders the auricular valves 

 unnecessary. 



Motions of the Heart and phenomena which accompany 

 them. The form of the contracted human heart is that 

 of a cone, of which the base is elliptical and the apex 

 rounded off; in the relaxed state the heart assumes the 

 form of the wedge-shaped space in which it is contained. 

 It approaches the anterior wall of the chest in ex- 

 piration, and recedes in inspiration. In systole the ven- 

 tricles suddenly draw themselves together towards a part 

 of the septum which is about two-thirds of the way from 

 the auriculo-ventricular groove to the apex. 



On grasping the contracting heart of an animal it is felt 

 to widen and become harder. The impulse is due to 

 these changes of form and consistency. It is felt most 

 strongly between the fifth and sixth cartilages. 



Each heart period is divided into two parts, the period 



