276 



THE CIRCULATION. 



can still be readily excited by touching the heart with the point of 

 a steel needle. If the heart be now held by its base between the 

 thumb and finger, with its point directed upward, it will be seen 

 to have a pyramidal or conical form, representing very nearly in 

 its outline an equilateral triangle (Fig. 85) ; its base, while in a 

 condition of rest, bulging out laterally, while the apex is compara- 

 tively obtuse. 



Fig! 85. 



Fig. 815. 



HEAKT op FROO 

 in a state of relaxa- 

 tion. 



HEAKT OF FKOO in cont action. 



Fig. 87, 



"\Yhen the heart, held in this position, is touched with the point 

 of a needle (Fig. 86), it starts up, becomes instantly narrower and 

 longer, its sides approximating and its point rising to an acute 

 angle. This contraction is immediately followed by a relaxation ; 

 the point of the heart sinks down, and its sides again bulge out- 

 ward. 



Let us now see in what manner this change in the figure of the 

 ventricles during contraction is produced. If the muscular fibres 



of the heart were arranged in the form of 

 simple loops, running parallel with the 

 axis of the organ, the contraction of these 

 fibres would merely have the effect of di- 

 minishing the size of the heart in every 

 direction. This effect can be seen in the 

 accompanying hypothetical diagram (Fig. 

 87), where the white outline represents 

 such simple looped fibres in a state of re- 

 laxation, and the dotted internal line indi- 

 cates the form which they would take in 

 contraction. In point of fact, however, 

 none of the muscular fibres of" the heart 



run parallel to its longitudinal axis. They are disposed, on the 

 contrary, in a direction partly spiral and partly circular. The most 

 superficial fibres start from the base of the ventricles, and pass 



Diagram of SIMPLE LOOPED 

 FIBRES, in relaxation and Con- 

 traction. 



