78 MOTION OF THE 



it is requisite that the passages indicated should be blocked 

 up, that the difference in point of strength and other things 

 between the two ventricles begin to be apparent: in the 

 altered circumstances the right has only to throw the blood 

 through the lungs, whilst the left has to impel it through the 

 whole body. 



There are further within the heart numerous braces, so to 

 speak, fleshy columns and fibrous bands, which Aristotle, in his 

 third book on Respiration, and the Parts of Animals, entitles 

 nerves. These are variously extended, and are either distinct 

 or contained in grooves in the walls and partition, where they 

 occasion numerous pits or depressions. They constitute a kind 

 of small muscles, which are superadded and supplementary to 

 the heart, assisting it to execute a more powerful and perfect 

 contraction, and so proving subservient to the complete expulsion 

 of the blood. They are in some sort like the elaborate and 

 artful arrangement of ropes in a ship, bracing the heart on every 

 side as it contracts, and so enabling it more effectually and 

 forcibly to expel the charge of blood from its ventricles. This 

 much is plain, at all events, that some animals have them 

 strongly marked, others have them less so; and, in all that have 

 them, they are more numerous and stronger in the left than in 

 the right ventricle ; and whilst some have them in the left, there 

 are yet none present in the right ventricle. In the human 

 subject, again, these fleshy columns and braces are more nu- 

 merous in the left than in the right ventricle, and they are more 

 abundant in the ventricles than in the auricles ; occasionally, 

 indeed, in the auricles there appear to be none present what- 

 soever. In large, more muscular and hardier bodies, as of 

 countrymen, they are numerous; in more slender frames and in 

 females they are fewer. 



In those animals in which the ventricles of the heart are 

 smooth within, and entirely without fibres or muscular bands, 

 or anything like foveae, as in almost all the smaller birds, the 

 partridge and the common fowl, serpents, frogs, tortoises, and 

 also fishes, for the major part, there are no chorda? tendinese, 

 nor bundles of fibres, neither are there any tricuspid valves in 

 the ventricles. 



Some animals have the right ventricle smooth internally, but 

 the left provided with fibrous bands, such as the goose, 

 swan, and larger birds ; and the reason here is still the same 



