430 



ANIMAL MECHANICS. 



The agreement in muscular type here shown between the 

 Emu and Cassowary is very interesting, and corroborates the 

 evidence as to their similarity drawn from other sources ; but 

 the muscular type here given is totally different from that of 

 the Ostrich and Rhea, and equally irreconcileable with that of 

 ordinary birds. 



From the foregoing facts, I think we are entitled to con- 

 clude that the fore limbs of the Struthionidae were never 

 intended to be used as wings, and that they are constructed 

 for some other purposes, which differ essentially in the two 

 groups, into which these remarkable birds may be divided. 



12. Principle of least Action applied to the Arrangement of 

 the muscular Fibres of the Heart. — At page 137 and following 

 pages, I have shown the amount of work done by the heart ; 

 and I propose here to inquire how that work is done, and to 

 show that it is done in conformity with the principle of 

 " least action." 



The heart is composed of innumerable muscular fibres 

 arranged in what Borelli calls a glomijile manner, similar to a 

 ball of twine. More correctly, we might compare the heart 

 to two balls of twine, each having a cavity in its centre, and 

 both surrounded by a third ball of twine enveloping the whole 

 structure. 



The muscular fibres of the heart are, however, not 

 continuous, as this illustration supposes, but are indepen- 

 dent of each other, the appearance of the whole being 



glomijile. 



The law of muscular contraction which has to be fulfilled 

 is the following : if I denote the length of a single fibre before 

 contraction, and V its length after contraction, whenever the 

 muscular fibre is perfectly free, it is found that V bears a con- 

 stant ratio to I ; or 



V = nl 



(109) 



