S82 Royal Society : — 



PROCEEDINGS OF LEARNED SOCIETIES. 



EOYAL SOCIETY. 



June 17, 1869. — Lieut.-General Sabine, President, in the Chair. 



" On some Elementary Principles in Animal Mechanics." — No. II. 

 By the Rev. Samuel Hatjghton, M.D. Dubhn, D.C.L. Oxon., 

 Fellow of Trinity College, Dublin. 



In a former communication to the Royal Society on this subject 

 (Annals, October 1867, p. 294), I endeavoured to establish the two 

 following principles : — 



I. That the force of a muscle is proportional to the area of its 

 cross section. 



II. That the force of a muscle is proportional to the cross section 

 of the tendon that co7weys its influence to a distant point. 



The first of these principles is true under all circumstances ; but 

 the second requires to be modified somewhat in its statement. If 

 the conditions as to friction of the tendons that convey the action of 

 the muscles to a distant point be the same, then the force of the 

 muscles will be proportional to the cross sections of the tendons ; 

 but if the tendons be subjected to different amounts of friction, 

 then the areas of their cross sections will cease to be proportional to 

 the forces of the muscles as represented by the areas of their cross 

 sections. 



In my former paper (No. I.), I selected, in illustration of prin- 

 ciple II., the long flexor tendons of the toes of the Rhea and other 

 struthious birds, and showed that the cross sections of the muscles 

 and tendons bore, approximately, a constant ratio to each other. 

 Now, in the Struthionidce the conditions as to friction of the long 

 flexor tendons of the toes are similar, although different in each 

 species ; and hence it was easy to prove that the ratios of the cross 

 sections of the muscles and tendons were nearly constant. 



"Wben, however, muscles and tendons, variously conditioned as to 

 friction, are compared together, the constancy (if the ratio of their 

 cross sections disappears, and undergoes a variation depending on 

 the friction to which both muscles and tendons are exposed. 



In order to ascertain the proportion of the cross section (or force) 

 of a muscle to the cross section (or strength) of its tendon in the 

 human subject, I made the following observations on the right arm 

 and hand of a well-developed male sxibject in the Royal College of 

 Surgeons in Ireland, in March 1868. 



I first ascertained the specific gravities of the muscles and tendons, 

 ,with the following results : — 



Muscles. 



Sp. gr. 



Biceps humeri {long head) 1 '050 



Biceps humeri 1 •054 



Brachialis I '053 



Mean 10523 



