ON MUSCULAR MOTION. 199 



This rapid (ketch of (he hiftory of mufcular ftru6lure has 

 been obtruded before (lie Royal Society to introduce the prin- 

 cipal experiments, and reafonings which are to follow: they 

 are not ordered with fo much exa6lnefs as becomes a more 

 deliberate efTay, bat the intention already ftated, and the limits 

 of a ledure are offered as the apology. 



Temperature has an efTential influence over the actions of Temperature of 

 raufcles, but it is not necefl^ary that the fame temperature"*" *^^^' 

 fliould fubfift in all mufcles during their actions; neither is it 

 elTential that all the mufcular parts of 'the fame animal fliould 

 be of uniform temperatures for the due performance of the 

 motive functions. 



It appears that all the claffes of animals are endowed with All animals do 

 fome power of producing thermometrical heat, lince it has^*" ""^^ " ' 

 been fo eftabliflied in the amphibia, pices, vermes, and infe6ta, 

 by Mr. John Hunter ; a fafl which has been verified to my 

 own experience ; the term '< cold-blooded*' is therefore only 

 relative. The ratio of this power is not, however, in thefe 

 examples, fufficient to preferve their equable temperature in 

 cold climates, fo that they yield to the changes of the at-but arealfoaf- 

 mofphere, or the medium in which they refide, and mofl of ^"^f ^^ ^^ ^^?"* 

 them become torpid, approaching to the degree of freezing tion, 

 water. Even the mammalia, and aves, pofTefs only a power 

 of relifting certain limited degrees of cold ; and their furfaces, 

 as well as their limbs, being diftant from the heart, and prin- 

 cipal blood. vcfleis, the mufcular parts fo fituated are fubjefl 

 toconfiderable variations in their temperature, the influence of 

 which is known. 



In thofe dalles of animals which have little power of gene- The colder anl- 

 raling he^-t, there are remarkable differences in the ftrudure^ 

 of their lungs, and in the corapolition of their blood, from the 

 mammalia and aves, 



Refpiration is one of the known caufes which influences the Refplratloni its 

 temperatures of animals : where thefe organs are extenfive, oShe/te^-'* 

 the refpiratlons are performed at regular intervals, and are not peratur* j 

 governed by the will, the whole mafs of blood being expofed 

 to the atmofphere in each circulation. In all fuch animals 

 living without the tropics, their temperature ranges above the 

 ordinary heat of the atmofphere, their blood contains more of 

 the red particles than in the other clafl^es, and their mufcular 

 irrilqibility ceafes morQ rapidly after violent death, 



Tlie 



