"VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 531 



of their continuance appears to be in the inverse proportion of the quantity of 

 contraction ; for muscles, whose usual functions consist in short contractions, 

 can go on for a long time, while those which are performed by long contractions 

 soon cease. 



In the muscles of a paralytic arm, their action, to a certain extent, is con- 

 tinued for years (the times of sleeping excepted), without any effect being pro- 

 duced on the constitution, or the parts themselves ; but in epileptic fits, in which 

 the actions are equally involuntary, only requiring longer contractions, they soon 

 cease, leaving the person greatly exhausted ; an effect which must arise from the 

 quantity, not the frequency, of the contractions. If we attend to the actions of 

 the involuntary muscles, we find that they are continued through life, but that 

 the quantity of contraction is very small ; and if from any circumstance the 

 quantity should be increased, it cannot be continued, the parts being unable to 

 sustain it for any length of time. The diaphragm, and intercostal muscles, act 

 constantly in performing the functions of respiration, but they do not exert them- 

 selves to their full extent. In laughing, which is likewise an involuntary action, 

 the contractions of these muscles are more extensive, therefore if continued be- 

 yond a very short period become so distressing, that a cessation necessarily 

 ensues. 



Muscular contraction is never made use of in an animal body, where any other 

 means can produce the same effect, and for this reason elastic ligaments are fre- 

 quently substituted for muscles; even where muscles are employed, various means 

 are applied to diminish the quantity of contraction. It is curious, in tracing the 

 different forms of muscles, and in considering the uses for which they are em- 

 ployed, to observe how variously the fibres are disposed, evidently for the purpose 

 of obviating the necessity of great contractions ; and the quantity of muscular 

 action saved by this mechanism is greater, in proportion to the frequency and im- 

 portance of the effect the muscle is intended to produce : this appears to be in- 

 variably the case. 



Muscles only occasionally called into action, have their fibres nearly straight, 

 which gives no mechanical advantage ; the sartorius is an instance of this kind. 

 Muscles frequently used are more complicated, as those of the fingers are half 

 penniform in their structure ; the muscle for raising the heel in walking is pen- 

 niform ; that which raises the shoulder, complex penniform ; and those of the 

 ribs, cruciform. That the 1 sets of intercostal muscles act at the same time, I 

 proved by experiment in the year 1776, I removed a portion of the external in- 

 tercostal muscles from the chest of a dog, and in that way saw very distinctly the 

 two sets of muscles in action. The fibres of both sets contracted exactly at the 

 same time. 



The particular structures of these different forms of muscles, and the mechani- 



3 Y 2 



