on Muscular Motion. 
213 
parts being unable to sustain it for any length of time. The 
diaphragm, and intercostal muscles, act constantly in perform- 
ing 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 beyond 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 frequently 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 employed, to ob- 
serve 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 importance of the 
effect the muscle is intended to produce : this appears to be 
invariably the case. 
Muscles only occasionally called into action, have their fi- 
bres 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 penniform ; that which raises 
the shoulder, complex penniform. ; and those of the ribs, cru- 
ciform. 
That the two sets of intercostal muscles act at the same 
time, I proved by experiment in the year 177b. I removed 
