APPLICATIONS OF THE GRAPHIC METHOD. 205 
the economy. There are variations, however, in the rapidity, 
force, regularity, and other qualities of the contraction in dif- 
ferent parts: thus, it is comparatively rapid in the iris, and ex- 
tremely powerful and regular in the uterus, serving to produce 
that prolonged yet intermittent pressure essential under the 
circumstances (expulsion of the foetus). 
Comparative-—Muscular contraction is relatively sluggish 
and prolonged among the invertebrates, to which, however, the 
movement of the wings of insects is a marked exception, some 
of them having been shown by the graphic method to vibrate 
some hundreds of times in a second. 
The slow movements of the snail are proverbial. Asa rule, 
the strength of the muscles of the invertebrates is incomparably 
greater than that of vertebrates, as witness the powerful grasp 
of a crab’s claw or a beetle’s jaws. 
These facts are in harmony with the generally slow metab- 
olism of most invertebrates and the lower vertebrates. 
The muscles of the tortoise contract tardily but with great 
power, resist fatigue well, retain their vitality under unfavor- 
able conditions, and after death for a very long period (days). 
Without resorting to elaborate experiments, the student 
may convince himself of the truth of most of the above state- 
ments by observing the movements of a water-snail attached 
to a glass vessel; the note made by the buzzing of an insect, 
and comparing it with one approaching it in pitch sounded by 
some instrument of music; the force necessary to withdraw 
the foot or tail of a tortoise; the peristaltic movements of the 
intestine and other organs in a freshly killed animal; or the 
action of a bee, wasp, or wood-boring beetle on the cork of a 
bottle in which one of them may be inclosed. 
SPECIAL CONSIDERATIONS. 
In the case of weakly (phthisical) persons a sharp tap on 
the chest will often produce a contraction of the muscles thus 
stimulated; but, in addition, a local contraction lasting some 
little time, known as a wheal or idio-muscular contraction, fol- 
lows. This phenomenon seems to be the result of a special 
irritability in such muscles. 
Cramp may arise under a great variety of circumstances, 
but it seems to be in all cases either a complete prolonged teta- 
nus, in which there is unusual muscular shortening in severe 
cases, at least, or the persistence of a contraction remainder. 
