96 The American Naturalist. (February, — a 
in regard to its anterior face, but the second overlaps the : 
first, the third, the second, and the fourth. In other words, 
the fourth has pulled both ways as the second has in the — 
shrimps, but in the latter the second segment is obviously the 
largest of the series, while the abdominal segments of Galathea 
are all of about the same size and strength. A 
We may, however, provisionally accept as a natural conse- 
quence and general rule, that the motion of a movable part 
naturally hollows out for itself a cavity in which to move — 
within the solid and fixed basis. This may be further illus- 
trated in the hard parts of the vertebrata. 
It is a well known fact that even when the muscles are said 
to be at rest, there is more or less of a strain, or pull exerted. 
When the human hand hangs loosely down, it rests half way ` 
between extreme flexion and extreme extension ; the same may 
be said of the fingers, although in some persons, as in sailors, 
where the flexor muscles are very strong the fingers hang as s0 
many “ hooks, ” owing to the pull of the more powerful flexor 
muscles. This is well shown in cases of the paralysis of onesetof 
muscles ; the parts are drawn to the side of the healthy muscles : 
even when no voluntary action is exerted by the normal mus- 
cles. This is too well known to medical men to require — 
further comment, and is especially illustrated in cases of facial _ 
paralysis where the face is drawn strongly to one side by the | 
normal muscles. 1 
Taking the action of the normal muscles when at so-called 
rest, it is not surprising to find that in the joints the lighter 
or more mobile part, being continually pulled against the 
fixed part and moved upon it, that a cavity should be 
formed in the fixed parts by the continual action and pres- 
sure of the mobile part. ‘Take for example, the humerus in 
_ man, the head is hemispherical and fits into a concave portion — 
of the scapula, a true ball and socket joint, and it is formed, if | 
there be any truth in the idea, by the humerus being pulled ` 
into the scapula by the powerful muscles of the shoulder. 
And as this is probably the first joint which was formed in the 
=~ evolution of the limb which then acted as a fin, the rotatory 
_ motion aided and finally established the ball and socket form 
