The name of Entrochi, or wheel stones, has with much propriety been applied to these insulated vertebræ. The perforations in the centre of these joints affording a facility for stringing them as beads, has caused them, in ancient times, to be used as rosaries. In the northern parts of England they still retain the appellation of St. Cuthbert's beads.
On a rock by Lindisfarn
Saint Cuthbert sits, and toils to frame
The sea-born beads, that bear his name.
MARMION.
Each of these presents a similar series of articulations,
varying as we ascend upwards through the body of the animal,
every joint being exactly adjusted, to give the requisite
amount of flexibility and strength. From one extremity of
the vertebral column to the other, and throughout the hands
within, the perpendicularly lengthened margin of the wider, a. b.; the outer crenelated edge of the narrower included vertebræ, articulate with the inner crenelated edge of the wider vertebræ, which thus surround them with a collar, that admits of more oblique flexion than the plane crenelated surfaces near the base of the column, Figs. 9, 10, and at the same time renders dislocation almost impossible.
To these is superadded a third contrivance, which still further increases the flexibility and strength of this portion of the column, viz. that of making the alternate larger joints, b. b. considerably thinner than the largest collar joints, a. a.
The figures numbered from 11 to 26 inclusive, represent single vertebræ taken from various portions of the column of Encrinites moniliformis. The joints at Figs. 11, 13, 15, 17, 19, 21, 23, 25, are of their natural size and in their natural horizontal position, and show, at the margin of each, a crenated edge, every tooth of which articulaled with a corresponding depression near the margin of the adjacent joint. The stellated figures (12, 14, 16, 18, 20, 22, 24, 26,) placed beneath the horizontal joints to which they respectively belong, are magnified representations of the various internal patterns presented by their articulating surfaces, variously covered with an alternate series of ridges and grooves, that like the cogs of two wheels, articulate with corresponding depressions and elevations on the surfaces of the adjacent vertebræ.
