These sheaths exhibit also two great modifications; in the one, as on the anterior part of the wrist, all the tendons are found contiguous, separated only by a kind of loose membrane which is placed between them; in the others, as on the posterior part of the wrist, under the general sheath, are found small fibrous partitions, which separate the tendons from each other. In general, the resistance of these sheaths is very considerable.
ARTICLE EIGHTH.
OF THE APONEUROSES.
We have distinguished aponeuroses as being of two classes, those for covering and those for insertion.
I. Of the Aponeuroses for Covering.
The aponeuroses for covering are general or partial.
Aponeuroses for General Covering.
They are found around the limbs, whose muscles they tie down. The arm, the fore-arm and the hand, the thigh, the leg and the foot, are provided with them.
Forms.
They are, in their conformation, analogous to the form of the limb, which they in part determine, and which they especially maintain, by preventing the displacement of the subjacent parts, a displacement which would continually take place, from the laxity of the cutaneous organ. Their thickness varies. In general, the greater the number of the muscles they cover, the greater their thickness; hence why the aponeurosis of the fascia lata is superior in this respect to the brachial; why that of the fore-arm is thicker in front than behind; why the plantar and palmar are so considerable, whilst hardly any fibres are found on the back part of the foot and the hand. There are however some exceptions to this rule; for example, the aponeurotic covering of the posterior part of the leg is not in proportion to the power of the gastrocnemii and solæus muscles; thus these muscles are more than all the others exposed to displacements, frequently very painful, which constitute cramp, and which it is necessary to distinguish from the pains or numbness which result from the compression of one of the nerves of the lower limbs, as of the sciatic, or the external plantar, a compression produced by a bad position, or any other analogous cause.