As in the great motions of the limbs, in which all the muscles are the most liable to be displaced, these are necessarily in action, they distend powerfully the aponeurosis, which thus reflects the motion that is communicated to it, and resists especially every displacement. When the limb is at rest, the tensor muscles cease their contraction, and the aponeurosis is relaxed. I would observe, that the muscles attached to the fibrous capsules, as to that of the humerus, for example, perform for them the same functions, that the tensor muscles do for their respective aponeuroses. The colour of these last is a brilliant white; in this respect they differ from all the fibrous organs thus far examined, and are analogous to the tendons, from which they differ a little however in their nature; in fact, they yield less quickly to maceration and ebullition; their fibres are more stiff and resisting. There are no aponeuroses exactly like the tendons, except those which are essentially formed by their expansion or which are at their origin, as those spread upon the anterior rectus of the thigh, those which are concealed in the fleshy fibres of a muscle, and afterwards go out of it to become a tendon. In certain parts of the limbs, as at the top of the arm, for example, the aponeuroses of general covering are insensibly lost in the cellular texture, without our being able to draw the line of demarcation. This arrangement is almost peculiar to the fibrous system; at least I know of no one which thus intermixes and loses its fibres in the cellular texture; it is so much the more remarkable, as the nature of the two textures is essentially different; they do not yield the same products, and they have not the same organic arrangement. The fibres of the general aponeuroses are only interlaced in two or three directions; this interlacing is almost always very evident to the naked eye. But I have observed that by plunging an aponeurosis into boiling water, and leaving it there for some time, its fibres, in the horny hardening they then undergo, become still much more evident. This observation is moreover applicable to the whole fibrous system, to its organs especially, whose texture but little apparent seems at first view to be homogeneous. In this way, we distinguish very well the fibres of the dura-mater. _Functions._