Similarly, the amount of this compound structure present in different parts appears to depend mainly on its uses.
Its offices of uniting the different textures, and of convoying the vessels and nerves, render it necessary that more or less of the tissue should always be present on the exterior of an organ ; and the same circumstances would lead us to expect a slight penetration of its surface.
In the interior of organs, however, its absence is by no means infrequent, and is very significant of its use. Thus, the minute elements of the osseous tissue are physically insusceptible of movement; the permissive and facilitating structure becomes unnecessary and impossible ; and is therefore absent. The highly delicate nervous pulp not only pos sesses no inherent mobility, but, by the ex treme delicacy of its structure, offers a physiological obstacle to movement of equal importance with the preceding, and is accom panied by a similar absence of the tissue. The intimate mutual connection of the mus cular fibres of the heart, and their association in a common and nearly simultaneous move ment, is associated with a like deprivation of this interstitial structure. The same absence at once of the necessity and of the tissue is seen in glandular organs, the situation of which shields them from injurious external force, as appears to be the case with the liver.
But where opposite circumstances obtain, where extent and variety of movement imply considerable mobility of the neighbouring muscles of a limb, or situation exposes an organ to external violence, a large quantity envelopes these different textures, penetrating between the different muscles and isolating their several fibres, or breaking up the gland into numerous subdivisions, moveable on each other : of this latter, the mamma is a familiar instan ce.
A similar relation might be traced in the wider circumstances of its application. Not only does it form a web of union to the whole body, but it also presents a special layer of considerable thickness, which invests its sur face, and partitions vvhich isolate its muscles. And something of a corresponding minimum is found in those animals whose locomotive movements are few and simple, or whose situation and habits little expose them to external violence. So that a rough gradation might be traced through fishes, cetaceans, and reptiles, to mammals ; in which last class man stands pre-eminent in the number and complexity of his voluntary motions, and in the remarkable amount of this subservient tissue.
An increase in the freedom of movement of contiguous parts is associated with an increased laxity of this web, the meshes of which become both longer and wider, so as to be more capable of stretching. They thus allow a greater amount of separation to take place between the parts which are attached to the extremities of their irregular net-work.