§ 54e. Returning from this parenthetical discussion to the subject of structure, we have to observe that besides facts presented by tissues and facts presented by organs, there are certain facts, less general than the one and more general than the other, which must now be noted. In the order of decreasing generality an account of organs should be preceded by an account of systems of organs. Some of these, as the muscular system and the osseous system, are co-extensive with tissues, but others of them are not. The nervous system, for example, contains more than one kind of tissue and is constituted of many different structures: besides afferent and efferent nerves there are the ganglia immediately controlling the viscera, and there are the spinal and cerebral masses, the last of which is divisible into numerous unlike parts. Then we have the vascular system made up of the heart, arteries, veins, and capillaries. The lymphatic system, too, with its scattered glands and ramifying channels has to be named. And then, not forgetting the respiratory system with its ancillary appliances, we have the highly heterogeneous alimentary system; including a great number of variously-constructed organs which work together. On contemplating these systems we see their common character to be that while as wholes they cooperate for the carrying on of the total life, each of them consists of cooperative parts: there is cooperation within cooperation. There is another general aspect under which structures must be contemplated. They are divisible into the universal and the particular--those which are everywhere present and those which occupy special places. The blood which a scratch brings out shows us that the vascular system sends branches into each spot. The sensation accompanying a scratch proves that the nervous system, too, has there some of its ultimate fibrils. Unobtrusive, and yet to be found at every point, are the ducts of the lymphatic system. And in all parts exists the connective tissue--an inert tough substance which, running through interspaces, wraps up and binds together the other tissues. As is implied by this description, these structures stand in contrast with local structures. Here is a bone, there is a muscle, in this place a gland, in that a sense-organ. Each has a limited extent and a particular duty. But through every one of them ramify branches of these universal structures. Every one of them has its arteries and veins and capillaries, its nerves, its lymphatics, its connective tissue.