160 CAPILLARY VESSELS.
nexion with the central portions of the nervous system by
means of a single uninterrupted cell. The different parts of
the body, however, are connected together by another kind
of uninterrupted secondary cell, namely, the capillaries. The
capillary system, generated from several primary cells, forms
one single secondary cell. The cavity of the secondary cell
communicates with that of the large vessels. Researches are
still required to decide the question whether these latter are
mere dilatations of the capillaries, or whether they are formed
simply by the junction of other elementary parts. In the
latter case the capillary vessels would open into a cavity altogether distinct from their own, just as a vegetable cell opens
into an intercellular space. It sometimes occurs that the cavities of certain vegetable cells open directly outwards, but such
instances are very rare.
As a primitive muscular fasciculus, a nervous fibre and a capillary vessel are corresponding formations in this class; we may also compare these structures with the elementary parts of other tissues. The elementary cells of all tissues correspond with one another, beg formed universally according to similar laws. A blood-corpuscle, an epithelial cell, a cartilage-cell, an elementary cell of areolar tissue (therefore, also a fasciculus of areolar tissue formed from it), correspond to an elementary cell of muscle, &c. There is no structure analogous to an entire primitive fasciculus of muscle or a secondary muscle-cell or a nervous fibre amongst the principal component parts of the tissues previously discussed, because with them the forma- tion of secondary cells only occurs as an exception. A mus- cular fasciculus differs, therefore, from a fasciculus of areolar tissue, and a primitive fibre of areolar tissue has no analogy with a primitive muscular fibre.
