EPITHELIAL FORMATIONS. 57
differs; for that is a real organ, in which we can distinguish at least four different tissues. We have in it the tela ossea properly so called, the cartilaginous layer, the stratum of connective tissue belonging to the periosteum, and the peculiar medullary tissue. These several parts again are exceedingly heterogeneous in their nature, inasmuch as, for example, vessels and nerves enter into the composition of the marrow, the periosteum, etc. All these must be taken together to constitute the entire organism of a bone. Before we come, therefore, to systems or apparatuses, properly so called, the special subject of descriptive anatomy, a long series of gradations must be traversed, and in discussions we must always begin by having a clear idea of what the question is. When bone and osseous tissue are confounded together, the extremest confusion is occasioned, and so also when it is sought to identify nervous with cerebral matter. The brain contains many things which are not of a nervous nature, and its physiological and pathological conditions cannot be comprehended if they are regarded as occurring in an aggregation of purely nervous parts, and no consideration is paid to the membranes, the interstitial substance, and the vessels, as well as the nerves.
If, now, we consider the first of the classes into which we have divided General Histology, namely, the simple cellular tissues, a little more attentively, we find that those of which we can best obtain a general idea are unquestionably the epithelial formations, such as we meet with in the epidermis and the rete Malpighii, upon the external surface of the body, and in the cylindrical and scaly epithelium of mucous and serous membranes. Their general plan is, that cell lies close to cell, so that in the most favorable specimens, as in plants, four-or six-sided cells lie in immediate apposition one to the
