EPITHELIAL FORMATIONS. 57 



differs ; for that is a real organ, in which we can distin- 

 guish 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 grada- 

 tions 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 to- 

 gether, 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 pathologi- 

 cal conditions cannot be comprehended if they are 

 regarded as occurring in an aggregation of purely ner- 

 vous 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 



