166 MANUAL OF HISTOLOGY. 



disturbances in the circulation of the latter may lead to its malforma- 

 tion. The nails, likewise, are shed from the extremities among rabbits, 

 as Steinruck's well-known experiments have shown, after division of the 

 sciatic nerve. The fact also observed by Koelliker is very interesting, 

 namely, that in those cases in which we find thickening and malforma- 

 tion oif the nails of elderly individuals, the capillaries of the anterior por- 

 tion of the matrix may be impervious, owing to a deposit in them of fatty 

 granules 



Finally, as to the first appearance of the nail in the embryo, we find 

 its rudiments in the third month of inter-uterine existence in the form of 

 a fold in the usual situation, which is clothed with the ordinary cells of the 

 embryonic skin. Then in the fourth month, under the embryonic epider- 

 mis, and above the rete Malpighii of the matrix, a layer of new cells 

 is seen, destined to become the horny cells of the future nail. Later 

 on, more of the same kind of strata are deposited on these, so that the 

 corneous layer, although still soft, acquires considerable thickness. At 

 the end of the fifth month the coating of simple epidermal scales has dis- 

 appeared from over the nail, and the latter lies freely exposed. In the 

 nail of the new-born child we may still recognise its cellular nature with- 

 out the aid of reagents, but after the first year the cells are of the same 

 constitution as in the adult body. 



C. Tissues belonging to the Connective- 

 Substance Group. 



101. 



Having discussed the epithelia, we now <turn to the consideration of 

 another natural group of textures, namely, the connective-substance group, 

 one of the most important, but at the same time most difficult chapters of 

 histology. 



This name has been given by the greater number of investigators of 

 our day to a series of tissues, all of which probably take their origin from 

 the so-called middle embryonic plate, and start from the same rudiments. 

 They usually, however, in the course of their farther development in 

 various directions, become separated further and further from one another, 

 taking on the most diverse forms, as well from a chemical as anatomical 

 point of view. Thus, in the mature organism, there occur in the connec- 

 tive-substance group masses which appear at the first glance to be sepa- 

 rated by a very wide gap. Among these may be reckoned cartilage, 

 mucoid or gelatinous tissues, recticular connective-substances, ordinary con- 

 nective tissue, fatty t'issue, bone, and the substance composing teeth or 

 dentine. 



The near relationship, however, of all these different tissues is not to 

 be denied. 



In the first place, we often see, though the typically marked varieties 

 of these several tissues may differ widely from one another, intermediate 

 forms, as, for instance, between gelatinous and ordinary connective tissue, 

 and between the latter and cartilage; so that a sharp line of demarcation 

 cannot possibly be drawn between the various members of the series. 



Again, in many regions of the body, these several tissues merge one into 

 another, as, for instance, in the case of those just mentioned. 



Further, a substitution or replacement of one tissue by another equi- 

 valent one has been remarked, and moreover of threefold nature. 



