116 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



within the parent-cell, or when set free by its rupture. The difference ob- 

 servable in the several cases that have been enumerated, and in others that 

 might be mentioned, seems to have reference chiefly to the degree of prepara- 

 tion that is effected in the nutriment with which the young cells are supplied ; 

 some drawing it directly from the blood ; whilst others receive it through 

 the medium of the parent-cell, which probably exerts a certain degree of pre- 

 paring influence upon it ; and others, again, requiring a further preparation 

 to be effected, by the elaborating or assimilating influence of a group of tem- 

 porary cells, expressly developed for this purpose. 



132. We shall find, as we proceed, that all the tissues most actively con- 

 cerned in the maintenance of the Vital functions of the Human body, both 

 those of a Vegetative nature, and those which are peculiarly Animal, are 

 composed of Cells which have undergone no considerable metamorphosis, 

 and of which one generation is produced after another with a rapidity that is 

 proportioned to the activity of the function. But there are other structures 

 of an accessory character, in which a departure from the original type is to 

 be traced, sometimes so complete, as to prevent their real nature from being 

 understood, except by a very careful scrutiny into their history. This depart- 

 ure is the result of various kinds of metamorphosis of the cells and of their 

 nuclei ; of which the following are the principal. The cells, originally sphe- 

 roidal, oval, or polygonal, may become elongated to such a degree, as to as- 

 sume the spindle or fusiform shape ; thus resembling woody fibres. They 

 may at the same time lose their nuclei ; and their cavities may be occupied 

 by internal deposits, so that they may be mistaken for solid fibres. Such 

 fusiform cells are often found in exudation-membranes. Again, the cells may 

 shoot out prolongations, either in a radiating manner, so that they assume a 

 stellate form ; or in no definite direction, so that their shape becomes altogether 

 irregular. Such forms are seen amongst the pigment-cells of the Batrachia 

 and Fishes, and among the vesicles of the gray matter of the nervous system. 

 Further, the original boundaries of the cells may be altogether lost, by their 

 coalescence with each other. This is the case with many membranes that 

 seem to have originated in a layer of flat cells ; the situation of which is rather 

 to be traced by their nuclei, than by their former boundaries, which have alto- 

 gether disappeared. It is often the case, too, with the horny cells, of which 

 the nails, hoof, &c., are made up ; and still more with the cells of shell, bone, 

 tooth, &c., which have been consolidated by the deposition of a calcifying 

 deposit. Lastly, the character of the original cell may be completely altered 

 by a solution in the continuity of its wall, in one or more spots, so that its 

 cavity is laid open, and coalesces with some other. In this manner, by the 

 disappearance of the partitions between cells laid in apposition, end to 

 end, may be formed a tube ; and this tube may coalesce with others, in like 

 manner, so as to form a capillary network for the circulation of the blood. 

 Or the tube may form a simple straight fibre ; and the nuclei of its component 

 cells may give origin to a new deposit, either in an amorphous condition, as 

 in the fibrous portion of nervous tissue, or in the form of an aggregation of 

 new cells, as in the most perfect kind of muscular fibre. In these cases, also, 

 the original composition of the tubes may be frequently traced by the nuclei 

 that remain in their interior. In the follicles of glands, the solution of con- 

 tinuity takes place at one point only, which establishes a communication be- 

 tween the cavity of the parent-cell, and some canal by which its contents may 

 be discharged ; and the nucleus situated at the blind or closed extremity of the 

 follicle, may then continue to form successive generations of secondary cells, 

 which are discharged by this outlet. 



133. The metamorphoses of the nucleus are not less important, though 

 not as numerous. In some instances we find it sending out radiating prolonga- 



