SELECTIVE POWER OF INDIVIDUAL PARTS. 595 



783. We have seen that, in some cases, the germs are prepared by pre- 

 viously-existing cells of the same kind; thus the Red and colourless corpuscles 

 of the Blood, the Cartilage-cells, the cells of Vesicular Nervous matter, and 

 those of many other tissues, appear to be the offspring of parents exactly 

 similar to themselves. In other cases, however, the germs seem to be fur- 

 nished by certain " nutritive centres," which appear to be constantly engaged 

 in the preparation of them, deriving their materials from the blood.* Thus 

 the Epidermic and Epithelial cells are produced, not from preceding cells of 

 a similar character (for these are thrown off without performing any such 

 reproductive act), but from germs derived from the basement or primary mem- 

 brane beneath; and, in like manner, the minute cells, of which the ultimate 



Jibrillse of Muscle are composed, appear to originate in nuclei or germinal 

 centres, belonging to the tubular Myolemma. But even these germinal centres 

 may probably be considered as nothing else than the nullei of certain parent- 

 cells, which, instead of producing their like, give origin to a new generation 

 having different properties. Thus, the basement or primary-membrane has 

 been already stated ( 135) to exhibit not unfrequently the indications of a 

 cellular constitution ; the germinal centres which it contains being the nuclei 

 of its component cells : and its character is particularly well seen where it is 

 inverted so as to form secreting follicles ; for, as we have seen ( 174), each 

 of these follicles may be regarded as a single parent-cell, which opens at the 

 extremity farthest from the nucleus, and continues to discharge from its orifice 

 successive generations of cells, having their origin in its nucleus, which thus 

 acts as a permanent " germinal centre." And in like manner, the germinal 

 centres of Muscular Fibre may be regarded as the nuclei of the cells, of which 

 it was originally composed. 



784. The Selecting power, which is possessed by the germs of each kind 

 of tissue, and which enables them to draw from the Blood the materials which 

 they severally require for their development, manifests itself also in the mode 

 in wKich substances, that are abnormally present in the Blood, affect the con- 

 dition and development of the solid tissues. Thus we find that the presence 

 of a certain quantity of Arsenic in the Blood, will produce a state of irritation 

 of all the Mucous membranes in the body. The continued introduction of 

 Lead into the circulating system, occasions a modification in the nutrition of 

 the extensor muscles of the forearm, producing the form of partial paralysis 

 commonly termed wrist-drop; and the existence of this modification is shown 

 by the fact (disclosed by Chemical analysis) of the actual presence of lead in 

 the palsied muscles. Here we have to remark the Symmetrical nature of the 

 affection, consequent upon the occurrence of the same disorder in the corre- 

 sponding parts of the two sides of the body; for these muscles appear to have 

 the same kind of tendency to attract Lead from the circulating current, in a 

 degree that is equal on the two sides, as they have to draw from the blood 

 the materials of their regular growth, and to develops themselves in an exactly 

 similar manner. In like manner, the cutaneous eruptions, which are occa- 

 sionally produced by the internal exhibition of iodide of potassium, are found 

 to be almost precisely symmetrical; the presence of the medicine in the blood 

 being the occasion of a disordered nutrition of certain parts of the skin; and 

 the selecting power of particular spots being evinced by the exact correspond- 

 ence of the parts affected on the two sides. 



785. The same appears to be the case with regard to substances, whose 

 presence in the blood is rather the result of a disordered condition of the 

 digestive and assimilating processes, than of their direct introduction from 

 without. Thus in Lepra and Psoriasis, chronic diseases of the Skin, which 



* See Gooclsir's Anatomical and Pathological Observations, Chap. I. 



