80 MANUAL OF HISTOLOGY. 



cellular nature and individuality of the cell will come under our notice 

 lower down. 



51. 



All structures of the body the tissue elements, and, in the case in 

 point, the cells show a transmutation of the matters of which they are 

 composed (p. 11) ; they present for our consideration an "interchange of 

 material" in connection with them. 



Simple microscopic investigation even affords us many proofs of this, 

 by showing that, beside the growth of the cell, its contents may become 

 eventually of a different nature from an optical point of view. Thus we 

 see, in glancing at embryonic processes in the first place, that the forma- 

 tive cell of tissues exchanges its previously homogeneous or finely granular 

 contents for more specific materials, in that, instead of the granules of the 

 yelk, fat globules, pigmentary matters, and blood-pigments, &c., may make 

 their appearance in its body. The same interchange of matter is also seen 

 in the mature animal system ; the white formative blood-corpuscles are 

 transformed into- red cells. The neutral fats, which, enveloped in a thin 

 layer of protoplasm, form the contents of the so-called fat-cells, may dis- 

 appear from the body of the latter in consequence of prolonged fasting or 

 exhausting disease, and be replaced by watery protoplasm, or, as it was 

 formerly expressed, by a "serous fluid." Again, in the interior of the 

 epithelial cells of the small intestine, certain fat globules may be ob- 

 served after every meal, which, in the course of a few hours, have regu- 

 larly disappeared again. And/ indeed, we might bring forward many 

 other examples. 



One more example of cellular transmutation may be mentioned here as 

 a recent discovery. 



In the inactive submaxillary gland are to be found cells which contain, 

 besides a small amount of protoplasm expanded peripherally and a nucleus, 

 a large drop of mucus. By the action of a continued electric stimulus, 

 these gland-cells may be made artificially to discharge the mucin, and on 

 doing so, are found to be granular throughout, and, freed of the gelatinous 

 substance, smaller. In a few hours the whole cell-body is observecl to be 

 formed entirely of protoplasm. 



Now, although we are in this way able to see, one might almost say, 

 with the naked eye, the transmutation of matter in the cell, nevertheless 

 great difficulties arise so soon as the question turns upon a more detailed 

 analysis of the same ; and this it is which causes the advance to appear 

 so inconsiderable which has been hitherto made in a field of inquiry so 

 important for general physiology. The very knowledge of the fact, also 

 (first observed by Graham), that crystalloids, but not colloids, can pass 

 through the envelopes and body of the cell, which consists of colloid mat- 

 ter, renders it difficult to comprehend how the nourishment and growth 

 of the latter take place, although it may explain, on the other hand, the 

 mode of excretion of the products of decomposition. 



When we are questioned as to the vigour with which this interchange 

 of matter takes place in the animal cell, we are only able to offer conjec- 

 tures. In the first place, different parts of the cell may be endowed with 

 different degrees of transformative energy. The membrane, for instance (if 

 the cell have acquired one), appears to be endowed with less than the other 

 portions, and to be the most stable of the whole, especially when it con- 

 sists of tough and indifferent elastic matter. On the other hand, everything 



