Septembek 6, 1900] 



NA TURE 



445 



ance when secretion is going on with that seen when the cells 

 are at rest, have shown that the cell plasm is much more 

 granular and opaque, and contains larger particles during 

 acti%nty than when the cell is passive ; the body of the cell swells 

 out from an increase in the contents of its plasm, and chemical 

 changes accompany the act of secretion. Ample evidence, 

 therefore, is at hand to support the position taken by John 

 Goodsir, nearly sixty years ago, that secretions are formed 

 within cells, and lie in that part of the cell which we now say 

 consists of the cell plasm ; that each secreting cell is endowed 

 with its own peculiar property, according to the organ in which 

 it is situated, so that bile is formed by the cells in the liver, milk 

 by those in the mamma, and so on. 



Intimately associated with the process of secretion is that of 

 nutrition. As the cell plasm lies at the periphery of a cell, and 

 as it is, alike both in secretion and nutrition, brought into closest 

 relation with the surrounding medium, from which the pabulum 

 is derived, it is necessarily associated with the nutritive activity. 

 Its position enables it to absorb nutritive material directly from 

 without, and in the process of growth it increases in amount by 

 interstitial changes and additions throughout its substance, and 

 not by mere accretions on its surface. 



Hitherto I have spoken of a cell as a unit, independent of its 

 neighbours as regards its nutrition and the other functions which 

 it has to discharge. The question has, however, been discussed, 

 whether in a tissue composed of cells closely packed together 

 cell plasm may not give origin to processes or threads which 

 are in contact or continuous with corresponding processes of 

 adjoining cells, and that cells may therefore, to some extent, 

 lose their individuality in the colony of which they are members. 

 Appearances were recognised between 1863 and 1870 bySchron 

 and others in the deeper cells of the epidermis and of some 

 mucous membranes which gave sanction to this view, and it 

 seems possible through contact or continuity of threads connect- 

 ing a cell with its neighbours, that cells may exercise a direct 

 influence on each other. 



Nageli, the botanist, as the foundation of a mechanico- 

 jihysiological theory of descent, considered that in plants a net- 

 work of cell plasm, named by him idio-plasm, extended 

 throughout the whole of the plant, forming its specific molecular 

 constitution, and that growth and activity were regulated by its 

 conditions of tension and movements (1884). 



The study of the structure of plants with special reference to 

 the presence of an intercellular network has for some years been 

 pursued by Walter Gardiner (1882-97), who has demonstrated 

 threads of cell plasm protruding through the walls of vegetable 

 cells and continuous with similar threads from adjoining cells. 

 Structurally, therefore, a plant may be conceived to be built up 

 of a nucleated cytoplasmic network, each nucleus with the 

 branching cell plasm surrounding it being a centre of activity. 

 On this view a cell would retain to some extent its individuality, 

 though, as Gardiner contends, the connecting threads would be 

 the medium for the conduction of impulses and of food from a 

 cell to those which lie around it. For the plant cell therefore, 

 as has long been accepted in the animal cell, the wall is reduced 

 to a secondary position, and the active constituent is the 

 nucleated cell plasm. It is not unlikely that the absence of a 

 controlling nervous system in plants requires the plasm of ad- 

 joining cells to be brought into more immediate contact and 

 continuity than is the case with the generality of animal cells, 

 so as to provide a mechanism for harmonising the nutritive and 

 other functional processes in the different areas in the body of 

 the plant. In this particular, it is of interest to note that the 

 epithelial tissues in animals, where somewhat similar connecting 

 arrangements occur, are only indirectly associated with the 

 nervous and vascular systems, so that, as in plants, the cells may 

 require, for nutritive and other purposes, to act and react 

 directly on each other. 



Nerve Cells. 

 Of recent years great attention has been paid to the intimate 

 structure of nerve cells, and to the appearance which they 

 present when in the exercise of their functional activity. A 

 nerve cell is not a secreting cell ; that is, it does not derive from 

 the blood or surrounding fluid a pabulum which it elaborates 

 into a visible, palpable secretion characteristic of the organ of 

 which the cell is a constituent element, to be in due course dis- 

 charged into a duct which conveys the secretion out of the 

 gland. Nerve cells, through the metabolic changes which take 

 place in them in connection with their nutrition, are associated 

 with the production of the form of energy specially exhibited by 



NO. 1610, VOL. 62] 



animals which possess a nervous system, termed nerve energy. 

 It has long been known that every nerve cell has a body m 

 which a relatively large nucleus is situated. A most important 

 discovery was the recognition that the body of every nerve cell 

 had one or more processes growing out from it. More recently 

 it has been proved, chiefly through the researches of Schultze, 

 His, Golgi and Ramon y Cajal, that at least one of the pro- 

 cesses, the axon of the nerve cell, is continued into the axial 

 cylinder of a nerve fibre, and that in the multipolar nerve cell 

 the other processes, or dendrites, branch and ramify for some 

 distance away from the body. A nerve fibre is therefore an 

 essential part of the cell with which it is continuous, and the cell, 

 its processes, ihe nerve fibre and the collaterals which arise from 

 the nerve fibre collectively form a neuron or structural nerve unit 

 (Waldeyer). The nucleated body of the nerve cell is the 

 physiological centre of the unit. 



The cell plasm occupies both the body of the nerve cell and 

 its processes. The intimate structure of the plasm has, by im- 

 proved methods of observation introduced during the last eight 

 years by Nissl, and conducted on similar lines by other investi- 

 gators, become more definitely understood. It has been 

 ascertained that it possesses two distinct characters which imply 

 different structures. One of these stains deeply on the addition 

 of certain dyes, and is named chromophile or chromatic sub- 

 stance ; the other, which does not possess a similar property, is 

 the achromatic network. The chromophile is found in the cell 

 body and the dendritic processes, but not in the axon. It 

 occurs in the form of granular particles, which may be scattered 

 throughout the plasm, or aggregated into little heaps which are 

 elongated or fusiform in shape and appear as distinct coloured 

 particles or masses. The achromatic network is found in the 

 cell body and the dendrites, and is continued also into the axon, 

 where it forms the axial cylinder of the nerve fibre. It consists 

 apparently of delicate threads or fibrillae, in the meshes of which 

 a homogeneous material, such as is found in the cell plasm 

 generally, is contained. In the nerve cells, as in other cells, 

 the plasm is without doubt concerned in the process of cell 

 nutrition. The achromatic fibrillar exercise an important influence 

 on the axon or nerve fibre with which they are continuous, and 

 probably they conduct the nerve impulses which manifest them- 

 selves in the form of nerve energy. The dendritic processes of a 

 multipolar nervecell ramify in close relation with similar processes 

 branching from other cells in the same group. The collaterals 

 and the free end of the axon fibre process branch and ramify in 

 association with the body of a nerve cell or of its dendrites. We 

 cannot say that these parts are directly continuous with each 

 other to form an intercellular network, but they are apparently 

 in apposition, and through contact exercise influence one on 

 the other in the transmission of nerve impulses. 



There is evidence to show that in the nerve cell the nucleus, 

 as well as the cell plasm, is an effective agent in nutrition. When 

 the cell is functionally active, both the cell body and the nucleus 

 increase in size (Vas, G. Mann, Lugaro) ; on the other hand, 

 when nerve cells are fatigued through excessive use, the nucleus 

 decreases in size and shrivels ; the cell plasm also shrinks, arid 

 its coloured or chromophile constituent becomes diminished in 

 quantity, as if it had been consumed during the prolonged use 

 of the cell (Hodge, Mann, Lugaro). It is interesting also to 

 note that in hibernating animals in the winter season, when their 

 functional activity is reduced to a minimum, the chromophile in 

 the plasm of the nerve cells is much smaller in amount than 

 when the animal is leading an active life in the spring and 

 summer (G. Levi). 



When a nerve cell has attained its normal size it does not seem 

 to be capable of reproducing new cells in its substance by a pro- 

 cess of karyokinesis, such as takes place when young cells arise 

 in the egg and in the tissues generally. It would appear that 

 nerve cells are so highly specialised in their association with the 

 evolution of nerve energy, that they have ceased to have the 

 power of reproducing their kind, and the metabolic changes both 

 in cell plasm and nucleus are needed to enable them to discharge 

 their very peculiar function. Hence it follows that when a 

 portion of the brain or other nerve-centre is destroyed, the 

 injury is not repaired by the production of fresh specimens 01 

 their characteristic cells, as would be the case in injuries to 

 bones and tendons. 



In our endeavours to diflTerentiate the function of the nucleus 

 from that of the cell plasm, we should not regard the former as 

 concerned only in the production of young cells, and the latter 

 as the exclusive agent in growth, nutrition, and, where gland 



