302 THE BODY AT WOEK 



known how impulses get across from the finest visible twigs of 

 arborizing axons to the dendrites of the neurones which they 

 influence. The wealth of structural detail which recent re- 

 search has revealed is an embarrassment to anyone who tries 

 to devise a scheme. Not improbably, pericellular nets are 

 intermediate factors in the exchange ; or, if not the nets, the 

 structures whose existence is indicated by the appearance of 

 the nets. In the case of many of the finer markings which 

 staining methods bring into view, it is impossible to say whether 

 they indicate the presence during life of the structure as it 

 appears to be, or whether the markings are due to coagulation 

 of plasma or to strain caused by shrinkage in coagulating 

 agents. In a sense this is not of much consequence. Coagu- 

 lation in a uniform pattern would mean the existence of an 

 architectural substructure which determines the pattern. We 

 may be looking at the cake or at the tin the cake was baked in. 

 There is a danger of seeing too much in a nerve-cell when 

 examining it under the highest powers of the microscope, and of 

 endeavouring to picture in too much detail the arrangements 

 which regulate the flow of impulses. Its markings are so com- 

 plicated as to suggest to the mind of the observer that it is 

 itself a microcosm a nervous system in miniature. Neuro- 

 fibrillae appear to offer many alternative paths within the cell. 

 It is unlikely that such a way of looking at the unit of struc- 

 ture is the right one. A certain motor cell of the spinal cord 

 is connected by its axon with thirty or forty separate muscle- 

 fibres ; but there is no reason for thinking that the fibres ever 

 contract save as a single group. The axon consists of parallel 

 fibrillae, but these do not appear to be needed as separate con- 

 ductors ; an impulse travels down the fascicle. It does not 

 appear to be necessary in the case of a motor cell, and pre- 

 sumably the statement holds good for the large cells of the 

 cerebellum and cerebrum to picture any arrangement for the 

 simultaneous conduction in its axon of several impulses, or 

 for the conduction of one impulse along one of its fibrillse 

 and a different one along another. What is necessary is that 

 this particular efferent path Z should be accessible from every 

 other part of the nervous system from A to Y. If, merely 

 for the sake of filling the space which would otherwise be blank 

 in the mental picture, we imagine a pericellular net connected 



