TRANSACTIONS OF SECTION I. 809 



explained. Wiiat, for example, is the real nature of elasticity ; what occurs in 

 dissolving a little sugar or common salt in water ; wliat is electrical conductivity ? 

 In no domain of science, except in mathematics, is our knowledge absolute ; and 

 physiology shares with the other sciences the possession of problems tliat, if I 

 may use a paradox, seem to be more insoluble the nearer we approach their 

 solution. 



The body of one of the higher animals — say that of man — is a highly complex 

 mechanism, consisting of systems of organs, of individual organs, and of tissues. 

 Physiologists have been able to give an explanation of the more obvious pheno- 

 mena. Thus locomotion, the circulation of the blood, respiration, digestion, the 

 mechanism of the senses, and the general phenomena of tha nervous system have 

 all been investigated, and in a general way they are understood. The same state- 

 ment maybe made as to the majority of individual organs. It is when we come 

 to the phenomena in the living tissues that we find ourselves in difficulties. The 

 changes happening in any living cell, let it be a connective tissue corpuscle, or a 

 secreting cell, or a nerve-ceU, are still imperfectly understood ; and yet it is upon 

 these changes that the phenomena of life depend. This has led the more thoughtful 

 physiologists in recent years back again to the study of the cell and of the simple 

 tissues tliat are formed from cells. Further, it is now recognised that if we are to 

 give an adequate explanation of the phenomena of life, we should study these, 

 not in the body of one of the lower organisms, as was at one time the fashion, 

 where there is little if any difl'erentiatiou of function — the whole body of an 

 amojboid organism showing capacities for locomotion, respiration, digestion. &c. — 

 but in the specialised tissue of one of the higher animals. Thus the muscle-cell 

 is specialised for contraction, and varieties of epithelium have highly specialised 

 functions. 



But when cells are examined with the highest microscopic powers, and with the 

 aid of the highly elaborated methods of modern histology, we do not seem to have 

 advanced very far towards an explanation of the ultimate phenomena. There 

 is the same feeling in the mind of the physiologist when he attacks the cell from 

 the chemical side. By using lai'ge numbers of cellular elements, or by the more 

 modern and fruitful methods of micro-chemistry, he resolves the cell-substance 

 into proteids, carbohydrates, fats, saline matter, and water, with possiblj' other 

 substances derived from the chemical changes happening in the cell while it was 

 alive; but he obtains little information as to how these proximate constituents, as 

 they are called, are built up into the living substance of the cell. But if we con- 

 eider the matter it will be evident that the phenomena of life depend on changes 

 occurring in the interactions of particles of matter far too small even to be seen by 

 the microscope. The physicist and the chemist have not been content with the 

 investigation of large masses of dead matter, but to explain many phenomena they 

 have had recourse to the conceptions of roolecules and atoms and of the dynamical 

 laws that regulate their movements. Thus the conception of a gas as consisting of 

 molecules having a to-and-fro motion, first advanced by Kronig in 1856 and by 

 Clausius in 1857, has enabled physicists to explain in a satisfactory manner the 

 general phenomena of gases, such as pressure, viscosity, diffusion, &c. In physio- 

 logy few attempts have been made in this direction, probably because it was felt 

 that data had not been collected in sutHcient numbers and with sufficient accuracy 

 to warrant any hypothesis of the molecular structure of living matter, and 

 physiologists have been content with the microscopic and chemical examination of 

 cells, of protoplasm, and of the simpler tissues formed from cells. An exception to 

 this general remark is the well-known hypothesis of Da Bois-Reymond as to the 

 existence in muscle of molecules having certain electrical properties, by which he 

 endeavoured to explain the more obvious electrical phenomena of muscle and 

 nerve. The conception of gemmules by Darwin and of biophors by Weismann 

 are examples also of a hypothetical method of discussing certain vital phenomena. 



Of all the properties of living matter assimilation must no doubt be regarded 

 as the most fundamental. On it depend all vital phenomena. Many physiologists 

 have endeavoured to give an explanation of assimilation by comparing it with 

 crystallisation. But the two processes are very different. The crystal grows by 



