PROTOPLASM. 21 



concerned with the translation of sensations from the outer 

 world into perceptions, and with the origination of move- 

 ments Irraecordance with those perceptions; and that nerve- 

 fibres ^re used as the paths by which sensations are con- 

 ducted to nerve-centres, and by which the resulting com- 

 mands are conducted to the various parts of the body. For 

 the same reason we give no account of the sense-organs, for 

 every one knows the purpose of an eye or an ear, and in 

 noting structure and position is not troubled with doubts as 

 to function, as often occurs when the internal organs are 

 being dissected out. 



In conclusion, it will perhaps be well to remark, that 

 when m the course of further studies the student meets with 

 organs which are called by the same name as those found in 

 man or in Mammals, as for example, the " liver " of the 

 Molluscs, he must be careful not to suppose that the 

 function of such a " liver " is the same as in Mammals, for 

 comparatively little investigation into the physiology of the 

 lower types of animal life has as yet been made. At the 

 same time he must clearly recognise that the great internal 

 activities are in a general way the same in all animals : thus, 

 respiration, whether accomplished by skin, or gills, or air 

 tubes, or lungs, by help of the red pigment (haemoglobin) of 

 the blood, or of some pigment which is not red, or occurring 

 without the presence of any blood at all, always means that 

 oxygen is absorbed almost like a kind of food by the tissues, 

 and that the carbonic acid gas which results from the 

 oxidation of part of the material of the tissues is removed. 



Modern Conception of Protoplasm. — The activities of 

 animals are ultimately due to physical and chemical changes 

 associated with the living matter or protoplasm. This is a 

 mere truism. We do not know the nature of this living 

 matter, in fact our most certain knowledge of it is that in 

 our brains its activity is expressed as thought. 

 ''^ But though we cannot analyse living matter, nor thoroughly 

 .explain the changes by which the material of the body breaks 

 down or is built up, we can trace, by chemical analysis, how 

 food passes through various transformations till it becomes 

 a useable part of the living body, and we can also catch 

 some of the waste-products formed when muscles or other 

 parts are active. 



