THE FROG 101 



structures, which support and nourish and, although they are not 

 absolutely essential for the organ to do its work, enable it to function 

 far more efficiently. These accessory parts may be derived from 

 the ectoderm also, as is the lens of the eye, but more often from 

 another and deeper lying set of cells, the mesoderm. 



This then completes an outline of the main structure and 

 functions of the nervous system and the sense organs. In the latter 

 we have highly specialised structures for the reception of impulses 

 or stimuli from the outside world, which can straightway send on the 

 messages to the brain. This wonderful organ forms, in the first place, a 

 great co-ordinating centre, in which all these messages, not only from 

 without but also from within, can be received, appreciated, and 

 brought into relation with one another. It further forms a place 

 in which messages can be originated and sent, via the nerves, to any 

 part of the body. Like any other part of the body, the nervous 

 system obtains the energy to carry on its work from the oxidation 

 of its own substance, and this consequently implies the formation 

 of waste substances. This wastage has to be made good by nutri- 

 ment, or the whole organisation would break down. . Local reflexes 

 play some part in the lives of all animals, but in general in complex 

 animals like vertebrates any ordinary stimulus needs more than just 

 a local response ; it needs a reply from the animal as a whole, and 

 the co-ordination of the various actions necessary to produce this 

 result is brought about by the brain. These replies are perceived 

 in ourselves, because they are nearly all conscious replies, but in addi- 

 tion to this, the brain is, in a way not noticed by ourselves, or, as we 

 say, subconsciously, controlling the various organs of the body, ac- 

 celerating or retarding their action to meet the varying needs of life. 

 It will readily be understood that an organ with such an important 

 part as the regulator, controller and co-ordinator of the other organs 

 of the body, is of sufficient interest to merit a branch of study all to 

 itself. This study of the function of the brain with all that it implies 

 will, memory, perception, consciousness, and so on is termed 

 Psychology, and is, save in a very general and vague way, outside 

 the scope of this work. 



Ductless Glands. 



The organs included under the present heading do not 

 compose a system, as do those treated of in the immediately preceding 

 pages, for they are isolated structurally and functionally independ- 

 ently, but they all have the one prominent feature in common that 

 has given them their name. Like all glands, they contain epithelial 

 cells, but unlike other glands, in which the secretion is poured, 

 either on to a free surface or else into an alveolus, whence it is carried 



