150 BIOLOGY: GENERAL AND MEDICAL 



functions may be carried on regularly and economically 

 digestive enzymes be passed into the digestive organs only 

 when there is something to digest, etc. 



As has been pointed out by Loeb, we are unable to 

 find any essential difference between nerve tissue and 

 other tissue except that it possesses greater irritability 

 and conductivity. It goes without saying, however, that 

 these particular qualities must be so adjusted as to fulfil 

 the requirements expressed above. 



In its most primitive form, with beginning differentia- 

 tion, the nervous tissue is scarcely to be recognized. In 

 the ectodermal cells of hydra, a few cells, not strikingly 

 different from the others, exceed their fellows in irritability 

 and contractility and seem to serve as guides or indicators, 

 by which the movements, especially of the tentacles, are 

 directed. The division of labor is not complete and the 

 action is scarcely if anything more than a manifestation 

 of tropism. 



Reflex Action. The specialization of irritability and 

 conductivity in nervous tissue would be to no purpose, 

 however, if no means were provided for utilizing these 

 special qualities. A nerve cell by itself can do nothing. 

 To be of use in the performance of the functions pre- 

 scribed i. e., direction, regulation, communication, and 

 coordination it must somehow get into relation with the 

 parts to be directed, regulated, intercommunicated, and 

 coordinated, and to do this certain morphological de- 

 velopment is necessary. This eventuates in a cell of 

 peculiar form known as a neuron, which constitutes the 

 essential structural element of the nervous system. It 

 consists of a body and certain processes, one, supposedly 

 of greatest importance, called the neuraxis, the others, 

 dendrites. 



The neuraxis a few cells possess two forms a rela- 

 tively long filament whose function is to collect impulses 

 and bring them to the cell, or to convey impulses originat- 

 ing in the cell to the structure upon which it acts. The 

 dendrites, of which there are a number, are relatively 



