September 6, 1912] 



SCIENCE 



303 



sometimes too much blood pumped into the 

 arteries, and so on. As the result of such 

 lack of cooperation the life of the whole 

 would cease to be normal and would even- 

 tually cease to be maintained. 



We have already seen what are the condi- 

 tions which are favorable for the mainte- 

 nance of life of the individual cell, no mat- 

 ter where situated. The principal condi- 

 tion is that it must be bathed by a 

 nutrient fluid of suitable and constant 

 composition. In higher animals this fluid 

 is the lymph, which bathes the tissue ele- 

 ments and is itself constantly supplied 

 with fresh nutriment and oxygen by the 

 blood. Some tissue-cells are directly 

 bathed by blood; and in invertebrates, in 

 which there is no special system of lymph- 

 vessels, all the tissues are thus nourished. 

 All cells both take from and give to the 

 blood, but not the same materials or to an 

 equal extent. Some, such as the absorbing 

 cells of the villi, almost exclusively give; 

 others, such as the cells of the renal tu- 

 bules, almost exclusively take. Neverthe- 

 less, the resultant of all the give and take 

 throughout the body serves to maintain the 

 composition of the blood constant under 

 all circumstances. In this way the first 

 condition of the maintenance of the life of 

 the aggregate is fulfilled by insuring that 

 the life of the individual cells composing it 

 is kept normal. 



The second essential condition for the 

 maintenance of life of the cell-aggregate is 

 the coordination of its parts and the due 

 regulation of their activity, so that they 

 may work together for the benefit of the 

 whole. In the animal body this is effected 

 in two ways: first, through the nervous 

 system; and second, by the action of spe- 

 cific chemical substances which are formed 

 in certain organs and carried by the blood 

 to other parts of the body, the cells of 

 which they excite to activity. These sub- 



stances have received the general designa- 

 tion of "hormones" {bpixiw, to stir up) a 

 term introduced by Professor Starling. 

 Their action, and indeed their very exist- 

 ence, has only been recognized of late 

 years, although the part which they play 

 in the physiology of animals appears to be 

 only second in importance to that of the 

 nervous system itself; indeed, maintenance 

 of life may become impossible in the ab- 

 sence of certain of these hormones. 



Before we consider the manner in which 

 the nervous system serves to coordinate the 

 life of the cell-aggregate, let us see how it 

 has become evolved. 



The first step in the process was taken 

 when certain of the cells of the external 

 layer became specially sensitive to stimuli 

 from outside, whether caused by mechan- 

 ical impressions (tactile and auditory 

 stimuli) or impressions of light and dark- 

 ness (visual stimuli) or chemical impres- 

 sions. The effects of such impressions were 

 probably at first simply communicated to 

 adjacent cells and spread from cell to cell 

 throughout the mass. An advance was 

 made when the more impressionable cells 

 threw out branching feelers amongst the 

 other cells of the organism. Such feelers 

 would convey the effects of stimuli with 

 greater rapidity and directness to distant 

 parts. They may at first have been re- 

 tractile, in this respect resembling the long 

 pseudopodia of certain rhizopoda. When 

 they became fixed they would be potential 

 nerve-fibers and would represent the be- 

 ginning of a nervous system. Even yet (as 

 Eoss Harrison has shown), in the course 

 of development of nerve-fibers, each fiber 

 makes its appearance as an amoeboid cell- 

 process which is at first retractile, but grad- 

 ually grows into the position it is eventu- 

 ally to occupy and in which it will become 

 fixed. 



In the further course of evolution a cer- 



