44 Elementary Biology. 



these limits may be considerably extended, 100 C, or 

 even 120 C-, being insufficient to cause death. 



Cold as a general rule retards, whilst gentle heat accele- 

 rates, amoeboid and other protoplasmic movements. A 

 species of coagulation, however, takes place if the tempera- 

 ture exceed 45 C. 



C. Pressure. The ordinary pressure of the atmosphere 

 at the sea-level is about 1473 Ibs. on the square inch, 

 which indicates the weight of a column of air of the same 

 sectional area. Manifestly the atmospheric pressure will 

 vary according to the height above the sea-level at which 

 the observation is made. Atmospheric pressure also varies 

 with the latitude and longitude, the temperature, the season 

 of the year, and the hour of the day. 



The total atmospheric pressure on the surface of the 

 body of an average-sized human adult is about 14 tons, a 

 pressure which is, however, equalised by the. outward pres- 

 sure of the air permeating the tissues. As a general rule, 

 higher organisms are constructed so as to subsist under an 

 atmospheric pressure not varying widely on either side of 

 the average, viz. 30 in. or 760 mm. of the mercurial baro- 

 meter. Many of the lower forms of life, however, can 

 tolerate without injury a much higher pressure than that. 

 With reference to aquatic, and more especially marine, 

 animals the limits of pressure are much wider. Many fish, 

 for example, live at a depth of from 300 to 2,000 fathoms, 

 that is to say, they are capable of accommodating themselves 

 to a pressure varying from one-half to two tons on the square 

 inch. Fish living at greater depths must, of course, be subject 

 to still greater pressures, amounting in some cases to as much 

 as four-and-a-half tons on the square inch, always of course 

 balanced by an equally great outward pressure of the water 

 with which their tissues are permeated. These forms, when 

 brought to the surface, are found to be greatly injured owing 

 to the effect on their bodies of the removal of much of the 

 external pressure at the surface. 



