he body is due to the accumulation of lymph at that place.

27 In certain small animals of the lowest types a single liquid,
serving as a medium of exchange between the cells and the body
surface, supplies all the needs of the organism. In larger animals,
however, where materials have to be moved from one part of the cell
group to another, a portion of the nutrient fluid is used for
purposes of transportation. This is confined in channels where it is
set in motion by suitable organs. The portion which remains outside
of the channels then transfers material between the cells, on the
one hand, and the moving liquid, on the other.

28 Surgeons in opening veins near the thoracic cavity have to be on
their guard to prevent air from being sucked into them, thereby
causing death.

29 Oxygen forms about 21 per cent of the atmosphere, nitrogen about 78
per cent, carbon dioxide about .03 per cent, and the recently
discovered element argon about 1 per cent. The oxygen is in a
_free_, or uncombined, condition--the form in which it can be used in
the body.

30 The peculiar work devolving upon the organs of respiration
necessitates a special plan of construction--one adapted to the
properties of the atmosphere. Being concerned in the movement of
air, a gaseous substance, they will naturally have a structure
different from the organs of circulation which move a liquid (the
blood). All the organs of the body are adapted by their structure to
the work which they perform.

31 In ordinary inspirations the force that causes the air to move
through the passages is scarcely an ounce to the square inch, while
in forced inspirations it does not exceed half a pound. On this
account the closing of any of the air passages by pressure, or by
the presence of foreign substances, would keep the air from reaching
some part of the lungs.

32 Coughing, which is a forceful expulsion of air, has for its purpose
the ejection of foreign substances from the throat and lungs.
Sneezing, on the other hand, has for its purpose the cleansing of
the nostrils. In coughing, the air is expelled through the mouth,
while in sneezing it is expelled through the nostrils.

33 The amount of dust suspended in what we ordinarily think of as pure
air is shown when a beam