412 ANIMAL PHYSIOLOGY. 
efferent nerves are the phrenics, intercostals, and others supply- 
ing the various muscles used in moving the chest-walls, etc, 
The respiratory center is automatic, but its action is sus- 
ceptible of modification through afferent influences taking a 
variety of paths.. The respiratory, vaso-motor, and cardio- 
inhibitory centers seem to act somewhat in concert. 
Blood-pressure is being constantly modified by the respira- 
tory act, rising with inspiration and sinking with expiration. 
In some animals the heart-beat also varies with these phases 
of respiration, becoming slow and irregular during expiration. 
Into the causation of these changes both mechanical and nerv- 
ous factors enter, and make a very complex mesh, which we 
can at present but imperfectly unravel. When the access of 
air to the tissues is prevented, a series of stages of respiratory 
activity and decline, accompanied by pronounced changes in 
the vascular system, are passed through, known as asphyxia, 
Three stages are distinguishable: one of dyspnoea, one of 
convulsions, and one of exhaustion—while at the same time 
there is a rise of blood-pressure during the first two, and a 
decline during the third, accompanied by marked alterations in 
the cardiac rhythm. 
PROTECTIVE AND EXCRETORY FUNCTIONS OF THE SKIN. 
As has been intimated from time to time, thus far, as a 
result of the metabolism of the tissues, certain products require 
constant removal from the blood to prevent poisonous effects. 
These substances are in all probability much more numerous 
than physiological chemistry has as yet distinctly recognized 
or, at all events, isolated. Quantitatively considered, the most 
important are carbonic anhydride, water, urea, and, of less im- 
portance, perhaps, certain salts. 
In many invertebrates and in all vertebrates several organs 
take part in this work of elimination of waste products or puri- 
fication of the blood, one set of which—the respiratory—we 
have just studied; and we now continue the consideration of 
the subject of excretion, this term being reserved for the pro- 
cess of separating harmful products from the blood and dis- 
charging them from the body. 
We strongly recommend the student to make the study of 
excretion comparative in the sense of noting how one organ 
engaged in the process supplements another. A clear under- 
