THE RESPIRATORY SYSTEM. 409 
A simple way of proving that CO, is exhaled is to breathe 
(blow) into a vessel containing some clear solution of quick- 
lime (CaO), the turbidity showing that an insoluble salt of lime 
(CaCO,) has been formed by the addition of this gas. 
The functions of most of the respiratory muscles, the phe- 
nomena of dyspncea, apncea (by a series of long breaths), partial 
asphyxia by holding the breath, and many other experiments, 
simple but convincing, will occur to the student who is willing 
to learn in this way. 
The observation of respiration in a dreaming animal (dog) 
will show how mental occurrences affect the respiratory center 
in the absence of all the usual outward influences, The respira- 
tion of the domestic animals, of the frog, turtle, snake, and fish 
are easily watched if these cold-blooded animals be placed for 
observation beneath a glass vessel. Their study will teach how 
manifold are the ways by which the one end is attained. Com- 
pare the tracings of Fig. 313. 
Evolution.— A. study of embryology shows that the respira- 
tory and circulatory systems develop together; that the vascu- 
lar system functions largely as a respiratory system also in cer- 
tain stages, and remains such, from a physiological point of 
view, throughout embryonic life. 
The changes that take place in the vascular system—the 
heart, especially—of the mammal when the lungs have become , 
functionally active at birth, show how one set of organs modi- 
fies the other. 
When one considers, in addition to these facts, that the 
digestive as well as the vascular and respiratory organs are 
represented in one group of structures in a jelly-fish, and that 
the lungs of the mammal are derived from the same mesoblast 
as gives rise to the digestive and circulatory organs, many of 
the relations of these systems in the highest groups of animals 
become intelligible; but unless there be descent with modifica- 
tion, these facts, clear enough from an evolutionary standpoint, 
are isolated and out of joint, bound together by no common 
_principle that satisfies a philosophical biology. 
It has been found that in hunting-dogs and wild rabbits the 
vagus is more efficient than in other races of dogs and in rab- 
bits kept in confinement; and possibly this may in part account 
for the greater speed and especially the endurance of the 
former. The very conformation of some animals, as the grey- 
hound, with his deep chest and capacious lungs, indicates an 
unusual respiratory capacity. 
