404 COMPARATIVE PHYSIOLOGY. 



blood may be imperfectly oxidized, giving rise to labored res- 

 piration. 



Personal Observation. — As hinted from time to time during 

 the treatment of this subject, there is a large number of facts 

 the student may verify for himself. 



A simple way of proving that COa is exhaled is to breathe 

 (blow) into a vessel containing somte clear solution of quick- 

 lime (CaO), the turbidity showing that an insoluble salt of lime 

 (OaCOs) has been formed by the addition of this gas. 



The functions of most of the respiratory muscles, the phe- 

 nomena of dyspnoea, apnoea (by a series of long breaths), par- 

 tial asphyxia by holding the breath, and many other experi- 

 ments, 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, and of the frog, turtle, snake, and 

 jftsh, is 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. 

 Compare the tracings of Fig. 305. 



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 gi^es 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 



