7 So CHEMISTR I ' OF RESPIRA TION. 
The Exchange of Gases between the Blood and the Tissues. 
Internal Respiration. 
From a comparative study 1 of the process of respiration, it is seen 
that the exchange of gases in the simplest forms of life is between the 
external medium and the protoplasm of the cell. 
In insects the smallest branches of the tracheal system cany oxygen to the 
individual cells,- which arc often the seat of a most energetic combustion. In 
no case is this more marked than in the luminous organ of the glowworm 
(Larrupyris gplendidula), where, as Max Schultze 3 has shown, there are 
special cells at the end of the tracheae. The phosphorescence still continues 
after the removal of the organ from the insect's body, and under the microscope 
is seen to appear first in those parts of the cells which are around the ends of 
the tracheae. The luminous cells have a great affinity for oxygen, as shown by 
the fact that they cease to give out light if confined in an atmosphere free 
from oxygen, 4 and readily reduce osmic acid. 
In the higher animals the blood is the medium which supplies the 
tissues with oxygen and removes their carbon dioxide and other 
waste products. Reference has already been made to the theories of 
Lavoisier and Crawford 5 concerning processes of oxidation in the blood, 
and we may proceed to consider the experimental evidence which has 
been advanced in favour of the view, that the blood is the chief seat of 
combustion. When blood is shed and kept at the temperature of the 
body, it becomes gradually poorer in oxygen, 6 and there is always a dis- 
tinct darkening in the colour of arterial blood, even within the first few 
minutes after it is shed. 7 These changes were investigated by Pfliiger in 
a series of determinations of the gases of the blood, and he found that 
arterial blood received directly into a large vacuum, surrounded by hot 
water, gave a percentage of oxygen from - 2 to 10 per cent, higher 
than the amount extracted by the slower method of the ordinary gas- 
pump. About the same time Alexander Schmidt 8 found that when the 
blood of an asphyxiated animal was exposed to a known quantity of 
oxygen, there was an absorption and disappearance of oxygen, and an 
increase in the amount of carbon dioxide. The capacity of blood to bring 
about this oxidation varied ; that taken from contracting muscles could 
consume from 3 to 4 per cent., that from the heart 2 per cent., and blood 
from the hepatic vein 0"8 per cent, oxygen. It was shown by Afanassiew 9 
that only the blood corpuscles and not the serum could take up oxygen 
in this way, and Tschiriew 10 found that lymph resembled the serum in 
containing no reducing substances. 
1 See Paul Bert, "Lecous sur la physiologie comparee tie la respiration," Paris, 1870 ; 
Johannes Miiller, "Elements of Physiology, " Baly's trans., vol. i. ; Pfliiger, Arch. f. d. yes. 
Physiol., Bonn, 1875, Bel. x. S. 270. 
- Finkler, Arch. f. d. yes. Physiol., Bonn, 1875, Bd. x. S. 273 ; Kupfl'er, Bcitr. ,:. Anat. 
u. Physiol, als Festgabe C. LudvAg, Leipzig, 1S75, S. 67. 
3 Arch. f. mikr. Ana/., Bonn, 1865, Bd. i. S. 124. 
4 Milne Edwards, " Lecons sur la physiologie et l'anatomie comparee," tome viii. pp. 
93-120. 5 See p. 756. 
6 Nawrocki, Stud. d. physiol. Just. :u Breslau, Leipzig, Bd. ii. S. 144 ; Sachs, Arch. f. 
Anat., Physiol, u. wissensch. Med., 1S63, S. 348. 
7 Pfliiger, Arch./, d. gcs. Physiol., Bonn, 1868, Bd. i. S. 61 ; Bernard, Journ. deVanat. 
et physiol. etc., Paris, 1858, tome i. S. 233. 
8 Per. d. k. sachs. Gesellsch. d. Wissensch. Math.-phys. 01., Leipzig, 1867, Bd. xix. S. 99 ; 
Centralbl. f. d. med. Wissensch., Berlin, 1867, S. 356. 
9 Ber. d. k. sachs. Gesellsch. d. JVissen9ch., Leipzig, 1872. Bd. xxiv. S. 253. 
10 Ibid., 1874, Bd. xxvi. S. 116. 
