86 ANATOMY OF THE RABBIT. 
What is commonly described as respiration, or the act of 
breathing, is a mechanical, muscular process accessory to respira- 
tion. It consists in the expansion of the thorax, so that a partial 
vacuum is created and the lungs fill with air, the expansion being 
followed by relaxation, in which the air is expelled. 
The first portion of this action, known as inspir- 
ation, is brought about by the contraction of the intercostal and 
related muscles, by which the ribs are raised, and by the contraction 
of the dome-shaped diaphragm, by which the posterior wall of the 
thorax is flattened, and incidentally the abdominal viscera displaced 
backward. Both actions tend to enlarge the thoracic space. The 
action of the diaphragm is 
controlled directly by the 
phrenic nerves, but all re- 
spiratory movements are de- 
Se pendent upon the cervical 
and thoracic spinal nerves, 
and there is also a respiratory 
control centre in the medulla. 
The expulsion of air, expira- 
tion, is accomplished without 
‘muscular contraction. 
BREATHING. 

Respiration as a general 
Fic. 45. The larger terminal ramifications of function is common _ to all 
the left inferior bronchial ramus, from the dorsal 5 
surface; metallic cast of the interior ; cf. Figs. organisms. Though always 
44 and 83, i. : : 
constructed for easy diffusion, 
the organs by which the function is discharged differ profoundly 
in the various groups. This is true even within the limits of the 
vertebrates, where lower forms are characterized by gills for 
LUNGS AND aquatic respiration, and the higher forms by 
GILLS. lungs for air respiration. The occurrence of a 
great variety of intermediate and _ transitional 
growth stages, in which gills are replaced by lungs, with no modifi- 
cation from one to the other, introduces a peculiar condition into 
the history of vertebrates. This condition is characterized by the 
appearance of gill structures in the embryos of all higher forms 
(Fig. 22) and by the gradual elaboration in the series of air sacs 
from a simple type, as illustrated in the frog, to the greatly branched 
lung tubes of mammals. 
