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 

 T p 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- 

 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. 



Respiration as a general 



Fig. 45- The larger terminal ramifications of f LlllCtion is COllimon to all 



the left inferior bronchial ramus, from the dorsal • -~ru„ U „1„r^,,r- 



surface; metallic cast of the interior ; cf. Figs. OrgaiUSlUS. 1 JlOUgll always 



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. 



