108 ANATOMY OF THE RABBIT 



fundamental exchange which really constitutes respiration. 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 inspiration, is brought about by the contraction 

 of the intercostal and related muscles in such a way that the ribs 

 are raised and by the simultaneous contraction of the dome- 

 shaped diaphragm, by which the posterior wall of the thorax is 

 flattened, and incidentally the abdominal viscera are displaced 

 backward. On account of the oblique position of the ribs when at 

 rest, these actions tend to enlarge the thoracic space in all three 

 dimensions, in consequence of which air passes in from the pharynx 

 and distends the lungs. The expulsion of air, or expiration, is ac- 

 complished by relaxation of the muscles mentioned above, assisted 

 by contraction of the transverse thoracic muscles (p. 323) and the 

 muscles of the ventral abdominal wall. The precise part played by 

 the different muscles in the co-ordinated act of breathing has been 

 shown to vary somewhat in different individual animals. 



The action of the diaphragm is controlled directly by the 

 phrenic nerves, but all respiratory movements are dependent upon 

 the cervical and thoracic spinal nerves. The excitation of these 

 nerves is regulated through a respiratory centre in the medulla 

 oblongata, comprising inspiratory and expiratory portions and 

 lying mainly under the back part of the fourth ventricle. The 

 centre is stimulated both directly by carbon dioxide in the blood 

 and reflexly by afferent impulses conveyed from the lungs, from 

 chemoreceptors in the blood-vessels, and from other sources through 

 fibres largely but not entirely in the vagus nerve. 



Lungs and Gills 

 Respiration as a general function is common to all organisms. 

 Though 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 aquatic respiration, and the 

 higher forms by 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 modification from one to 



