RESPIRATORY SYSTEM 209 



cutaneously into animals cause constriction of the arterioles and 

 a rise of blood pressure in the large arteries. As a remote 

 effect there is observed stimulation of the heart muscle, of un- 

 striped muscle generally, and inhibition of the movements of 

 respiration by interference with the centre in the medulla ; but 

 it is very doubtful whether such an amount of so powerful a 

 substance is ever obtainable at one time from the normal glands. 



IV. Respiratory System. 



The stages in the evolution of the organs of respiration 

 throughout the animal kingdom have been already dealt with 

 from the anatomical point of view : the mechanism of respira- 

 tion may be considered as follows (disregarding those Protozoa 

 in whom the respiratory exchange takes place indifferently over 

 the whole surface of the body) : 



(a) Animals with a water-vascular system where sea-water 

 containing about 2\ per cent, of oxygen (by volume) circulates 

 through the channels of this system to the internal organs and 

 diffuses throughout the tissues. 



(b) Tracheata with their elastic trachea branching in every 

 direction throughout the entire organism, so that the finest sub- 

 divisions reach individual cells and, according to Cuvier, the air 

 comes directly into contact with the tissues. 



(c) Branchiate standing a step higher in development with 

 the capillary blood vessels lying immediately under the super- 

 ficial epithelium of the gill, each division of which may be 

 regarded as a lung turned inside out. 



(d) The highest stage is reached in the Pulmonary system^ 

 which is found in gradually increasing development from 

 amphibia, birds and reptiles, up to the mammals, and finally 

 man. These have all alike internal lungs in which the gaseous 

 exchange is carried on, the air being sucked in and driven out 

 by the mechanism of the diaphragm and costal muscles. 



TABLE OF RESPIRATION-RATES PER MiNUTE. 1 



Horse (resting) . . . .6-10 

 Ox 10-15 



1 Sussdorf in Ellenberger's Physiologic, ii., p. 616. 

 14 



