RESPIRATORY SYSTEM u 3 



V. Respiratory System. 



For the maintenance of life, the animal organism requires 

 not only that the tissues be constantly saturated with nutritious 

 juice, but also that the gases suspended in cells and tissues be 

 constantly renewed. The organs which perform this function 

 have all been formed on one and the same plan and gradually 

 perfected by progressive development. The fundamental prin- 

 ciple of the exchange of gases may be traced from the proto- 

 zoa up to man. Broadly speaking, there is no great difference 

 between the lowest and highest creature ; the method of modifi- 

 cation may be seen by studying the details. 



Among the lowest forms of protozoa the entire surface of 

 the body performs the office of absorbing and discharging 



FIG. 59. Lymph FIG. 60. White blood corpuscles, mobile 

 cells. and immobile. (Ranke.) 



FIG. 61. Red corpuscles of 

 human blood, a, whole 

 surface ; b, seen edge- 

 wise ; c, arranged like 

 piles of coins ; d, ren- 

 dered globular by water ; 

 e, deprived of colour ; 

 /, coagulated. (Kol- 

 liker, Gewebelehre.) 



gases, but the amceba, and still more the infusoria, possess 

 contractile vesicles filled with fluid matter, and these may safely 

 be assumed to be organs specially designed for the exchange 

 of gases. The coelenterata have progressed further and pos- 

 sess a system of vesicles which pass through the body and are 

 filled with a compound of chyme and water, being apparently 

 designed for respiration as well as for alimentation. 



Proceeding to the higher classes of animals we find rudi- 

 mentary breathing organs. The lower worms, it is true, do not 

 possess such, the function of respiration being performed by 

 the integument and by means of the water which penetrates 

 into the abdominal cavity. But the tentacles of the Bryozoa 



