II. Organs. 8. Respiratory Organic. 



29 



reservoir, whence it flows all over the body. This is the usual 

 arrangement in known animals with a respiratory apparatus and a 

 complete vascular system, but great variations occur within these 

 limits. Among many Invertebrates (Mollusca, Crustacea), the arterial 

 blood spaces are represented by the heart, which, as it receives blood 

 from the gills, is an arterial heart, and drives it all over 

 the body ; the impure blood is carried into a large non-contractile 

 reservoir, a venous blood sinus, whence again it returns to the gills. 



Fig 23. Diagram illustrating the usual relations of the respiratory organs a to the 

 vascular system. Arterial blood light, venous blood dark. See text. — Orig. 



In Pisces it is entirely different : here the heart is represented by a 

 venous reservoir, which receives blood from the body and sends it to 

 the gills ; it is, therefore, a venous heart. Blood goes from the 

 gills to the aorta, a large non-pulsatile vessel corresponding to the 

 arterial reservoir, and passes thence to the body. In Mammalia and 

 Aves, again, there is another arrangement, for, functionally, t nv o 

 hearts are present, of which one, the right side of the organ (right 

 auricle and right ventricle) represents the venous reservoir, and 

 receives blood from the body and sends it to the lungs; whilst the 

 other (left auricle and left ventricle) replaces the arterial reservoir, 

 receives blood from the respiratory apparatus and distributes it all 

 over the organism. 



It must be noticed here that the formation of special respiratory 

 organs is not accompanied by cessation of cutaneous and intestinal 

 respiration. These are, it is true, of very slight significance in many 

 animals, e.g., Mammalia ; in others, however, they play an important 

 part, especially cutaneous respiration. 



A red substance, hsemoglobin, present in the coloured coi-puscles of Vertebi-ates, 

 is of great respiratory importance, for most of the oxygen absorbed is not simply 

 dissolved in the blood-plasma, but enters into a loose chemical combination witb 

 the hsemoglobin, from which it is easily separated again, and is seized upon by 

 the tissues as the blood travels through the capillaries. When the hEemoo-lobin 

 is combined vrith oxygen, the blood is of a bright red coloTir, and is called arterial 

 blood ; when the oxygen has been given up, the blood looks dark red, and is 

 known as venous blood. The same, or a similar, substance is found in the red 



