84 ORGANIC FUNCTIONING 



the blood coming from the intestine and going to the general 

 circulation first passes through the liver, thtn, via the great veins, 

 it reaches the heart. In all the higher animals the circulatory 

 system has the same essential plan of structure : from the heart 

 blood is propelled, through arterial vessels, to the respiratory 

 organs, gills or lungs. There it takes up oxygen and eliminates 

 CO 2. The circulation to the respiratory organs may form part 

 of a single scheme, or it may be part of a double scheme. 



In the Mammal. In whatever precise ways these purely 

 hydraulic mechanisms are disposed the termini of the blood- 

 stream are the living, functioning tissues. All these, whether 

 muscles, or glands, or brain, etc., are permeated, in the minute 

 parts, by a network of capillary vessels which distribute blood to 

 them. Between the tissue-cells are lymph channels, chinks, 

 irregular passages of obscure forms, etc., and the liquid part of 

 the blood — the plasma — passes through the capillary walls, flows 

 through the interstices between the tissue elements, gives up 

 to the latter food substances and oxygen and is finally collected 

 again by lymph channels and returned to the general blood-vessels. 

 It is in this way that the functioning, growing, or repairing tissues 

 are " fed." 



30. ON THE ORGANS OF RESPIRATION 



Whatever be the precise structures of the organs indicated in 

 section {i2c) this is always the case : 



(?) There is a respiratory membrane. In the air-breathing 

 higher animal this is the epithelium that lines the air-sacs of the 

 lungs. In the fish, or other water-breathing animal, it is the 

 epithelium covering the gill-lamellae, etc. 



{ii) One side of the membrane is exposed to the oxygen- 

 containing medium — air or water. On the other side is a dense 

 network of capillary blood-vessels. Blood that is venous, or 

 " spent " as regards its O-content, is circulated through this 

 network. 



{Hi) The blood contains an oxygen-carrier. This is haemog- 

 lobin, or some analogous substance. It may be contained in cells 

 — the red blood-corpuscles — or it may be dissolved in the fluid 

 blood. 



{iv). There is a gaseous interchange between the blood and the 



