BLOOD AND RESPIRATION. 7 



is the colouring matter of the blood, acts as the air-carrier of 

 oxygen to the tissues. On meeting with oxygen (as in the 

 air cells), it forms oxyhaemoglobin, which on being brought 

 into the presence of carbonic acid (as in the capillaries of the 

 tissues) parts with its oxygen. The bright red colour of 

 arterial blood of the general circulation, as when an artery of 

 the external muscles is cut, is due to the oxyhaemoglobin in 

 it. When haemoglobin is uncombined with oxygen, it is of a 

 purple hue, as we may see in ordinary venous blood. 



Under usual circumstances of health, the quantity of 

 blood in the body is more or less evenly distributed through- 

 out the system. As this supply of blood is just sufficient for 

 the requirements of the animal, it follows that if there is an 

 unusually large accumulation (congestion] of blood in one 

 part, there will be too little blood (ancemia or bloodlessness) 

 in the remainder of the system ; and vice versa. Such local 

 changes can be borne with impunity, if they are restricted 

 within comparatively moderate limits of time and extent. It 

 is, however, impossible to fix the limits which divide conges- 

 tion and anaemia from health and disease. Cold sets up 

 anaemia by causing contraction of the blood-vessels of the 

 part. Heat and exercise have the opposite effect. As a rule, 

 there is less blood near the surface of the body than in the 

 deeper-lying tissues, owing to the cooling effect of the atmo- 

 sphere, and to the fact that the work done by the internal 

 organs is more continuous than that of the superficial muscles. 

 Although the terms anaemia and congestion are generally con- 

 nected with disease, we may correctly apply them to certain 

 healthy conditions, as for instance, the anaemia of the brain 

 during sleep, and the congestion of the blood-vessels of the 

 stomach during the digestion of food in that organ. 



BODILY SUPPLY AND REMOVAL. 



The living body suffers constant loss of substance in the 

 performance of its functions, and to make up for this loss and 



