THE BLOOD 



27 



microscopic, and in connection with this question some of the most 

 acute and fatal infectious diseases of animals are caused by organisms 

 of this class, of which rinderpest, foot and mouth disease, rabies, and 

 African ' horse sickness ' are examples. Still, in spite of the fact 

 that these microbes have not been seen, their existence is undoubted, 

 the best evidence of which is that some of them are sufficiently large 

 to be caught in the pores of a filter, leaving the filtrate sterile. Other 

 organisms attack the blood-cells, either from without or within — for 

 example, the important group of Trypanosomes, the malaria para- 

 site, the organism of Texas fever, and such like. In these cases the 

 product of red-cell destruction may show itself by the discoloured 

 urine, and is evident in the tissues — for example, the liver and spleen. 



Compared with the red corpuscles, the white are seldom affected 

 with disease, but there are certain pathological conditions associated 

 with a great increase in their number (leucocytosis) , and others in 

 which the white cells are reduced in number (leucopcenia) . 



There are other conditions affecting the blood — for instance, 

 Purpura — which cannot be attributed to parasitic agency. In this 

 disease, either from defects in the blood or vessel-wall, haemorrhage 

 takes place into the tissues. No organ appears to be able to escape, 

 though probably the subcutaneous and muscular tissues are the 

 most frequent seat of the haemorrhage. 



Quite as strange and obscure is the dietetic disease of equines 

 known as haemoglobinuria, in which the animal in the middle of work 

 suddenly falls paralysed ; the urine becomes coffee-coloured and 

 loaded with methaemoglobin, in consequence of the destruction of 

 the red cells. What the destructive agent is, is at present unknown, 

 but it is probably one of the poisonous products of proteid disintegra- 

 tion, which will be found dealt with in the chapter on Digestion. 



Blood-letting in the treatment of disease was at one time so uni- 

 versal that it came to be regarded as the ' sheet-anchor ' of life, and 

 animals were regularly bled in order to keep them in health. ' Blood- 

 letting ' was killed by abuse ; it is now a question whether the pendulum 

 has not travelled too far in the other direction, and the employment 

 of a physiological means in the treatment of disease been too long 

 neglected. Towards the end of the eighteenth century a full blood- 

 letting for the horse was from 4 to 5 pints. During the first half of 

 the nineteenth century 8 pints were considered a moderate bleeding. 

 Under pressure of acute disease 3 gallons were drawn, and Percivall 

 tells us he had heard of 4 gallons being taken (see p. 25). 



This is the abuse we allude to as having caused the fashion to 

 change. Such heavy blood-lettings must have been responsible 

 for considerable mortality. Percivall describes the 'impression on 

 the system,' which was considered a necessary indication if blood- 

 letting was to prove beneficial. As the pulse began to sink, the 

 horse became very uneasy, jerking the head up and down, moving 

 backwards until finding support for the hind-quarters ; respirations 

 increased, deep sighing, the body rocking from side to side, in 

 danger of falling headlong ; shivering ; and, after the operation, 

 sweating. A second and even third bleeding was employed. 



The effect of bleeding healthy animals to improve nutrition was 

 fully accepted, and Percivall declares his opinion that, if continued 

 in, it became necessary for preserving health. An increased disposi- 

 tion to fatten was observed in young animals submitted to moderate 

 bleeding, and farmers employed this regularly for their calves. 



