iv THE BLOOD: FOKMED CONSTITUENTS 93 



external world, and to modify these relations in various ways by 

 means of the organs of motion. It consists accordingly of the 

 central and peripheral nervous system, i.e. the sensory and 

 conducting organs, and of the muscular and skeletal system, i.e. 

 the active and passive apparatus of movement. 



The blood is the centre and objective of all the functions of 

 the vegetative system ; the brain is the central seat and focus of 

 the functions of the animal system. 



A third order of physiological processes must further be dis- 

 tinguished from the special functions of the vegetative and animal 

 systems, in which both these and, in a certain sense, the entire 

 organism participate. These are the physiological phenomena 

 of general metabolism and the regulation of the balance of output 

 and intake ; thermogenesis and the regulation of the heat balance ; 

 sexual and reproductive functions ; the physiology of the embryo, 

 and of the different stages of uterine life. 



II. As centre of the vegetative system, the blood contains all 

 the histos;enic substances destined to nourish and renew the 



o 



tissues, and all the histolytic products of consumption, useless or 

 noxious residues, to be eliminated. The first, which filter through 

 the living walls of the capillaries, pass in the form of lymph into 

 the interstitial plasma-spaces of the tissues for which they provide 

 aliment ; the second, secreted by the tissue cells, pass into the 

 blood by way of the lymph vessels, and are thence eliminated by 

 the kidneys, lungs, skin, and liver. 



From the histological standpoint the blood may be regarded 

 as a tissue. It contains a number of formed elements, represented 

 by the corpuscles, and an intercellular substance, the plasma, 

 which is essentially a product elaborated and secreted by all the 

 cells which take part in haematopoiesis and haematolysis. The 

 blood is distinguished from the other tissues by the fact that it 

 is fluid and that it circulates, and is therefore capable of exerting 

 its action on all the fixed tissues, bringing them into relation 

 and binding them together. It thus functions as the centre of 

 the vegetative system, and is the agent of metabolism, i.e. of the 

 material exchanges of the whole body. 



III. To compress within a few lines the historical development 

 of our physiological knowledge of the blood would be a work of 

 difficulty. In this field there is no one great discovery to be 

 recorded, only the gradual acquisition of separate facts due to 

 the labours of a vast number of observers. We shall confine 

 ourselves to enumerating a few of the principal dates and names 

 as landmarks. 



The Italian Malpighi (1661) was the first who saw the red 

 corpuscles, while the Dutch Leeuwenhoek (1673) first described 

 them accurately. In England Hewson (1770) also observed the 

 leucocytes, found that many salts delayed or inhibited coagulation, 



