572 



NATURE 



[July 31, 191: 



the plasma — which holds in suspension living cellular 

 bodies, called cells or corpuscles. These are of two 

 kinds, red and white corpuscles. The red are by far 

 the more numerous, and in man there are about 

 5,000,000 of them to a cubic millimetre of blood, but 

 this number varies enormously under the influence 

 of parasites. To these red corpuscles is due the red 

 colour of the blood, and they are the carriers of 

 oxygen, acquired by the aeration of the blood in the 

 lungs, to the tissues. We breathe in order that they 

 may breathe, for we only care about oxygen in so 

 far as they care about it. 



The other kind of corpuscles are the white, or 

 leucocytes, and of these in health there are about 

 7500 per cubic millimetre. A few years ago it was 

 enough to know that there were red and white cor- 

 puscles, but now we have to know more. Through 

 the work of Ehrlich we know that there are at least 

 five different kinds of leucocytes in normal blood, 

 which I will just indicate to you. 



(1) Lymphocytes. — These are the smallest cells, and 

 contain a relatively very large nucleus. 



(2) Large Mononuclears. — These are large, and are 

 called macrophages, as they possess the power of 

 being able to absorb and digest parasites and other 

 foreign bodies. 



(3) Polynuclears. — These are characterised by the 

 irregular, moniliform aspect of their nucleus, and 

 they are called microphages for the same reason that 

 the large mononuclears are called macrophages. Both 

 of these are also called generally, phagocytes, on 

 account of their power of ingesting and digesting 

 foreign bodies. 



(4) Eosinophils. — These are characterised by a 

 bilobed nucleus, and by granulations which colour 

 deeply with eosin and other acid colours. 



(5) Labrocytes or Mastzellen. — These are rare, and 

 are characterised by large granulations which stain 

 with basic colours. 



In parasitic diseases these corpuscles are profoundly 

 modified and altered, numerically and morphologic- 

 ally, and other new elements may make their appear- 

 ance in the blood. 



The blood is essentially the same in all animals, 

 but it varies within certain limits. For instance, the 

 red corpuscles are not of the same size and shape in 

 every animal, and in birds and fishes they are 

 nucleated ; in us they are only nucleated in foetal life 

 and in disease. The mononuclear and polynuclear 

 leucocytes are really separate organisms living in us, 

 and they have qualities which it is very difficult to 

 call anything else but consciousness; so that it is a 

 subtle distinction to draw the line between the para- 

 sites — which these leucocytes are, in a way — which 

 are part of us, and those that are not. When the 

 balance of power is well preserved amongst our leuco- 

 cytes, when they are working well together, then all 

 is well with us ; if we are ill, it is because they are 

 quarrelling' with themselves or with an invader, and 

 we send for Sir Almroth Wright to pacify or chastise 

 them with his vaccines. 



So that, as Darwin said: "An organic being is a 

 microcosm, a little universe, formed of a host of 

 self-propagating organisms, inconceivably minute and 

 numerous as the stars in heaven " — as we ourselves 

 are but parts of life at large. 



The three main functions of the blood are : that 

 it is a means of respiration, a means of nutrition, 

 and a defence acainst invading organisms. 



And now to these latter. A blood-parasite proper 

 is a living being, vegetable or animal, passing part 

 or the whole of its existence in the blood of another 

 living being, upon which it lives, this being obligatory 

 and necessary to its life-cycle. 



It was in i8ji that the first blood-parasite was seen 

 NO. 2283, VOL. 91] 



by Valentin in the blood of a fish, and two years later 

 Gruby gave the name trypanosoma to an organism 

 he found in the blood of a frog. But since Laveran's 

 discovery of the malarial parasite in 1880, we have 

 learnt to differentiate many other parasites as causal 

 agents of such diseases as I shall mention later in 

 connection with the various parasites. But we know- 

 as yet dangerously little about most of them, so that 

 we have strenuously to resist the temptation to make 

 our account of them sound too harmonious, before 

 we have found half the notes of the chord we are 

 trying to play. We speak, as it were, with authorised 

 uncertainty, and there are parts of our science which, 

 after all, are only expressions for our ignorance of 

 our own ignorance. These parasites have a very com- 

 plicated life-history; part ot their life-cycle is passed 

 in the blood of man or beast, and part in various 

 parts of the body of some blood-sucking invertebrate, 

 such as a fly, mosquito, or tick, which transfers the 

 parasite to another animal whilst feeding from him. 

 It was thought formerly that blood-parasites would 

 be a restricted order, but the work of recent years 

 has shown that they have an enormous distribution 

 both geographically and as regards their hosts. For 

 instance, during the last five years I have had the 

 opportunity of examining all the animals (in the large 

 sense of the word) which have died in the Zoological 

 Gardens. I have examined the blood of more than 

 8000 animals, coming from all parts of the world, and 

 I have found parasites in the blood of 587 of them, 

 that is in about 7 per cent., and in 295 species of 

 animals I have found them for the first time. I 

 mention this just to give you some numerical idea 

 of their occurrence and distribution. 



It will be better to take first those parasites which 

 live in the plasma, and then those that live in the 

 corpuscles, rather than to attempt to take them in 

 their, at present rather uncertain, biological order; 

 and I will begin at the bottom, biologkallv speak- 

 ing, that is with the bacteria which are plants. These 

 only require mention, since they do not live in the 

 blood as parasites proper, but only as accidental para- 

 sites — that is, parasitism is not necessary to their life- 

 cycle; they get into the blood in the later, or in 

 certain, stages of certain diseases. 



An example is the blood of a Senegal turtle-dove 

 which died in twenty-six hours from fowl cholera. 

 This bacillus was discovered by Pasteur, and is in- 

 teresting, as it was his work upon it which led to his 

 discovery of the attentuation of a virus, and of its 

 transformation thereby into a protective vaccine. 



The first parasites proper I shall mention are the 

 Spirochetes. These have at present rather an in- 

 secure position in our idea of nature ; they were 

 formerly classed close to the bacteria, but now they 

 are placed tentatively among animals, and they are 

 not yet quite sure of their place. But they, never- 

 theless, although insecure of their place in the books, 

 produce grave diseases, such as relapsing fever, tick 

 fever of man, the spirochetoses of horses, oxen, and 

 birds, syphilis, and yaws. They, with the exception 

 of the last two, are carried bv, and developed in, 

 ticks and bugs; and in tick fever the parasite is 

 also found in the nymph form of the tick, and this 

 is one of the rare instances of heredity of a parasite. 



The spirochete of relapsing fever in man was dis- 

 covered by Obermeier in 1868, and he died from 

 inoculating himself with the blood of a oatient with 

 the disease. He was one of «the fir^t scientific 

 martvrs ; he established our knowledge of the cause 

 of this disease at the expense of his own life. 



We will now take a long jump to the Filariae. 

 These are nematode worms, the embryo forms of 

 which live in the blood ; the parent forms, being too 

 large to get through the capillaries, live in manv 



