THE UNIQUENESS OF THE INDIVIDUAL 



grafts simply cannot get through it — unless, indeed, the cornea 

 should become vascularized by accident, in which case a graft 

 transplanted to the cornea will usually fail. Brain and cornea 

 are therefore both privileged sites of grafting, but for entirely 

 different reasons. A simple experiment will make the distinc- 

 tion clear. A homograft in the brain will certainly be destroyed 

 if the animal into which it is transplanted is immunized by 

 some other, efficacious route — for example, by a skin homo- 

 graft transplanted upon or beneath the skin. It is therefore 

 entirely vulnerable to an immunological reaction; it owes its 

 privilege only to the fact that it cannot set such a reaction 

 going. But a homograft in the cornea, provided the cornea is 

 unvascularized, will survive in the face of a fulminating 

 immunity directed against it; it survives because the state of 

 immunity cannot take effect. 



So much for what may be called the bye-laws of tissue 

 transplantation, the special rules that govern the behaviour of 

 homografts in special positions in the body. I have mentioned 

 two or three, but others remain to be discovered. No one has 

 yet put forward a plausible explanation of why it is that homo- 

 grafts of certain endocrine glands — of the ovary, for example, 

 in so far as it is a source of hormones, or the adrenal cortex — 

 sometimes survive when homografts of skin demonstrably fail 

 to do so. But a general solution of the homograft problem must 

 turn upon the last of the four expedients which I mentioned in 

 the introduction to this section, i.e. upon changing the host 

 in such a way that its reaction against homografts is done away 

 with or at least enfeebled. 



There are half a dozen ways in which the intended recipient 

 of a homograft can be treated in order to prolong the homo- 

 graft ""s normal lease of life; most are temporary, but one can be 

 permanent; some could be applied in surgical practice if it were 

 worthwhile doing so, others not; a few are innocuous, but most 

 are harmful. By far the most important distinction, however, 



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