It was formerly thought that possibly the products of hæmolysis were themselves toxic and capable of producing the symptoms described. This seems, however, to have been disproved by Bayliss, who has shown that in the dog and cat the hæmolysed blood of the same species is, with extremely rare exceptions, innocuous. Another possible cause of similar symptoms is the sodium citrate used as an anticoagulant in one of the methods of transfusion subsequently to be described. But the symptoms, if due to this cause, will not be accompanied by any signs of hæmolysis, are usually not severe, and are always very transient. This will be referred to again later on. The symptoms of incompatibility begin to be apparent so quickly that the worst results can be avoided by the exercise of caution. If for any reason it has been necessary to use an untested blood donor, the first 100 cc. of blood should be injected very slowly. If no untoward symptoms result, the remainder of the blood can be injected with greater confidence. Little can be said as to the treatment of this condition, for prevention is far better than cure. When the symptoms have developed, the damage has been done, and cannot be undone. The ordinary measures for combating severe collapse may be used. A lesser danger of transfusion is that of administering the blood too rapidly. Sometimes during a transfusion the patient complains of difficulty in breathing and a sensation of tightness in the chest; this should always be regarded as a warning that the blood must be given more slowly or perhaps that enough has been given and that the transfusion should be discontinued. Usually the symptom amounts to nothing more than discomfort, and will disappear if caution be exercised. The explanation is to be found in the too rapid filling of the venous side of an impaired circulation with overloading, and perhaps temporary dilatation, of the right side of the heart. I have never seen these symptoms occur to an alarming degree, but actual loss of consciousness with a very rapid and feeble pulse has been recorded by other writers. Directions as to the amount of blood which should be given and the rate at which it should be injected so that these symptoms may be avoided will be found under the description of methods given in a later chapter. CHAPTER V PHYSIOLOGY AND PATHOLOGY OF BLOOD GROUPS In the foregoing chapter the reactions between the blood groups and the morbid symptoms which may follow the injection of incompatible blood have been described. In the present chapter some account will be given of the more general physiology and pathology of the groups.