§ 36. To prove by chemical and physical methods that a certain stain is that of blood, is often only one step in the inquiry, the next question being whether the blood is that of man or of animals. The blood-corpuscles of man are larger than those of any domestic animal inhabiting Europe. The diameter of the average red blood-corpuscle is about the 1/126 of a millimetre, or 7·9 µ.[55] The corpuscles of man and of mammals, generally speaking, are round, those of birds and reptiles oval, so that there can be no confusion between man and birds, fishes or reptiles; if the corpuscles are circular in shape the blood will be that of a mammal. By careful measurements, Dr. Richardson, of Pennsylvania, affirms that it is quite possible to distinguish human blood from that of all common animals. He maintains, and it is true, that, by using very high magnifying powers and taking much trouble, an expert can satisfactorily identify human blood, if he has some half-dozen drops of blood from different animals--such as the sheep, goat, horse, dog, cat, &c., all fresh at hand for comparison, and _if the human blood is normal_. However, when we come to the blood of persons suffering from disease, there are changes in the diameter and even the form of the corpuscles which much complicate the matter; while, in blood-stains of any age, the blood-corpuscles, even with the most artfully-contrived solvent, are so distorted in shape that he would be a bold man who should venture on any definite conclusion as to whether the blood was certainly human, more especially if he had to give evidence in a criminal case. [55] 1/3200 of an inch; the Greek letter µ is the micro-millimetre, or 1000th of a millimetre, ·00003937 inch. Neumann affirms that the pattern which the fibrin or coagulum of the blood forms is peculiar to each animal, and Dr. Day, of Geelong, has independently confirmed his researches: this very interesting observation perhaps has not received the attention it merits.