THE BLOOD. 237 



Hewson described the red globules as being flattened, and 

 was the first to discover distinctly, the dark spot in the centre, 

 formed by the central nucleus. Modern improvements in the 

 manner of using the microscope, have confirmed his vjews, 

 and shown that the globules of blood m different animals, are 

 always flattened, and that they vary. in their outline, from the 

 elliptical to the circular form. 



Fig. 175.* In the mammalia, including man, they are circu- 

 FLCJ. a lar disks ; that is, globules flattened upon the sides. 

 In birds, reptiles, and fishes, they are elliptical ; the 

 long diameter being twice that of the transverse. 

 In the globules of human blood, their thickness is 

 about one-fourth or one-fifth of their transverse 

 diameter. The size of the red globules varies very 

 much in different animals, and to some extent 

 among themselves, even in the same animal. But 

 in man and most other animals, the variation in 

 size, never amounts to a doubling of the diameter. The size 

 of the red globules in the amphibia, is greater than in any 

 other class of animals. The long diameter of the red globules 

 in the frog, is four times that of the circular diameter of those 

 of man. The diameter of the blood corpuscles in man have 

 been variously stated, from the ^ 50 th to the ? i ss th part of an 

 English inch. 



Fig. 176.f In the centre of all the red particles there is 

 Fiq d a dark spot, which is elliptical in shape, in all 

 animals in which the general shape of the par- 

 ticle is elliptical ; and round in those in which 

 /7*^lti^ ^ e P art id es are circular and flattened. These 

 I spots or nuclei, when viewed with the light in 

 a particular situation, reflect it, and present the 

 appearance of holes, which at one time induced the belief, that 



* Fig. 175. a, Globule of human blood, b, Of sheep's blood, c, Of the 

 blood of a sparrow. These globules are magnified a thousand times in diame- 

 ter. P. 



f Fig. 176. d, Globule of frog's blood, magnified seven hundred times, and 

 seen in profile, e, Front view of the same, in which the envelop or coloring 

 matter is torn, so as to exhibit the central nucleus. P. 



