118 PHYSIOLOGY CHAP. 



described by Hayem under the name of haematoblasts, because 

 they were erroneously considered to be the precursors, or early 

 stages of development, of the erythrocytes (Fig. 41). They are in 

 the form of circular flat discs and consist of a finely granulated, 

 highly refrangible substance, colourless (hence entirely destitute of 

 haemoglobin), and staining fairly intensely with aniline dyes. They 

 are two to three times smaller than the erythrocytes (2-3 p ; see 



Fig. 41). Their number varies 

 from 200,000 to 500,000 per 

 c.mm. The numerical relation 

 of the leucocytes to the plate- 

 lets is about 1 : 40, and of 

 the platelets to the erythro- 



FIG. 41. Blood-platelets viewed from ih.- surface CytCS about 1:25. Their Slir- 

 and laterally : highly magnified. In the centre is S , } , , 



an erythrocyte for comparison of size. lace mgillj Vlb lib, ctllt 



stagnant blood they agglutin- 

 ate, forming granulated heaps which readily break up and dissolve 

 in the plasma. 



Lowit is of opinion that the platelets are formed by disintegra- 

 tion of the leucocytes, and are not pre-existent in the blood before 

 it is extracted from the vessels. Bizzozero, however, proved that 

 they can easily be seen in the mesenteric vessels of guinea-pigs, 

 and in the wings of bats, on retarding the circulation. Osier, in 

 investigating the mesentery of the mouse (Fig. 42), confirmed this 

 observation. But the fact that they are found in living, circulating 

 blood does not seem a sufficient argument for regarding them 

 as distinct morphological and physio- 

 logical individuals. Lilienfeld's later 

 researches proved that the platelets 

 contain nuclein in the form of iiucleo- 

 albumin, the micro-chemical reactions 

 of which are similar to those of the 



nuclei Of the leUCOCyteS. It Seems not FIG. 4->. Erythrocytes and blood- 



improbable that they are derived from fSZ*ff.y m 

 the latter, owing to disintegration of 



cellular protoplasm. Besides blood-platelets, the older observers, 

 detected granules and irregular protoplasmic fragments of various 

 dimensions (and quite distinct from the fat drops that dissolve 

 in ether) in the blood, which evidently originate in the disintegra- 

 tion of the protoplasm of the lymphatic cells or leucocytes; on 

 this they founded the hypothesis that the platelets too are derived 

 from the disintegration of leucocytes. 



In accordance with this theory, Fano has demonstrated that 

 there are scarcely any platelets in dog's lymph. Probably this is 

 due to the fact that the younger lymphatic cells predominate in 

 lymph, and that their protoplasm disintegrates less readily. It 

 should also be added that blood-platelets of characteristic form 



