THE BLOOD. 223 



blood of a Snail in a vessel for some days, it I'emained liquid 

 and entire, not separating, in the manner of human blood, into 

 two portions of vmequal densities ; but, when he applied 

 heat, it readily congealed into an opaque bluish coagulum, 

 just as the human serum would have done under the same 

 circumstances. But Lister knew well that the blood of 

 these creatures was not homogeneous; for he adds, that with 

 a good microscope it is easily shown to consist of globules 

 swimming in a limpid fluid; that these globules are truly 

 round, and considerably exceed in size those of human 

 blood; they are also heavier than the fluid part, since they 

 gradually sink to the bottom when kept still in a glass 

 tube.* The late experiments of Prevost and Dumas have 

 confirmed those of the old English naturalist : they have 

 ascertained that the globules of the Snail have a diameter 

 one third greater than those of man-j- and quadrupeds; and, 

 what is more remarkable, they found the globules to be 

 really spherical, as Lister has asserted, although analogy 

 would have led us to a different conclusion ; for they are 

 elliptical in birds, reptiles, and fishes, to which the mol- 

 lusca are certainly more nearly allied than they are to the 

 mammalia. J The globules in the bivalve mollvisca are also, 

 according to Poli, much larger than in man ; so that he con- 

 siders the latter to be to the former as hemp-seed to millet- 

 seed. § The red colour of blood has been attributed to the 



* Excr. Anat. dc Cocli. p. 95, and Exer. Anat. tcrt. p. xxxiii. — " Si tamen 

 in bono microscopio examinetur, id est, syplione capillari vitreo, vena alicui 

 intruso, globulos opacos vere orbiculares baud paiicos videbis ; ac sanguineos 

 nostros globnlos magnitudine pluiimum excedentcs. Hi vcro globuli, ut prse 

 sanguinis globulis pauci sunt, ita aqua quaedam limpida innatant, et paulatim 

 pra3 gravitate ad imum syphonem descendunt. Idem quoque experimentum 

 de succo vitali in Cocbleis fluviatilibus feci ; idemque coagulum subcoeru- 

 leum, igni admotus, dedit." 



+ Tlie red globules of human blood, according to the observations of Mr, 

 Bauer, as corrected by Kater and others, are one five-thousandth part of an 

 inch in diameter. — Home's Comp. Anat. vol. iii. p. 4., compared with p. 12. 

 But in the foetus, the globules, say Prevost and Dimias, differ in their form 

 and volume from those of the adult ; the former being double the size of the 

 latter. Bostock's Pfu/siologi/,vo\. ii. p. 200; and approximating nearer, of 

 course, to the size of those of mollusca. The fact is curious, when consi- 

 dered in relation to some speculations of Carus. 



X Zoological Journal, i. 178. 



§ Rudolphi's Physiology, trans, i. p. 132. — Mr. Lister found the particles 

 of blood in tlie Ascidiaj to be not uniform in size or shape, but they were 

 mostly between '00025 and "0002 inch in diameter, and approaching to glo- 

 bular. — Phil. Trans, for 1834, p. 380. Milne-Edwards says that the blood- 

 globules of invertebrated animals differ greatly from those of the vertebrated : 

 their size is very variable in the same individual, their surface appears wrin- 

 kled, neither a central nucleus nor an exterior envelope can be distinguished, 

 and their form is in general spherical. — Elcm. de Zoologie, p. 21, 



