126 ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



more distinct; the structure of the nucleus being so evident without the addi- 

 tion of acetic acid, that its granules can be counted.* 



146. The form of the Red Corpuscles is not unfrequently seen to 

 change during their circulation ; but this is generally in consequence of 

 pressure : from the effects of which, however, they quickly recover them- 

 selves. In the narrow capillary vessels, they sometimes become suddenly 

 elongated, twisted, or bent, through a narrowing of the channel ; and this 

 may take place to such a degree, as to enable the disc to pass through an 

 aperture, which appears very minute in proportion to its diameter. When 

 undergoing spontaneous decomposition, the blood-discs become granulated, 

 and sometimes (as long ago noticed by Hewson) even mulberry-shaped ; and 

 particles in which these changes appear to be commencing, may be found in 

 the blood at all times. It has been ascertained that bile and urea exert a pecu- 

 liar solvent power on the blood-corpuscles ; and hence we can understand 

 one of the modes in which a retention of these substances in the circulating 

 fluid (Chap. XV., Sect. 1) proves so injurious. The size of the blood-discs 

 is liable to considerable variation, even in the same individual ; some being 

 met with as much as one-third larger, whilst others are one-third smaller, than 

 the average. The diameter of the corpuscles bears no constant relation to the 

 size of the animal, even within the limits of the same class ; thus, although 

 those of the Elephant are the largest among Mammalia (as far as is hitherto 

 known), those of the Mouse tribe are far from being the smallest, being in 

 fact more than three times the diameter of those of the Musk Deer. There 

 is, however, a more uniform relation between the size of the animal and that 

 of its blood-discs, when the comparison is made within the limits of the same 

 order. In Man, the diameter varies from about 1 -4000th to l-2800th of an 

 inch; the average diameter is probably about l-3200th. 



a. The following measurements of the blood-discs of various animals are chiefly given 

 on the authority of Mr. Gulliver. The diameter of the corpuscles in the Quadrumana is 

 generally about the same with that of the Human blood-discs ; there is, however, a slight 

 diminution among the Lemurs, and there is more variation among them, than among the 

 Monkeys. Among the Cheiroptera, the diameter of the corpuscles is somewhat less than in 

 the preceding order, the average being about l-4300th of an inch. Passing to the Insecti- 

 vora, we find the blood-discs of the Mole to be still smaller, averaging only the l-4747th of 

 an inch; those of the Hedgehog, however, are larger, being about l-40S5th. In the corpus- 

 cles of the different families of the Carnivora, there is such a well-marked diversity in the 

 size of the corpuscles, that the fact may be used as a help to classification."}" In the Seals, 



* As Professor Owen's interesting account of the blood-discs of the Siren may not be 

 generally accessible (Penny Cyclopfedia, Art. Siren"), the leading facts in it will be here 

 stated. This animal agrees with the Proteus and other species in being perennibranchiate 

 ( 32); and, as in all its congeners yet examined, the blood-discs are of very large dimen- 

 sions. They are usually of an oval form, the long diameter being nearly twice the short; 

 and the nucleus projects slightly from each of the flattened surfaces. Considerable variety 

 in the form of the disc presents itself, some of the corpuscles being much less oval than 

 others; but the nuclei do not partake of these variations in nearly the same degree. The 

 nucleus is clearly seen to consist of a number of moderately-bright spherical granules, of 

 which from 20 to 30 could be seen in one plane or focus, the total number being of course 

 much greater. When removed from the capsule, the nuclei are colourless, nnd the compo- 

 nent granules have a high refracting power. Viewed in situ, they present a tinge of colour 

 lighter than that of the surrounding fluid, and dependent upon the thin layer of that fluid 

 interposed between the nucleus and the capsule. As the fluid contents of the blood-disc in 

 part evaporate during the process of desiccation, the capsule falls into fiilds in the interspace 

 between the nucleus and the outer margin; these folds generally take the direction of straight 

 lines, three to seven in number, radiating from the nucleus. 



f Two facts of much interest in Zoology have been brought to light by Mr. Gulliver's 

 examination of the diameter of the blood-corpuscles of this tribe. The difference between 

 those of the Dog and the Wolf is not greater than that which exists among the varieties of 

 the Dog; whilst the discs of the Fox are much smaller. The discs of the Hyaena are far 

 more approximate to those of the Canidce, than they are to those of the Felida . 



