94 



NORMAL HISTOLOGY. 



adipose tissue they are irregularly polygonal ; in the liver-lobules they con- 

 verge; while in the subepithelial papillae of the skin and mucous membranes 

 the capillaries commonly form loops. In general, the greater the functional 

 activity of an organ, the closer is its capillary network. Organs actively 

 engaged in excretion, as the kidneys, or in the elimination of substances 

 from the blood, as the lungs and liver, as well as organs producing substances 

 directly entering the circulation (organs of internal secretion), as the thyroid 

 gland, are provided with exceptionally rich and close networks. 



THE BLOOD. 



The fluid circulating within all parts of the blood-vascular system 

 consists of a clear, almost colorless plasma or liquor sanguinis, in which are 

 suspended vast numbers of small free corpuscular elements, the blood-cells. 

 The latter are of two chief kinds, the colored cells or erythrocytes, and the 

 colorless cells or leucocytes. The characteristic appearance of the blood is 

 due to the presence of hemoglobin contained within the erythrocytes which, 

 while individually only faintly tinted, collectively impart the familiar hue, as 

 well as a certain degree of opacity. That the characteristic pigment is 

 limited to the cells is shown by the lack of color and transparency of the 



plasma when examined under the 

 microscope, although to the unaided 

 eye the blood appears uniformly red 

 and somewhat opaque. 



The Colored Blood-Cells. 

 ^k As usually seen, the mature colored 

 blood-cells, erythrocytes or red cor- 

 puscles, of man and other mammals 

 (except those of the camel family, 

 which are elliptical in outline) are 

 small biconcave circular nonnucleated 

 disks, with smooth contour and rounded 

 edges. When viewed by transmitted 

 light, the individual ' ' red ' ' cells pos- 

 sess a pale greenish-yellow tint, and 

 only when they are collected in masses 

 or in several layers is the distinctive 

 blood-color evident. The peculiar 

 form of the corpuscle as ordinarily 

 seen biconcave in the centre and biconvex at the margin renders accurate 

 focussing of all parts of its surface at one time impossible, the cell 

 appearing, according to focal adjustment, either as a dark ring enclosing a 

 light centre or vice versa. Viewed in profile, the disk presents a figure some- 

 what resembling a dumb-bell, the thicker margins of the cell being connected 

 by the thinner concave centre. Although the biconcave discoidal form of 

 the mammalian erythrocytes is the one ordinarily exhibited, during 

 life, after certain fixatives the red blood-cells appear cup-shaped, which 

 led, for a time, to the assumption that this was the normal shape of the 

 living corpuscles. 



The structure of the red blood-cell has long been and still is a subject 

 dispute. According to one view, the cell consists of a soft pliable 



FIG. 131. Human colored bipod-cells, spread into 

 a single layer and dried. X 1000. 



of 



envelope enclosing a fluid contents containing the coloring matter, the hemo- 

 globin. The other view regards the corpuscle as composed of an extremely 



