STRUCTURE OF CAPILLARIES. 



clxxvii 



Fig. XCV. INJECTED CAPILLARY VESSELS OP MUSCLE, 



SEEX WITH A LOW MAGNIPYIfcG POWER. 



Fig. XCVI. NET-WORK OP CAPILLARY VESSELS OP THE 

 AIR-CELLS OP THE HORSE'S LUNG, MAGNIFIED. 



a, a, capillaries proceeding from b, b, terminal branches 

 of the pulmonary artery (after Frey). 



retina, and in such, cases it is easy to display their independent membra- 

 nous parietes. The 



Fig. XCV. Fig. XCVI. number as well as 



the structure of the 

 coats of the capillaries 

 differs according to 

 the size of the vessels. 

 Capillaries of a diame- 

 ter less than ^ ff 

 of an inch, were 

 until lately believed 

 to have but a single 

 coat, formed of simple 

 homogeneous trans- 

 parent membrane, 

 with nucleiform cor- 

 puscles attached to it 

 or inclosed in its sub- 

 stance ; but from re- 

 cent researches (by 

 Auerbach, Eberth, and 

 Chrzonszczewsky,) it 

 has been ascertained 

 that they are fur- 

 nished with an epi- 

 thelium, to which the 

 nuclei of the capillary 



coat really belong. The cells of the epithelium are, as in the larger 

 vessels, flattened into scales and form but a single layer, in which the 

 outline of the scales, or their lines of junction one with another, may be 

 made apparent by nitrate of silver injection ; after which the nuclei may be 

 brought into view by acetic acid or carmine (fig. xcvii.). The epithelium 

 scales, which are polygonal in the small arteiies and veins (A, v), gradually 

 become oblong or spindle-shaped as they pass into the capillaries, and 

 throughout these vessels the epithelium presents the same characters, only 

 the flattened cells or scales become longer and narrower in the smaller 

 capillaries, and fewer in number in the circumference of the tube. In the 

 brain usually only two cells are to be seen in the cross section of a capillary, 

 but in the large capillaries of the kidney and bladder the number may rise 

 to four or five, and in this case the scales are shorter and broader. At the 

 points of junction of the capillaries the cells are much broader and not 

 spindle-shaped but radiate, with three or four pointed branches fitting in 

 between the cells of the three or four adjoining vessels which meet at the 

 spot (fig. xcvii., c c'). 



Auerbach describes the capillary wall as formed entirely of flattened 

 epithelium- cells fitted together at their edges into a continuous and coherent 

 membrane, without any further supporting structure ; but Chrzonszczewsky 

 discovered portions of the capillaries in which the epithelium-cells had been 

 displaced, and where spaces of some extent were left entirely devoid of 

 nuclei and of the outline markings of the cells ; and in these parts the out- 

 line of the capillary wall was still entire and continuous, and its substance 

 quite structureless. He concludes, therefore, that there is a homogeneous 

 coat, and within this an epithelium, to which the nuclei belong. 



