STRUCTURE OF CAPILLARIES. 



371 



muscular tissue, in form of the usual long contractile fibre-cells, which are directed 

 across the length of the vessel. The elongated nuclei of these cells may be 

 brought into view by means of acetic acid or by staining fluids (fig. 430). This 

 layer corresponds with the middle coat of the larger vessels. In the smallest 

 vessels in which it appears the muscular cells are few and apart, and a single long 

 cell may turn spirally round the tube (Lister) ; in larger vessels, especially those of 



Fig. 429. A SMALL AKTEBT A, 

 AND VEIN V, FROM THE SCB- 

 CUTANEOUSCONNECTIVE TISSUE 

 OF THE RAT. TREATED WITH 

 NITRATE OF SILVER. 175 



DIAMETERS. (E. A. S. ) 



a, a', epithelium-cells -with b, 

 b', their nuclei ; m, w, transverse 

 markings due to staining of sub- 

 stance between the muscular fibre- 

 cells ; c, c, nuclei of connective 

 tissue corpuscles attached to ex- 

 terior of vessel. 



the arterial system, the mus- 

 cular cells are more closely 

 arranged. Outside the mus- 

 cular coat is the areolar or 

 connective tissue coat, con- 

 taining fibres and connec- 

 tive tissue corpuscles, with 

 longitudinally placed nuclei . 

 In vessels of -^ of an 

 inch in diameter, or even 

 less, the elastic layers of 

 the inner coat may be dis- 

 covered (fig. 430, A, 8), in 

 the form generally of homo- 

 geneous or fenestrated mem- 

 brane, more rarely of longi- 

 tudinal reticulating elastic 

 fibres. The small veins 

 differ from arteries of cor- 

 responding size, chiefly in 

 the inferior development of 

 their muscular tissue ; the lining cells of the arteries also are very much longer 

 and narrower than those of the veins. These differences, as well as the compara- 

 tive size of corresponding vessels, are well shown in the accompanying figures (429 

 and 430). 



The only open communication between the arteries and the veins, is by means of capillary 

 vessels as above described, unless in the maternal part of the placenta and in the interior of 

 erectile organs, in which small arteries may open directly into wide venous cavities without 

 the intervention of capillaries. Moreover, in the spleen the arterial capillaries do not at once 

 pass into the commencements of the veins, but open into the interstices of the organ, from 

 which the minute veins collect the blood. 



But it would appear that in certain parts small arteries may pass into small veins without 

 the intervention of true capillaries (Sucquet, Hoyer). 



Arterial glands. At the upper end of the common carotid {carotid gland*) and in front of 

 the apex of the coccyx (coccygcal gland. Luschka), are found small solid-looking bodies of a 



