146 



MANUAL OF HISTOLOGY. 



average thickness of 1 to 2 micro-millimetres (0.001 0.002 

 mm.). This membrane forms a tubule, the parietes of which 

 are studded at intervals with rounded or oval nuclei, often 

 containing one or more bright nucleoli. When oval, these nu- 

 clei have their long axis parallel with the direction of the ves- 

 sel. Their average size is 0.0056 to 0.0074 mm. They possess 

 the property of eagerly imbibing most of the staining fluids 

 employed in histology, and of resisting the action of dilute 

 acids, alkalies, and other reagents. (See Fig. 59.) 



Besides nuclei, the capillary wall contains at various points 

 peculiar granules, which indicate its protoplasmic nature. In 

 addition, Strieker and Eberth have described lateral processes 

 and pointed prolongations jutting out from the parietes of the 



Fio. 60. Capillaries of the lungs of the frog, with irregularly dentated cells : a, vascular meshes. 

 Eberth. 



capillary tubes. In growing tissue these are readily demonstra- 

 ble, often forming thread-like connecting bridges between neigh- 

 boring vessels ; at a later period they are hollowed out into 

 true capillaries. The shorter sprouts are also protoplasmic 

 buds, capable of further development into similar vessels. (See 

 Fig. 61.) By employing weak solutions of silver nitrate, the 

 capillary-wall may be shown to consist of variously shaped 

 areas, each one corresponding to a nucleated cell. They are 

 the endothelia, and represent, as already stated, the sole essen- 

 tial constituents of all capillaries. Their form varies with the 

 calibre of the vessel, the smaller capillaries being composed of 



