364 



MANUAL OF HISTOLOGY. 



ever, if, for the future, we make use of the term primary vascular mem- 

 brane in referring to this cellular tube. 



In regard to the cells themselves, they are presented to us according to 



the breadth of the tube, either under a 

 more or less fusiform or polygonal form. 

 The first variety (fig. 356), bounded by 

 delicate serrated or undulating lines, have 

 a length of 0-0756-0'0977 mm., and 

 breadth of -009 9-0 -050 mm. Such ele- 

 ments are to be found forming the walls 

 of the finest capillaries, arranged either 

 parallel to the axis of the vessel, or more 

 rarely obliquely, as regards the latter. In 

 transverse sections of the vessels two or 

 three, or less frequently four of them, may 

 be remarked. 



In many of the finest vessels portions 

 of the tube are formed of one single cell 

 alone, its two edges meeting around the 

 lumen. Such cases may be found among the capillaries of the brain, the 

 retina, the muscles, and the skin. 



Capillaries of larger calibre are made up of cells of the second variety. 

 We encounter here either regular polygons, as, for instance, in the 

 clwrio-capillaris of the cat and iris of the bird's eye, or more irregular 



Fig. 357. Capillary network from tlie 

 lung of the frog treated with solution 

 of nitrate of silver. 6, vascular cells; 

 a, nuclei of the same. 



Fig. 358. Capillary ves- 

 sel from the mesen- 

 tery of the frog treated 

 with nitrate of silver. 

 Between the vascular 

 cells at a, a and 6, the 

 "stomala" are to be 

 seen. 



Fig. 359. Capillaries and finer trunks 

 from a mammal, a, capillary from 

 the brain ; 6, from a lymphatic gland; 

 c, a somewhat stronger branch, with 

 a lymphatic sheath, from the small 

 intestine; and d, transverse section 

 of a small artery of a lymphatic 

 gland. 



plates (fig. 357), in many instances giving off long processes. In the 

 transverse section of the vessel we may have two or four of these. In 

 size they are naturally subject to great variation, and may attain a diameter 

 at certain points of 0'0749-0'01737 mm. 



The interdigitation of their 



