387 



are black and opaque throughout ; they average 1-lOOOOth of 

 an inch in diameter. 



VI. Photograph representing a minute artery, with part 

 of the adjoining network of capillaries, from the muscular 



coat of the urinary bladder of the frog. Negative No. 220, 

 New Sei'ies. From preparation No. 3378, Microscojjic Section. 

 Magnified 400 diameters by Wales's -^th objective. The field 

 is crossed by a small artery, 1- 1700th of an inch in diameter. 

 Its ej^ithelial cells are longer in proportion to their width 

 than those of the veins. They average 1 -400th of an inch in 

 length, and have nuclei similar to those of the venous epithe- 

 lium. Wherever the capillaries come into focus the epithelium 

 of their walls is also plainly shown. In the intervascular 

 spaces the nuclei of the muscle and connective tissue appear 

 as in the first photograph. The cut presents an outline of a 

 part of the picture ; a, a, are the nuclei of the vascular epi- 

 thelium ; b, b, those of the muscle ; c, c, those of the connec- 

 tive tissue. 



VII. Photograph representing a portion of the view pre- 

 sented by the last picture. Negative No. 22S, New Series. 

 Magnified 1000 diameters by Powell and Leland's immersion 

 l-16th objective. The spindle-shaped forms of the epithelial 

 cells of the arteries and the characters of the nuclei are 

 plainly shown. 



