XVI CHOICE OF METHODS 



much valuable information can be secured by direct examination of the retinal 

 blood vessels. Even here their distance from the surface is considerable and 

 magnification is therefore limited. As far as we know at present the best that 

 can be done is to take advantage of a discovery, made by Lombard (W. P., Am. 

 J. Physiol., 1911-12, 29, 335-362) that the epidermis can be rendered transparent 

 by the addition of a little highly refractile oil without noticeably injuring it or 

 disturbing the underlying tissues. By this means the blood vessels of the dermal 

 papillae in the fold of skin over the nail bed, which are very near to the surface, 

 can be studied directly at fairly high magnification and over long periods of time 

 thus permitting the making of excellent pictures. See review of literature by 

 Wright, I. S. and Duryee, A. W., Arch. Int. Med., 1933, 52, 545-575. 



That the lymphatics in the human skin can be made visible in vivo by the 

 injection of small amounts of Patent Blue V has been demonstrated by Hudack, 

 S. S. and McMaster, P. D., J. Exp. Med., 1933, 57, 751-774. The vessels in 

 the ears of living mice can readily be seen without any surgical procedure. It 

 is even possible to directly watch the dye, Chicago blue, after intravenous 

 injection elsewhere in the body, leak out into the tissues especially through the 

 walls of the venules (Smith, F. and Rous, P., J. Exp. Med., 1931, 54, 499-514). 

 Ideas as to the relative hydrogen ion concentrations of some of the tissues visible 

 from without can be secured by the injection of Hydrogen Ion Indicators (Rous, 

 P., J. Exp. Med., 1925, 41, 739-759). The opportunities are many especially 

 in animal experimentation. 



Another way to examine structure in vivo is to record the structure by x-ray 

 photographs and to magnify the photographs, see Microradiographic examina- 

 tion. 



2. To Examine through Windows in Vivo 



The construction of windows in the skin or body wall through which the 

 tissues can be examined in vivo is a less ideal technique because it involves 

 surgical interference with the body. In the most used of these techniques a hole 

 is made through a rabbit's ear from one surface to the other. A glass chamber 

 is then sewed into the hole in such a way that a blood vessel is included between 

 a thin layer of glass (serving as a cover glass) and a thicker one serving as a slide. 

 After a time the epidermis adheres to the edges of the chamber and blood vessels, 

 nerves and other tissues grow into it where they can be studied under oil immer- 

 sion objectives. This technique was first reported by Sandison (J. C, Anat. 

 Rec, 1924, 28, 281) working under Dr. E. R. Clark at the University of Penn- 

 sylvania. It has since been very greatly improved (Clark, E. R., et al., Anat. 

 Rec, 1930, 47, 187-211 and Abell, R. G., and Clark, E. R., Anat. Rec, 1932, 

 53, 121-140) by the introduction of "round table" and "moat" chambers. 



To place a window in the wall of the skull and to observe what is going on 

 within has been done with more or less success on several occasions. The tech- 

 nique devised by Forbes (H. S., Arch. Neurol, and Psych., 1928, 19, 75) permits 

 direct observation at low magnification of the blood vessels over the cerebral 



