CHOICE OF METHODS 



The selection uill depend upon what it is desired to do. In most cases a 

 particular kind of information is sought. Feasible methods of obtaining it 

 with the materials available are needed and it is important that the information 

 secured be trustworthy ha\dng a minimum of experimental error. A brief out- 

 line of what can be done is presented in the hope that some of the techniques 

 mentioned will be suitable or will suggest satisfactory ways to proceed. Further 

 data are given in the bodj^ of this book concerning the subjects given in bold 

 face type. 



1. To Examine Directly in Vivo 



The ideal arrangement is to look into the body and to study its parts as they 

 function without causing any disturbance. Y/ith protozoa and certain small 

 transparent invertebrates this is relatively simple. The web of a frog's foot is 

 thin and can easily be looked through without seriousl}^ interfering with the 

 frog. Some other parts of the bodies of various aquatic lower forms lend them- 

 selves to direct examination in vivo; but there are definite limitations in such a 

 study of what is going on in the human body. It is possible to peer into the 

 various apertures but to get close enough to the living tissues to use high mag- 

 nifications is not feasible. The cornea and lens of the eye are transparent and 

 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 underh' ing 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 directl}" 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 readih^ be seen without any surgical procedure. It 

 is even ]jossible 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, 



1 



