CHOICE OF METHODS Xxiii 



others. Intracerebral and intratesticular inoculations are often made and, 

 again, young animals are in general most susceptible. 



The culture of Bacteria and Protozoa has for generations been a fine art based 

 on meticulous study of their needs. These relatively simple organisms provide 

 wonderful material for the investigation of the most basic of vital phenomena. 



7. To Investigate Composition by Chemical Means 



This cannot be done blindly — by just taking a chunk of tissue and analysing 

 it. The investigations must be guided by knowledge of the structure and func- 

 tion of the materials analysed. Blood can, for example, be collected in suf- 

 ficient volume for routine chemical analysis; but the results will differ depending 

 upon whether it is arterial blood, portal venous blood from the intestines, or 

 venous blood from the extremities. Analyses of whole skin are practically 

 worthless because the skin is a structure made up of two parts: avascular epi- 

 dermis of ectodermal origin and underlying dermis made up of connective tissue 

 differing in vascularity, fiber, fat, tissue fluid and gland contents in various 

 regions of the body. Only since a technique has been devised whereby whole 

 Epidermis freed from dermis can be obtained in a condition suitable for analysis, 

 not having been exposed to any fluids, has progress been possible. 



Results of direct chemical analysis of any tissue may be misleading unless 

 interpreted in terms of its structural make up and of what has happened to it 

 since it existed in vivo. Among the experimental errors to be guarded against 

 are variability in sacrificing the animal, or the manner of death of the patient, 

 in excision of tissue allowing more or less blood and other fluids to drain out or 

 evaporate, in time and in temperature, in age, sex, and in conditions before 

 death. 



The extracellular and intracellular fluids or phases, are large in volume, w^hen 

 all are taken together, but difficult to get at directly. To obtain data "the 

 deducive histochemical method" is suggested. This is described by Lowry, 

 0. H. and Hastings, A. B. in Cowdry's Problems of Ageing, 1942, 728-755. 



Those wishing to analyse extremely small volumes of fluid which by contrast 

 can be collected for direct determinations cannot do better than to familarize 

 themselves with the techniques elaborated by A. N. Richards and his associates 

 at the University of Pennsylvania for the study of glomerular urine. 



By the useful technique of Microincineration minerals which are not volatilized 

 at high temperature can be directly studied in the tissues in the positions which 

 they previously occupied in living organisms. They appear as shining particles 

 when viewed by the Dark Field Microscope. Microincineration is truly a 

 microchemical method for the localization of structure which is microscopic 

 in its fineness. 



Quite a number of Microchemical Reactions capable of demonstrating the 

 precise location in the cells of minerals, fats, carbohydrates and proteins are 

 available. 



