6 
INTRODUCTION 
tor, but nevertheless the question remained: 
How is the core heat conserved? The answer 
Scholander found was that the veins in the whale 
form a concentric circle around the arteries. The 
arteries run centrally and the veins are confluent 
around them. This is an ideal means for transfer 
of heat from artery to vein, since the gradient ex- 
ists from the core out to the periphery. The flow 
of heat, however, is counter to the flow of 
blood. This observation formed the basis for the 
counter-current theory which has come to play 
an immensely practical role in our understand- 
ing of human renal function and human renal 
disease. 
The marine mammals were apparently ter- 
restrial at one point in their evolution and 
then reverted back to marine existence, but on 
returning to the marine environment they kept 
their standard mammalian pulmonary appara- 
tus. Their adaptation has included an ability 
to dive to 600 feet and to surface without de- 
veloping the bends. This certainly is a model 
worth studying. Some can also remain under 
water for periods of up to 30 or 40 minutes. 
This latter phenomenon cannot be accounted 
for on the basis of oxygen storage in the lungs 
or other places. The secret lies in marine mam- 
mals' ability to regulate blood flow to the muscles 
and peripheral systems. Fundamentally they are 
able to shut off blood flow to the viscera and to 
the muscles while they are underwater and keep 
the flow open to the brain and to the heart. They 
can, therefore, build up a huge oxygen debt and a 
huge lactic acid excess. These are superb mod- 
els, then, for the study of the regional distribu- 
tion of blood flow. 
The comparative analysis of these well- 
deflned animal characteristics can lead to bene- 
fits in areas not specifically related to clinical 
medicine as well. The field of bionics, for ex- 
ample, borrows from the characteristics of ani- 
mal systems to assist in the design of physical 
systems or circuits. The rods and cones of the 
retina, for instance, have been copied as models 
in the design of receptor circuits. The exquisite 
sensitivity of the cochlea can respond to physical 
displacement of a distance less than the diam- 
eter of a hydrogen molecule and this has served 
for models for development of displacement 
transducers. In neither of these cases can engi- 
neers challenge the sensitivity of the biological 
circuit, but these biological models can serve as 
ideals to which engineers can aspire. 
Animal species have also exhibited a number 
of well-developed characteristics with which 
man has only recently become familiar. In a 
sense, man has only caught up with some of the 
lower animals. The infrared heat sensors in a 
snake, the pit viper, can sense the presence of a 
mouse at a distance of about six feet. Presum- 
ably pit vipers have preyed on mice using this 
technique for eons before man ever thought 
about developing heat-seeking missiles. Bats 
used sonar to navigate and to locate their vic- 
tims long before we invented sonar and, con- 
versely, moths learned to jam the bat's radar 
before we learned about jamming devices. The 
list of animals which exhibit physiological or 
pathological characteristics that could be capi- 
talized on is by no means exhausted. It is only 
the imagination of the researchers that has been 
slow in responding. 
A related area to which the role of research 
animals in clinical medicine can be extended is 
the general development of models of disease. 
A direct approach to the study of human disease 
would be the study of man himself, but to a 
large extent this is not possible. 
There is a very practical need to develop 
analogs of human disease. Many such analogs 
occur naturally. They represent pathological 
examples of the "experiments in nature" that I 
mention. Schmidt-Neilsen, for example, has 
shown that the sand rat is naturally diabetic, 
which represents a very interesting model of 
human diabetes. Scrapies is a neurological dis- 
ease in the chimpanzee which bears a striking 
resemblance to some of the demyelinating and 
sclerosing diseases in man. Investigators have 
shown that scrapies is transmissable, a major 
breakthrough in the etiological or epidemiologi- 
cal considerations of neurological diseases. 
Disease models are not necessarily limited 
to mammals. Scarpelli has published a paper 
in which he has surveyed the spontaneous and 
experimental disease processes in lower animals 
and even in invertebrates. He has identified 
analogs of more than two hundred diseases in 
a great variety of lower animals, many rarely 
seen in laboratories. The disease processes 
