In the present study of Respiratory and cardio-
vascular reflexes of the lungs attention has been
particularly directed to the afferent mechanisms
involved. This was considered necessary since there
is little definite information about the nature of
these fibres although much is now known about the
jpossible reflexes in which they may participate.
with this object a first attack on the problem was
made as described in the first part of this thesis
by the determination of the conduction velocities of
the known vagal thoracic afferents. It is hoped
that information so gained will be of value in the
interpretation of results of previous investigators.
The second part of the thesis presents the
results of investigation into the afferent fibre
composition of the vagal rootlets as studied by the
electrical recording of nerve impulses. This was
undertaken because it was hoped that a functional
differentiation of the vagal afferents might occur
at this level as suggested by the work Beer & Kreidl
(1895) and Cadman (I9OO), and thereby provide a means
of selective stimulation. This was found not to be
the case and further investigation in that field
was therefore discontinued.
From the work of Dawes, Matt, and Widdicombe
(I951) it appeared that certain araidines such as
phenyl diguanide are capable of producing a reflex
respiratory inhibition which is blocked by cooling
the vagi to 31°C. Because of the low blocking
temperature it seems possible that this drug affords
the means of selective stimulation of small afferents
of trie vagus and as will be shown in the 3rd section,
this is certainly true. Further in order to test
the hypothesis of Dawes et al that the pulmonary
vascular afferents described by Whitteridge (1948)
were probably concerned in the above reflex, the
action of phenyl diguanide on these and other
afferents has been studied. Evidence will be
presented to indicate that none of the known vagal
afferents including the pulmonary vascular ones are
sensitized or stimulated by phenyl diguanide
