U.S. patent number 3,841,305 [Application Number 05/300,571] was granted by the patent office on 1974-10-15 for external inductive neural stimulator system.
This patent grant is currently assigned to Iowa State University Research Foundation Inc.. Invention is credited to Richard C. Hallgren.
United States Patent |
3,841,305 |
Hallgren |
October 15, 1974 |
EXTERNAL INDUCTIVE NEURAL STIMULATOR SYSTEM
Abstract
A system for external stimulation of a nerve includes a coil of
wire with a flux-concentrating core in the lumen of the coil. The
core preferably has a T-shape, with the base of the T extending
through the lumen of the coil providing the area of stimulation.
The coil is pulsed by a discharging capacitor and circuitry is
disclosed for charging the capacitor and generating discharge
pulses of alternate polarity.
Inventors: |
Hallgren; Richard C. (Ames,
IA) |
Assignee: |
Iowa State University Research
Foundation Inc. (Ames, IA)
|
Family
ID: |
23159661 |
Appl.
No.: |
05/300,571 |
Filed: |
October 25, 1972 |
Current U.S.
Class: |
600/13;
219/770 |
Current CPC
Class: |
A61N
1/40 (20130101) |
Current International
Class: |
A61N
1/40 (20060101); A61n 001/42 () |
Field of
Search: |
;128/1-5,2.1R,419E,419R,421,423,424,404 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Maass et al., "IEEE Transactions on Magnetics," Vol. Mag-6, No. 2,
June, 1970, pp. 322-326..
|
Primary Examiner: Kamm; William E.
Attorney, Agent or Firm: Dawson, Tilton, Fallon &
Lungmus
Claims
I claim:
1. A system for external tetanic stimulation of nerves comprising:
a coil of conductive wire wound into a generally flat, circular
shape providing a central lumen; magnetic core means extending
through said lumen for concentrating the flux lines in the core
extending through the lumen and providing an application surface
for contacting an external body surface to apply electromagnetic
energy, said application surface extending transverse of said core
means to transversely cut the flux lines concentrated therein; and
circuit means including a source of electrical energy, a discharge
capacitor, first control switch means for selectively coupling said
capacitor to said coil, timing circuit means for periodically
triggering said first switch means to discharge said capacitor
means through said coil, and detector means responsive to the
charge on said capacitor and including second control switch means
for charging said capacitor at a time when said first switch means
is not discharging the same and for electrically isolating said
capacitor from said source means when said first switch means is
discharging said capacitor.
2. The system of claim 1 wherein said core means comprises a
plurality of T-shaped laminations stacked together and providing a
base and a cross bar, the bases of said T-plates extending through
the lumen of said coil, the collective bottoms of said legs of said
T-plates providing said application surface extending perpendicular
to the axis of the base of said T-plates.
3. In a system for the electromagnetic stimulation of nerves, the
combination comprising a conductive coil of wire providing a
central lumen; core means extending through said lumen for
concentrating the magnetic flux generated by said coil; a discharge
capacitor; first switch means for selectively connecting said
capacitor to said coil; a source of periodically alternating
voltage; second switch means for selectively connecting said source
to charge said capacitor; polarity detector circuit means
responsive to the polarity of voltage on said capacitor for
selectively energizing said second switch means to charge said
capacitor to a peak voltage having the same polarity as the
existing voltage on said capacitor; and pulse control circuit means
for selectively energizing said first switch means to discharge
said capacitor through said coil to generate a stimulating pulse in
mutually exclusive time relation with said polarity detector means,
whereby said capacitor is isolated from said source when said
capacitor is discharged.
4. The system of claim 3 wherein said first switch means includes
first and second control switch means connected between said
capacitor and said coil for coupling respectively current of
forward and reverse polarity to said coil; and wherein said pulse
control circuit means includes logic counter circuit means
responsive to said source voltage for generating a control signal
every predetermined number of half cycles thereof; monostable
circuit means responsive to said control signal for generating an
output pulse when said capacitor is isolated from its associated
charging voltage; and means for coupling said output pulse of said
monostable circuit to said first and second control switch means to
cause the same to conduct, said control signal of said logic
circuit means occurring approximately at a time when said
alternating voltage is experiencing a zero crossover and the
discharge current through said coil ringing only in one polarity,
thereby leaving a residual voltage on said capacitor of polarity
opposite to the original voltage for sensing by said polarity
detector circuit means.
5. The system of claim 3 wherein said second switch means includes
third and fourth control switches; and wherein said polarity
detector circuit means comprises first and second channel circuit
means for generating a charge control signal respectively during
alternate half cycles of said source voltage; and means for
coupling the output signals of each of said channel circuit means
respectively to said third and fourth control switch means, said
third and fourth control switch means being connected in circuit
between said source and said capacitor for coupling respectively
current of forward and reverse polarity to said capacitor.
6. The system of claim 5 wherein each of said channels of said
polarity detector circuit means includes means for generating a
unipolar signal, optical isolation means responsive to said
unipolar signal for generating a second electrical signal
proportional thereto; and circuit means including threshold circuit
means coupled to said capacitor for generating an output signal as
long as the voltage on said capacitor is above a predetermined
value, said output signal occurring exclusive of times immediately
adjacent zero crossover voltages thereof.
Description
BACKGROUND AND SUMMARY
The present invention relates to a system for stimulating nerves.
In the eighteenth century, Galvani demonstrated that he could make
a frog's leg twitch by exposing the nerve and contacting it with
two electrodes charged at different potentials, thereby causing a
current to flow in the nerve. Since that time, the stimulation of
nerves electrically has become a useful therapeutic and diagnostic
tool in medicine. However, most of the stimulation has been
effected by the technique first demonstrated by Galvani -- using
metal electrodes to physically contact the nerve, either by
exposing the nerve or piercing the skin with needle electrodes.
The presence of a metallic electrode presents a number of serious
problems to a living system. Over a period of time, the electrode
deteriorates, and the resulting metallic salt may poison the
system. Trauma is caused both by the surgical procedure required to
expose the site of stimulation and the introduction of the
stimulating electrodes. Improper attachment of electrodes can cause
constriction and atrophy of a neural trunk.
Further, if the nerve is to be exposed, there is need for a surgeon
to expose the nerve. Even if needle electrodes are inserted, so as
to avoid the danger of infection, chemical action occurs at the
interface between the body fluids and the exposed metallic surface
of the electrodes. Other undesirable effects produced by the use of
metallic electrodes contacting the nerves or other body tissue
include an uncomfortable sensation of electric shock and pain
during application. Plate electrodes must be properly attached to
the tissue in order to provide good electrical continuity if areas
of high current density and resulting skin burns are to be
avoided.
Work has also been conducted on an inductive transducer for
stimulating nerves, as reported by Maass and Asa in a paper
entitled "Contactless Nerve Stimulation and Signal Detection by
Inductive Transducer," IEEE Transactions on Magnetics, Vol. Mag-6,
No. 2, June, 1970, p. 322.
The present invention provides a coil of wire which is excited by a
discharging capacitor. In the lumen of the coil there is a
laminated core, preferably formed from plates having a T-shape. The
base of the T is placed through the center of the coil, and the
bottom of the base of the T is placed in proximity to the nerve
desired to be stimulated.
The present invention uses inductive stimulation of a nerve wherein
the stimulating voltage is applied to the nervous structure by
means of an induced electric field. This completely eliminates any
metallic contact between the nervous structure and the stimulating
device and it is, therefore, advantageous in use as compared with a
metallic electrode system. The induced electric field may be
generated by discharging a capacitor through the coil that
surrounds the base of the T-plate laminated core.
I have developed a circuit which discharges a capacitor through the
stimulation coil. This circuit has been found to operate well to
discharge capacitors of relatively high value and it is thus highly
advantageous in the practice of the invention.
Other features and advantages of the present invention will be
apparent to persons skilled in the art from the following detailed
description of a preferred embodiment accompanied by the attached
drawing.
THE DRAWING
FIG. 1 is a perspective view of a neural stimulating device
constructed according to the present invention with a portion of
the wire coil cut away;
FIG. 2 is an idealized graph illustrating the voltage waveform of
the 60 cycle sinusoidal input voltage and the voltage across the
discharge capacitor;
FIG. 3 is a circuit schematic diagram, partly in functional block
form, of a preferred system for energizing the stimulator of FIG.
1;
FIG. 4 is a circuit schematic diagram partly in functional block
form, of the capacity polarity detector of FIG. 3;
FIG. 5 is a circuit schematic diagram, partly in functional block
form, of the pulse control circuit of FIG. 3; and
FIG. 6 is a timing diagram for explaining the operation of the
circuitry of FIG. 5.
DETAILED DESCRIPTION
By way of general description, the neural stimulator of the present
invention applies a stimulating current to a nerve structure by
means of an induced electric field produced by a time-varying
magnetic field. The device is external to the body, and need be
placed only in proximity to the nerve that is to be stimulated. To
obtain the magnetic field, a capacitor is discharged through a coil
which is provided with a laminated core having a branch which
extends through the lumen in the coil.
Neither exposure of the nerve to be stimulated nor contact between
the stimulator and the nerve is required for proper operation.
The unit has been in operation and has provided consistent
stimulation of neural pathways in both humans and dogs, with no
apparent ill effects. Stimulation of a human test subject has
resulted in excitation of phrenic, ulnar and femoral nerve trunks
and numerous nerves in face, neck shoulders, arms and hands. Use of
the instant device has resulted in no sensation of pain or electric
shock to the subject even though no anesthesia is applied.
Referring now to FIG. 1, a neural stimulating device or stimulator
is generally designated by reference numeral 10, and it includes a
coil 11 (one quarter of which is broken away for clarity) and a
laminated magnetic core 12.
The coil 11 is formed into a generally toroidal shape from a
continuous piece of wire. In one embodiment, the coil 11 included
twenty turns of No. 16 enameled copper wire, and it had a
self-inductance of 16.4 microhenrys. The coil had an internal
diameter of 2.00 cm. and an outside diameter of 4.00 cm. The ends
of the coil 11 are connected to first and second insulated lead
wires 13 and 14 which conduct the stimulating current pulse to the
coil 11; and the coil may be coated with an epoxy coating 15 for
insulation purposes.
The magnetic core 12 preferably takes the form of a plurality of
T-shaped plates, each plate including a base 12a, and an upper
cross bar 12b. The width of the base 12a may be approximately twice
the width of the cross bar 12b because magnetic flux paths are
generated from both lateral ends of the cross bar 12b, and the flux
returns through the base 12a. With the addition of the magnetic
core 12, wherein each plate was similarly dimensioned to have an
overall length of the cross bar of approximately 2 inches and a
height of approximately 1 inch, the self-inductance of the
stimulator coil increased to 24.5 microhenrys. It will be observed
that the bases 12a of each of the lamination plates, when they are
stacked, is placed into the lumen 16 of the coil 11. The function
of the metallic core 12 is to concentrate the magnetic flux. For
stimulation the coil and the laminated core are placed such that
the nerve 18 is located in a plane perpendicular to the axis of the
base 12a of the core, such as is diagrammatically illustrated by
the nerve.
The magnetic flux generated by current flowing in the coil 11
emanates from the bottom 17 of the base 12a of the T-plate
laminations, and it spreads out in planes which extend radially of
the axis of coil 11. The flux then re-enters the core at the outer
ends of the cross bar 12, which ends are collectibly designated by
reference numeral 19. The excitation current, as will be discussed
in greater detail presently, generates pulses which alternately
change polarity; and these pulses occur in a periodic manner.
Hence, the resulting magnetic flux changes in direction
periodically.
Turning now to FIG. 3, reference numeral 20 denotes two inputs to
which a conventional 60-cycle 110 volt alternating current source
is connected. The input terminals 20 are connected to the primary
winding of a transformer 21 and to the primary winding of a
step-down transformer 22. The secondary winding of the transformer
22 is connected to the input of a pulse control circuit 23 which is
responsive to the timing of the alternating input voltage to
generate trigger pulses, as discussed in more detail in connection
with FIGS. 5 and 6. One such pulse is transmitted to the terminals
C, C' and the other is transmitted to the terminals D, D'.
The trigger pulses of the pulse control circuit 23 control the
firing of two silicon control rectifiers (SRC) designated
respectively by reference numerals 24 and 25 in the upper
right-hand corner of FIG. 3. The anode of the SCR 24 and the
cathode of SCR 25 are connected together and to one terminal of the
coil 11. The cathode of SCR 24 is connected to the anode of SCR 25
and directly to one terminal of a storage capacitor 27. The other
terminal of capacitor 27 is connected in common to the other
terminal of the coil 11 and to one terminal of the secondary
winding of transformer 21.
Connected across the capacitor 27 is a capacitor polarity detector
functionally shown in the block designated 28 (and seen in more
detail in FIG. 4) which senses the polarity of the charge stored on
the capacitor 27, and depending upon the sensed polarity, generates
an output signal on either the output lines A, A' (if the polarity
is positive according to the convention marked) or on the lines B,
B' if the polarity on the capacitor 27 is negative relative to the
convention indicated.
The other terminal of the secondary winding of transformer 21 is
connected directly to the cathode of a third silicon control
rectifier 30 and to the anode of a fourth silicon control rectifier
31. The SCR 30 is triggered by a positive signal on the lines B, B'
from the capacity polarity detector 28; and the SCR 31 is triggered
by a positive signal generated on the lines A, A' of the polarity
detector 28.
Turning now to FIG. 4, there is seen in more detail the circuitry
of the capacity polarity detector 28. In this drawing, the
discharge capacitor is again shown at 27. The capacity polarity
detector is divided into two channels--one being generally
designated by reference numeral 38, and the other being generally
designated by reference numeral 39. These channels are similar, and
their function is to generate output signals which are the logical
inverses of each other.
The channel 38 includes a diode 40 having its anode connected to
one terminal of discharge capacitor 27, a resistor 41 in series
with the diode 40, and an optical isolator generally designated 42.
The optical isolator 42 comprises a neon light 43 and a
light-sensitive resistor 44 packaged together, as illustrated by
the dashed line 45.
The resistor 44 decreases in value in accordance with an increase
in the intensity of the light of the source 43 to provide very high
electrical isolation. Other optical isolators such as a solid state
device including a light-emitting diode and light-sensitive solid
state detector may be used with like results. The resistor 44 is
connected in series with a source of D.C. voltage 46, and the
resulting signal is fed to a differentiator circuit 47, the output
of which is fed to a pulse shaper circuit 48, which is a high gain
conventional limiting amplifier. The output of the pulse shaper
circuit 48 controls a power supply 49, the output terminals of
which are designated A, A' which are the same as the
correspondingly designated output lines and control lines in FIG.
3.
Similarly, the channel 39 includes an input diode 50 having its
cathode connected to the anode of the diode 40, and in series with
a resistor 51 and a second isolating optical element generally
designated 52. The resistor 51 is in series with a neon light
source 53 of the device 52; and the photo-sensitive resistance 54
is connected in series with a battery 55, the circuit feeding a
differentiator circuit 56. The output of the differentiator circuit
56 feeds a pulse shaper circuit 57 which, in turn, controls a power
supply circuit 58, the output of which supplies power to the lines
B, B'.
Turning now to FIG. 5, there is shown a functional block diagram
for the pulse control circuit 23. It includes a 60 Hz source
generally designated by reference numeral 61 from which it derives
both power and timing. The source 61 may include the previously
described transformer 22. The 60 Hz source feeds a full wave
rectifier circuit 62 which in turn feeds a Schmitt trigger circuit
63. The Schmitt trigger circuit 63 is of conventional design, and
it is a low-threshold circuit for forming square wave pulses from
the half sine wave output signals of the full wave rectifier 62,
such as are seen on line 1 of FIG. 6 and designated 64. The pulses
64, however, are illustrated with somewhat exaggerated separation
for clarity of illustration.
Returning to FIG. 5, the output of the Schmitt trigger 63 feeds a
digital counter circuit 64 which may be conventional flip-flop
circuits arranged to serially count the input pulses. In the
present case, the digital counter circuit contains two flip-flops
and is capable of counting up to four input pulses. The "1" outputs
of the two flip-flop circuits in the digital counter circuit 64a
are fed to the inputs of an AND gate 65, the output of which is fed
back along a line 66 to reset the digital counter circuit. Hence,
the combination of the digital counter circuit 64a, AND gate 65 and
reset line 66 is to provide a "count three" circuit since after the
outputs of each of the flip-flops have gone to a "1," the digital
counter circuit is reset. This is illustrated in lines 2 and 3 of
FIG. 6 which are the outputs respectively of the two flip-flop
circuits in the digital counter 64a, line 2 illustrating the output
of the lowest order flip-flop and line 3 indicating the line of the
higher order flip-flop. The flip-flops trigger on a negative-going
pulse. Hence, the first flip-flop is set by the trailing edge of
the first of the pulses 64, and the output of this first flip-flop
is illustrated by the pulse 67. The first flip-flop is reset at the
end of the second occuring pulse 64, and as the first flip-flop
resets, the second flip-flop sets so as to produce pulse 68. At the
end of the third pulse 64, the first flip-flop again sets, but the
combination of a set output at both of the flip-flops causes the
AND gate 65 to reset the counter 64a. The cycle is illustrated a
second time for this same operation.
The output of the AND gate 65 also feeds a monostable circuit 70
having an output pulse lasting for a predetermined time interval.
This output pulse energizes a power amplifier 71 which, in turn,
energizes first and second pulse transformers 72 and 73. The
outputs of the pulse transformers 72, 73 may be amplified, if
desired. The resulting signals energize respectively the line pairs
C, C' and D, D', as illustrated, which are connected, as seen in
FIG. 3, to trigger the SCRs 24, 25. The SCRs 24, 25 are connected
in opposing polarity between the capacitor 27 and coil 11 so that
the charge on the capacitor will flow through the coil irrespective
of its polarity.
OPERATION
Before describing the overall system operation, the operation of
the capacity polarity detector 28 as seen in FIG. 4 will be
described first. When the voltage on the discharge capacitor 27
goes positive, the diode 40 is forward biased. When the voltage
across neon lamp 43 reaches a sufficient value, the lamp will
conduct and emit light, thereby reducing the resistance of the
photo-sensitive resistor 44. Current will then flow to the input of
the differentiator circuit 47 from the battery 46. The
differentiator circuit 47 causes the resulting signal to have a
sharper rise time, and the pulse shaper circuit 48, causes the
resulting signal to have faster rise and fall times to trigger the
power supply 49. The lamp 43 does not conduct unitl the voltage
across it reaches 70-80 volts; hence this threshold provides a
delay between the time the input waveform crosses zero volts and
the time that the capacitor 27 is charged. It is during this delay
that the capacitor 27 is discharged. Similarly, the channel 39
produces power at the terminals B, B' when the voltage on the
discharge capacitor reverses polarity.
Turning now to the pulse control circuit 23, as seen in FIG. 5, the
full wave rectifier circuit 62 produces a half sine wave for each
half cycle of the periodic input source 61. The Schmitt trigger
circuit 63 forms these half sine waves, which are positive, into
square waves such as those seen on line 1 of FIG. 6; and these are
fed to the digital counter circuit 64a. This circuit, as already
mentioned, in combination with the AND gate 65 results in a "count
three" circuit which energizes the monostable circuit at the
termination of every third pulse. It will be appreciated that the
input sine wave from the 60 cycle source crosses zero at a time
inbetween adjacent ones of the pulses 64. Hence, the resulting
signal from the monostable circuit 70 occurs at the end of every
third half cycle of the input periodic waveform and at a time when
it is crossing zero voltage, or shortly thereafter. This has the
effect of isolating the charging of the capacitor 27 from the
discharging thereof into the stimulating coil 11 on a time basis,
as explained above. The power amplifier 71 amplifies the output
signal of the monostable circuit 70 to energize the pulse
transformers 72, 73 for the short duration during which the
monostable circuit 70 generates a pulse. The width of this pulse is
narrow, as explained more fully below, so that the capacitor 27 is
not discharged at the same time that it is being charged to avoid
shorting of the charging source.
Turning now to FIG. 2, the continuous 60-cycle sinusoidal input
voltage is designated by reference numeral 35; and it is shown on
the same time scale as the voltage, V.sub.c which is the voltage
across the discharge capacitor 27. The capacitor 27 is a
non-polarized capacitor, and the inductance 11 is a schematic
showing of the inductance of the stimulating transducer 10. The
function of the pulse control circuit 23 is to trigger the SCRs 24,
25 to thereby discharge the capacitor 27 at every third zero
crossover of the periodic input voltage 35. Because this occurs at
odd half cycles, the subsequent polarities for charging the
capacitor 27 will alternate.
The SCRs 30, 31 are connected in circuit with the input transformer
21 in opposing polarity; and the function of these two switches is
to selectively charge the capacitor 27 in response to the output
signals of the polarity detector 28.
The following description of the operation of the circuit of FIG. 3
will assume a pulse rate of 45 pulses per second -- that is, one
pulse for every third half cycle of the sinusoidal waveform 35.
Assuming that at time t = t.sub.0, the polarity of the voltage on
capacitor 27 is positive, the polarity detector 28 generates a
voltage at the terminals A, A', as already discussed, to cause SCR
31 to conduct and thereby charge the capacitor 27 positively along
the portion 36a of the capacitor voltage curve 36, as seen in FIG.
2. These voltage waveforms are idealized, as has already been
mentioned. The conduction of the charging SCRs 30, 31 occurs at a
time after the discharging SCRs 24, 25 have ceased conducting.
During the next positive half cycle of the voltage V.sub.L, namely
at the time t.sub.3 of FIG. 2, the polarity detector 28 again
causes the SCR 31 to conduct to charge the capacitor 27 along the
portion 36b of the curve 36 to a higher positive potential. It will
be observed that this occurs prior to the time of the termination
of the third count of the "count three" circuitry in the pulse
control circuit 23. Hence, at the next zero crossover of the
60-cycle voltage V.sub.L, namely at time t.sub.4 in FIG. 2, the
pulse control circuit 23 generates an output pulse at the terminals
C, C' and D, D' thereby causing SCR 25 to conduct and discharging
capacitor 27 through the stimulator coil 11. During this conduction
time, the input or charging SCRs 30, 31 are non-conducting; and the
capacitor 27 is thereby isolated from the input voltage.
It will be observed that during discharge, the capacitor 27 forms a
series RLC circuit including the inductance of coil 11. Hence, as
seen in FIG. 2, the voltage across the discharge capacitor will
actually go negative due to the energy stored in the inductance as
the capacitor discharges. The time duration of the pulse which
causes SCRs 24, 25 to conduct is shorter than the time required for
the voltage on the discharge capacitor to reach the maximum value
at the reverse polarity. Discharge of capacitor 27 ceases when SCR
25 becomes reverse-biased due to the inductor-capacitor elements in
the circuit which cause a ringing effect in the current. This
results in leaving a charge on the capacitor of a polarity inverse
to that which it carried prior to discharge; and this residual
charge then is sensed by the capacitor polarity detector and
enhanced. That is, during the next negative half cycle of the input
line voltage, the capacitor 27 is charged still further negatively
according to the portion 36c of the curve 36 of FIG. 2. This, of
course, is caused by the circuitry in channel 39 of the capacity
polarity detector 28, as seen in FIG. 4, and the resulting output
voltage at the terminals B, B' which causes the switch 30 (FIG. 3)
to conduct, thereby connecting the capacitor 27 in series with the
secondary of the input transformer 21.
During the next negative half cycle of the line voltage, the
capacitor 27 is charged still further negatively according to the
portion 36d of the waveform 36; and at a subsequent zero crossover
(namely, time t.sub.9), the pulse control circuit 23 generates a
pulse along the output lines C, C' and D, D' to discharge the
negatively-charged capacitor 27 through SCR 24 and the stimulator
coil 11.
The sequence of operation continues in the manner described,
wherein the capacitor 27 is charged and discharged at mutually
exclusive times and in alternate polarity at the rate of 45 cycles
per second. This design overcomes a principal problem in charging
and discharging a large capacitor (the capacitor 27 may be of the
order of 500 microfarads) and charged to 150 volts at rates as high
as 45 times per second. The present invention has been able to
charge and discharge a capacitor to values as high as 300 volts,
and with straightforward changes in the count logic, it will be
appreciated that the repetition rate for pulsing the stimulation
coil 11 may be changed without changing the frequency of the
primary 60-cycle source. Alternatively, although the system may
become somewhat more complicated, a variable frequency input source
could be used in place of the 60-cycle source as disclosed.
With the described system, successful tetanic stimulation of the
ulnar, femoral and phrenic nerve trunks and numerous small fibers
and trunks in the face, neck, hands, shoulders, etc., has been
accomplished on humans without any accompanying sensation of pain
or shock and with no apparent harmful physical effects.
Having thus described in detail a preferred embodiment of the
present invention, persons skilled in the art will be able to
modify certain of the structure which has been described and to
substitute equivalent elements for those disclosed while continuing
to practice the principle of the invention; and it is, therefore,
intended that all such modifications and substitutions be covered
as they are embraced within the spirit and scope of the appended
claims.
* * * * *