What does the following lines mean from Macbeth Act V, Scene III?
Macbeth: Throw physic to the dos, I'll non of it.
Come, put mine armor on. Give me my staff.
Seyton, send out. ---Doctor, the thanes fly from me.---
Come, sir, dispatch. If thou couldst, doctor, cast
The water of my land, find her disease
And purge it to a sound and pristine health,
I would applaud thee to the very echo,
That should applaud again. --Pull't off, I say--
What rhubarb, senna, or what purgative drug,
Would scour these English hence? Hear'st thou of them?
The first line refers to his inability to convince the doctor to cure his crazy wife with medicine. He says she needs a priest for that.
The second line shows Macbeth's readiness, finally, to prepare for battle. This is significant because he has believed so much in the thanes prophecies that he has NOT been preparing when he should have.
The third line reveals that many of his men are deserting him, probably because he keeps babbling about the trees moving and nobody being borne of woman.
The remaining lines are his plea to the doctor to find some herb or magic potion to cure his wife. He has completely neglected medicine and science and is grasping at straws to have his wife's mental breakdown cured.
In this opening line, Macbeth tells the doctor to "throw physic to the dogs" because the doctor has admitted that he can do nothing for Lady Macbeth's madness. In fact, he recommends that Lady Macbeth examine her conscience as a means of bringing about a cure (he senses that her visions are caused by guilt). By uttering these words, Macbeth expresses his scorn at such an opinion and states that no such thing will happen ("I'll none of it").
As Macbeth prepares for battle, he urges the doctor to find out what his wrong with his country. He asks the doctor to examine the "water," by which he means urine, as a means of diagnosing its problems, just like a patient. Note how Shakespeare uses personification to liken the country to a sick woman. He also asks for some medicine to "purge" the country of English invaders and he references some contemporary examples, like rhubarb and senna. Both of these items were used by medieval doctors to relieve a sick patient.
Knowing that the doctor cannot cure a sick country, Macbeth continues his preparations for battle. Notice the irony here: Macbeth's despotic rule is the cause of his country's problems but he cannot see this truth since he is blinded by his own ambition.