Theory:

7. I didn’t jump. I didn’t tremble. I didn’t cry out. There was no time to do any such thing. The snake slithered along my shoulder and coiled around my left arm above the elbow. The hood was spread out and its head was hardly three or four inches from my face!

It would not be correct to say merely that I sat there holding my breath. I was turned to stone. But my mind was very active. The door opened into darkness. The room was surrounded by darkness. In the light of the lamp I sat there like a stone image in the flesh.
Explanation:

The doctor was shocked, but he couldn't do anything. His careless attitude brought a threat to his life. Generally, if a snake is around a person, behaving calm and still would help them save themselves, as there are chances that it would leave the place without biting. But, it would attack someone if provoked. So, the doctor remained calm.

Cobra is the world's largest venomous snake and is considered as one of the most intelligent ones. Its bite delivers a tremendous amount of paralysis-inducing neurotoxins. And, the snake's venom is so voluminous and dangerous that it is enough to kill an elephant.

The doctor did not react, jump or tremble, and he did not scream or shiver, as he felt it would lead to his death. Moreover, the snake's encounter with the doctor happened so fast that he did not have any time to react. The snake slithered up the doctor's shoulder and wrapped itself around his left arm just above the elbow. Its head was very close to the doctor, spreading out its hood, hardly $$3-4$$ inches from the doctor's face. One can easily imagine what a dreadful situation the doctor was in, as the presence of a cobra near anyone would definitely make a person's blood freeze.