Read the following statements about the internal structure of the human heart.
I. Humans have a an auto - excitable , chambered , muscular and myogenic heat.
II. The heart is mesodermal in origin and is present symmetrically between two lungs in the thoracic cavity.
III. The opening between the right atrium and left ventricle is guarded by a tricuspid valve.
IV. Only the sino - atrial node of the heart has hte ability to generate action potentials without any external stimuli.
How many statement/s is/ are incorrect from the above ?

Options

A. Two
B. Three
C. Four
D. One

Answer

To determine how many statements about the internal structure of the human heart are incorrect, let's analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement I**: "Humans have an auto-excitable, chambered, muscular, and myogenic heart."
- **Analysis**: This statement is correct. The human heart is indeed auto-excitable (it can generate its own electrical impulses), it is chambered (consisting of four chambers: two atria and two ventricles), muscular (made up of cardiac muscle), and myogenic (the contraction is initiated by the heart muscle itself).

2. **Statement II**: "The heart is mesodermal in origin and is present symmetrically between two lungs in the thoracic cavity."
- **Analysis**: This statement is also correct. The heart develops from the mesoderm during embryonic development and is located symmetrically between the two lungs in the thoracic cavity.

3. **Statement III**: "The opening between the right atrium and left ventricle is guarded by a tricuspid valve."
- **Analysis**: This statement is incorrect. The opening between the right atrium and right ventricle is guarded by the tricuspid valve, while the opening between the left atrium and left ventricle is guarded by the mitral (or bicuspid) valve.

4. **Statement IV**: "Only the sino-atrial node of the heart has the ability to generate action potentials without any external stimuli."
- **Analysis**: This statement is correct. The sinoatrial (SA) node is known as the natural pacemaker of the heart and is capable of generating action potentials independently.

### Conclusion:
- From the analysis, we find that **only Statement III is incorrect**. Therefore, the number of incorrect statements is **1**.

### Final Answer:
**1 statement is incorrect.**