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 th 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 solve the question regarding the internal structure of the human heart and identify the incorrect statements, we will 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 due to the presence of pacemaker cells, it has chambers (two atria and two ventricles), and it is a muscular organ. The term "myogenic" indicates that the heart muscle generates its own contractions without needing external nerve impulses.
- **Conclusion**: Correct.

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 it is located in the thoracic cavity, positioned symmetrically between the two lungs.
- **Conclusion**: Correct.

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.
- **Conclusion**: Incorrect.

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 sino-atrial (SA) node is known as the natural pacemaker of the heart and is capable of generating action potentials autonomously, which leads to heart contractions.
- **Conclusion**: Correct.

### Summary of Findings:
- **Correct Statements**: I, II, IV
- **Incorrect Statement**: III

### Final Answer:
Only **one statement** (Statement III) is incorrect.

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