With possible exception of the atomic clock, the heart may be the most perfect machine ever devised. How it develops from a simple embryonic tube is a fascinating story of biology and lends a great deal of insight into the source of heart defects that affect children and adults alike. Central to this entire lecture is the fact that the fetus resides in an aquatic environment. Oxygenated blood arrives from the placenta and deoxygenated returns to the placenta. The lungs play no role in delivering oxygen or removing carbon dioxide to or from the circulation. Thus, the fetus mainly (but not...

Histologically, muscle is conveniently divided into two groups, striated and nonstriated, based on whether the cells exhibit cross-striations in the light microscope (Figure 3). Smooth muscle is involuntary: its contraction is controlled by the autonomic nervous system. Striated muscle includes both cardiac (involuntary) and skeletal (voluntary). The former is innervated by visceral efferent fibers of the autonomic nervous system, whereas the latter is innervated by somatic efferent fibers, most of which have their cell bodies in the ventral, motor horn of the spinal cord. Smooth muscle is...