The Idea of an Essay, Volume 2

95 main function of the jugular vein valves is to prevent regurgitation of blood from the inferior vena cava and right atrium into the jugular vein and the number of valves [was concluded] to be fixed in utero” (p. 1). Without the presence of these valves, the blood would flow into the jugular vein, which is a vein that carries deoxygenated blood from the brain back to the heart, and cause the blood to re-enter the giraffe’s head while it is lowered. This must be prevented. In order to keep the blood from flowing the opposite direction through the jugular vein and to maintain the cardiac output (the amount of blood pumped throughout the giraffe’s body), the jugular valves must be fully functional (Mitchell, Van Sittert & Skinner, 2009). These jugular valves would have to evolve simultaneously with the long neck in order for the giraffe to live. According to Mitchell, Van Sitter & Skinner (2009), “… [the valves’] effectiveness in preventing regurgitation will depend on their capacity to withstand the hydrostatic pressure generated by venous return and the column of blood in the jugular veins…and position” (p. 179). In other words the valves must be strong and well-developed in order to withstand the pressure of the blood. If the valves are evolving over time in the neck as it elongates, then the valves which are only partially developed will not be strong enough to hold the blood and the giraffe would die. The effectiveness of the valves also depends on position (near or away from the heart). If the giraffe with the intermediate- length neck only developed the valves closer to the head instead of the valves closer to the heart, then the valves would not do their job, and the giraffe would die. The jugular valves are also seen in the giraffe during gestation. In dissections preformed on fetal giraffes, according to research done by Mitchell, Van Sittert & Skinner (2009), “the number of valves [per unit length] in each segment of the fetuses was the same as in adults, and so the number appears to be fixed in utero” (p. 180). The giraffe is born with this feature because, if absent, the baby giraffe would die at birth. The fact that the baby giraffe is born with this feature shows the importance of the feature for its survival. The giraffes that would have been born with only half-evolved systems would have died immediately after birth. The complexity and coherence of these mechanisms, which must be in place in order for the long-necked giraffe to live, point to a design that was present from the beginning of the giraffe’s existence.