Abstract
Sensory terminations of aortic nerve acting reducingly on blood pressure are in the fetal life dispersed over a large area centered on the lower edge of arcus aortae i.e. over the entire of arcus aortae, aorta ascendens, the emergent parts of truncus brachiocephalicus and a. carotis comm. sin. as well as a. pulmonalis. Their development, however, is more remarkable in the earlier half of the fetal life, coming into degenerative change in the later stage, especially in a. pulmonalis.
Sensory terminations of aortic nerve found in arcus aortae and aorta ascendens, widely unlike their counterpart in adult, are formed along the smooth muscle fibres of the media and of uncomparably simpler formation. But in quantity, they surpass by far such terminations in adult. Consequently, these cannot be looked upon as corresponding to the Type I terminations in adult.
As almost no blood vessel is found penetrating the media, no sensory terminations corresponding to the Type II terminations in adult are formed in fetal stage. They are likely to be formed gradually after birth.
Sensory terminations found in the incipient parts of truncus brachio-cephalicus and a. carotis comm. sin. of human embryo pass from the externa into the media and sometimes even the intima and their terminal formation is similar to those seen in aorta. Consequently, they must undoubtedly have their origin in aortic nerve.
In the pulmonary artery also is observed the existence of sensory terminations of similar conformation. They are discovered in a large quantity in 4th month embryo, but in 8th month embryo they are found degenerated to a large extent and remaining as a few simple branched terminations in the externa.
The above described terminations of aortic nerve in embryo are believed to change over into the Type I and Type II terminations after birth. The process of transition, I presume, consists in that the terminations in the media so gradually degenerate and disappear, that the majority of their stem fibres retrocedes to be only limited to the externa of the lower edge of arcus aortae, and then the tips of stem fibres newly develop peculiar terminations coming under the Types I and II.
The reason why the sensory fibres of aortic nerve extended over a large area of major arteries terminate in the media thereof is thought to lie in the fact that the circulation activity in the embryo has to keep up incessant and extremely flexible adaptation to the fluctuation in the flow of blood in the maternal arteries.
