Human Anatomy, Part 1 - K. A. Dyubenko, A. K. Kolomiyytsev, Yu. B. Chaykovsky 2002

Special Part
Splanchnology, splanchnologia [the study of viscera] — Development of the digestive and respiratory systems
Development and transformation of the embryonic branchial apparatus

Although mammals evolved from Water to land, a branchial (gill) apparatus is established during their Embryogenesis. In mammals, some PARTS OF THE branchial apparatus regress, while others give rise to Organs and Tissues not directly related to gill Respiration. Thus, The formation of the branchial apparatus is a manifestation of the biogenetic law, which makes it possible to distinguish the branchial region of the embryo at certain stages of embryonic development. Here, on both sides of the HEAD end of the embryo, caudal to the primary oral opening, four pairs of protrusions (branchial arches) are formed, each including a mesenchymal core, a cranial nerve, a Blood vessel (aortic arch), and the primordium of Skeletal Tissues.

Externally, the branchial arches are separated from each other by ectodermal depressions—branchial grooves or clefts—and internally by evaginations of the pharyngeal gut lining—pharyngeal pouches. The latter grow toward the branchial clefts and come into contact with the ectoderm, forming, together with it and sometimes a small layer of mesenchyme, the so-called branchial membranes. In mammalian embryos, some of these membranes may rupture, resulting in the formation of through branchial clefts. However, they are short-lived and soon close.

The anterior (pharyngeal) gut also belongs to the branchial region. This is the most cranially located section of the intestinal tube, situated posterior to the pharyngeal membrane. Unlike the more caudal parts of the gut, the pharyngeal gut is a flattened tube—compressed anteroposteriorly and expanded laterally.

The first branchial arch, as noted above, limits the entrance to the primary Oral Cavity from below. It forms the upper and lower jaws. Innervation of the arch is provided by the trigeminal (V) nerve, which supplies the masticatory Muscles developing within it.

The second branchial arch (hyoid arch) is so named because it gives rise to certain parts of the Hyoid bone. It also forms the Muscles of facial expression and other muscles innervated by the facial (VII) nerve.

The third branchial arch participates in the Formation of the hyoid bone and a Muscle innervated by the glossopharyngeal (IX) cranial nerve.

The fourth branchial arch, supplied by the vagus (X) nerve, forms A number of cartilages and Muscles of the Larynx and the lower part of the Pharynx.

Regarding the branchial clefts, the first branchial cleft transforms into the external acoustic meatus, while the II–IV clefts are covered by the greatly overgrown second (hyoid) arch, causing the smoothing of the neck contours and their subsequent disappearance.