As discussed in Chapter 18, a number of bars of tissue are found on the anterior surface of the developing embryo, which are referred to as pharyngeal arches. The first pharyngeal arch is the mandibular arch, the second is the hyoid arch, and the remainder are numbered III, IV, and VI (V disappears). Just above the first arch and extending down behind all the arches is a hollow tube, the digestive tract. Before the end of the fourth embryonic week, this tube is closed off at the upper end by the buccopharyngeal membrane, which separates the upper end or foregut from the primitive oral cavity or stomodeum. The epithelium anterior to the buccopharyngeal membrane develops from the outer germ layer or ectoderm. The tube behind the buccopharyngeal membrane develops from the inner germ layer or entoderm. At about 4½ weeks the buccopharyngeal membrane ruptures, but the epithelium in that area still comes from two distinct germ layers. It is at this point that the tongue starts to develop as a swelling that arises out of the back part of the pharyngeal arches (Fig. 24-1). This swelling develops from the future floor of the mouth. The epithelial covering of the tongue develops from ectoderm and entoderm—the anterior two thirds from ectoderm and the posterior one third from entoderm. The tongue is basically a sac of epithelium filled with muscles. These muscles arise from the middle germ layer of the embryo, the mesoderm.
Each pharyngeal arch is associated with a particular cranial nerve; Chapter 34 on the nervous system discusses from which pharyngeal arch the various parts of the tongue develop. If you refer back to Chapter 18 you will find that the anterior two thirds of the tongue develops from two lateral lingual swellings and a midline tuberculum impar. These are both from the first arch. The posterior one third of the tongue develops from the copula (hypobranchial eminence) and the third arch. The root of the tongue and epiglottis develops from the epiglottal swelling of the fourth arch.
As mentioned before, the tongue is an epithelial sac filled with muscles and connective tissue. These muscles can be controlled willfully and are generally referred to as skeletal muscle or voluntary striated muscle. They are divided into two groupings: intrinsic and extrinsic muscles. Those that start and end wholly within the tongue are referred to as intrinsic muscles and include the four following groups: