Human Embryology and Morphology 17

From Embryology

Keith, A. Human Embryology And Morphology (1921) Longmans, Green & Co.:New York.

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures


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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter XVII. Phraynx and Neck

In previous chapters the origin of various pharyngeal structures has been touched on. We have seen that a forward prolongation of the archenteron during the 3rd week gives rise to the fore-gut (Fig. 18), that the anterior or pharyngeal part of the fore-gut is separated from the primitive mouth or stomodaeum by the oral plate (Fig. 102), that the notochord is laid down along the dorsal wall of the pharynx (Fig. 102) and that the heart lies under its floor, while the aortic arches encircle it (Fig. 80). Mention has been made of its cartilaginous skeleton (Fig. 150), of the segmentation of its mesoderm (Fig. M9) and of its nerves (Fig. 93). In this chapter we have to knit these isolated statements together by following the developmental changes which transform the simple fish-like pharynx of the embryo into the complex of structures found in the neck and throat of the adult.


Evolution of the Pharyngeal Region

In the latter part of the first month and opening part of the second the neck of the human embryo undergoes a very remarkable transformation. In the 5th week, when the human embryo is about 5 mm. in length, representations of four gill clefts and five gill or branchial arches are plainly to be seen in the region of the neck or pharynx (Fig. 248) ; the elevation caused by the heart reaches forwards almost to the mandibular arch ; properly speaking, there is no neck at the ith week ; as in a fish, the head is fixed directly to the body. By the beginning of the 7th week (Fig. 43, p. 46) all traces of the branchial arches have disappeared ; the head of the embryo is now extended and lifted away from the thoracic region, which now contains the heart. Before the branchial arches have begun to disappear in the 6th week, a pouch has grown out from the floor of the pharynx to form the larynx, trachea, bronchi and lungs (Fig. 250). In the passage from the 6th to the 7th week of development we see the human embryo evolve from a stage in which the parts are adapted for a branchial respiration, as in fishes, to a higher one in which its parts are fitted for breathing air. Pharyngeal glands, such as the tonsil, thyroid and thymus, originally developed in connection with the visceral or gill arches, become modified in structure and position to suit the new conditions of life. With the evolution of the mammalian method of mastication and swallowing, the pharynx, originally a respiratory structure, was further modified. The tongue became difierentiated from parts in the floor of the pharynx, and muscles, which were at first designed to move the branchial arches, became converted into muscles of deglutition.


Pharynx of the Embryo

There is very little resemblance between the pharynx and neck of a human embryo in the third week and that of the adult. Indeed, in the 4th week the human pharynx resembles closely that of a fish (Figs. 248, 249). In both the human embryo and fish the pharynx is bounded by visceral or branchial arches, which are separated by depressions (human embryos) or clefts (fishes) ; in both the heart is situated under the pharynx, and from the ventral aorta, aortic arches pass up on each side, one in each visceral arch, to terminate in the dorsal aortae. In fishes the aortic arches give off vessels to the gills, in which the blood is arterialized. In the human embryo the blood passes directly through the aortic arches. The walls of the pharynx were, therefore, primarily respiratory in function.

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Fig. 248. Showing the Visceral Arches and Cleft-depressions in the Pharyngeal Wall of a Human Embryo at the beginning of the 5th week. Each Visceral Arch contains an Aortic Arch. (After His.)

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Fig. 249. Showing the position of the Heart, Visceral and Aortic Arches in a Fish. (Diagrammatic — after Gegenbaur.)


A considerable part of the human neck lying in front of the vertebral column and between the mouth above and the thorax and clavicles below, with the bounding walls of the adult pharynx, is formed from the embryonic visceral arches. A knowledge of the transformation of the embryonic to the adult pharynx is of the utmost practical importance : it explains the occurrence of fistulae and cysts found in the neck ; it accounts for the peculiar courses taken by nerves, such as the recurrent laryngeal and phrenic ; it explains the peculiar distribution of nerves to the pharynx ; and throws light on the nature and anomalies of the thymus, thyroid and tonsil. As may be seen from Fig. 248, the floor of the pharynx of the human embryo rests on the dorsal wall of the pericardium ; in the adult the pharynx and pericardium are separated by the whole length of the neck.

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Fig. 250. Showing the Primitive Pharynx of a 5th week Embryo in Sagittal Section, bounded by the Visceral Arches. After Wilhelm His (1831-1904)

Visceral Arches

The visceral arches bound and form the whole thickness of the wall of the primitive pharynx, which is flattened dorso-ventrally, so that its cavity forms a transverse cleft when seen in cross-sections of the embryo. Four arches, each bounded behind by a depression, are to be recognized superficially on each side of the pharynx of the 5th week human embryo (Fig. 248), but behind the 4th. cleft are fifth and sixth arches which, however, never become raised or superficially differentiated from the body wall behind (Fig. 251). Sagittal and coronal sections of the primitive pharynx (Figs. 250 and 251) give a better idea of the arrangement and constitution of the visceral arches than can be had from a surface view. They are developed round the most anterior part of the fore-gut, which forms the lining membrane of the primitive pharynx. The pharyngeal lining membrane, therefore, is the same as that of the alimentary canal from which spring all the organs and glands of digestion and assimilation.


Visceral Clefts

The epithelium or entoderm, which lines the primitive pharynx, covers the inner aspects of the arches and passes outwards in the recesses between them and there, for a short time, comes in contact with the epithelial covering of the body (ectoderm) which dips in to meet it (Fig. 251). The membrane thus formed by the union of the ectoderm and entoderm in the recesses between the arches, may be named the " cleft membrane." It is never ruptured nor disappears in the development of mammals, but is invaded by the mesoderm of neighbouring arches ; in fishes it disappears and real clefts are formed between the arches. On the outer side of the membrane is the " cleft depression," on its inner side a " pharyngeal recess," presently developed into a pouch. From the entodermal Iming of the pharyngeal pouches we shall see that the tonsil, thyroid and thymus arise : from the external depressions are formed the various branchial cysts and fistulae, which occasionally occur in the neck of the adult. Further, at the upper end of each cleft depression there develop remarkable sense-organs, known as the epibranchial placodes. In each arch there develop exactly the same elements as are to be seen in the gill arches of fishes, namely : (a) A skeletal basis of cartilage ; (b) an aortic or vascular arcii ; (c) a larger nerve along its anterior border and a smaller along its posterior ; (d) a muscle element.

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Fig. 251. Showing the Floor of the Pharynx of a 5th week Human Embryo. (After His.)


In Fig. 149 (p. 155) a schematic transverse section of a vertebrate embryo lias already been given to show the relationship of the tissues of a branchial arch or branchiomere to the segments of the head.


The first visceral arch is known as the mandibular, the second as the hyoid (Fig. 251). The remaining four are branchial arches, having been at one stage of evolution devoted solely to the purpose of carrying gills. The hyoid arch is specialized in fishes, to protect the branchial arches, and assist in the circulation of blood through the gills and water through the pharynx. The mandibular arch bounds part of the buccal cavity in all vertebrates, and forms part of the apparatus of mastication.


Formation of the Cervical Sinus

The first arch especially, and also the second, grow and increase at a much greater rate than the branchial arches. The second arch (hyoid) which in fishes forms the operculum for the gills, grows over and buries the third and fourth in the human embryo. Already at the end of the 5th week (Fig. 251) there is clear evidence of the sinking in of the hinder arches, and it is easy to see that as the hyoid arch grows backwards over them, an ectodermal space will become covered over and form the cervical sinus — representing the gill cavity of fishes. Its formation is effected in the 6th week. In Fig. 252 a model of the lining membrane of an embryo at the end of the 6th week of development is depicted as seen on its ventral aspect. The enclosed pocket of ectoderm — the cervical sinus — is shown to be connected with the 3rd pouch by a vesicular prolongation — the cervical vesicle, also by extensions to the 2nd or tonsillar pouch and to the 4th. The last named connection is short lived ; indeed, before the end of the 2nd month all traces of the sinus should have disappeared.

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Fig. 252. The Lining Membrane of the Pliarynx of a Human Embryo at the end of the 6th week of development (10 mm.) viewed on its ventral aspect. (Grosser.)

Although the cervical sinus usually disappears, it may remain and form a cyst in the neck, which opens on the anterior border of the sterno-mastoid a short distance above the sterno-clavicular joint. It may be drawn out into a trumpet-shaped tube, which ends in contact with the tonsillar recess, passing between the internal and external carotid arteries or in contact with the pharynx behind the hyoid (Fig. 253), connections which are explained by the origin of the sinus (Fig. 252). Often the cutaneous orifice is marked by a tag of skin representing a rudimentary external ear, which encloses a piece of cartilage.[1] If the outer cleft depression in front of or behind the third arch persist, it must open in the cervical sinus.

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Fig. 253. Diagram to illustrate the various parts of the Visceral Clefts which may persist. The 2n(i inner cleft recess gives rise to the tonsil ; the recess may be in contact with an epithelial tube derived from the cervical sinus. The 3rd inner recess gives rise to the thymus and carotid bodies.

What becomes of the Visceral Clefts

By the end of the second month the clefts, or, to be more exact, the representatives of clefts in the human embryo, have disappeared, except the upper part of the first. From the external depression of this part a soKd ingrowth of epithelium takes place which, ultimately becoming canaliculized, forms the external auditory meatus (Fig. 230). In connection with the upper or dorsal parts of the first and second cleft depressions the Eustachian tube and tympanum are formed, the membrana tympani remaining approximately in the position of a cleft membrane (p. 232).


If traces of the other clefts remain as fistulae or cysts they will occur in the positions shown in Fig. 254. Part of the second cleft is marked in the goat by an opemng and auricular appendage. As already pointed out, superficial remnants of the second and third clefts are rare ; they are usually included with the cervical sinus beneath the hyoid operculum.


Within the pharynx traces of inner cleft recesses are to be seen besides the Eustachian opening (see Figs. 253, 266). The tonsil is developed in the second cleft ; the anterior pillar of the fauces represents only the position of the second arch. The lateral recess of the pharynx (fossa of Rosenmiiller), behind the Eustachian tube, although sometimes regarded as a derivative of the second cleft, is, as we have seen (p. 228), a secondary formation. The pyriform fossa, at each side of the laryngeal aperture, represents the position of the fourth and fifth clefts (see Fig. 253).

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Fig. 254. Showing the position of the External Cleft Depressions in the Adult. (For explanation, see text.)

The Cartilages of the Arches

(Fig. 255).— The history of the skeletal basis of the first arch (Meckel's cartilage) has been already traced (p. 175). The cartilage of the 2nd or hyoid arch forms (Fig. 255) : (1) The tympano-hyal, embedded in the petro-mastoid, and originally continuous with the ear ossicles (Fig. 237). (2) The stylo-hyal, which ossifies in the early years of life and becomes joined to the tympano-hyal to form the styloid process. (3) The segment below, the epi-hyal, becomes ligamentous, and forms the stylo-hyoid ligament, but it also may become ossified. (4) The lowest segment, the cerato-hyal, forming the small horn of the hyoid. The epi-hyal lies behind and outside the tonsil, and when ossified has been excised under the belief that it was a foreign body. The body of the hyoid (basi-hyal) represents the fused ventral parts (copulae) of the 2nd and 3rd cartilages ; in the floor of the embryonic pharynx (Fig. 251) the ventral ends of the 2nd and 3rd arches end in a common or mesobranchial field. In this area the body of the hyoid develops.


Prof. Parsons[2] has drawn attention to the fact that there is a ridge of bone on the upper surface of the body of the hyoid, which may occasionally form an almost separate bar. It lies between the lesser horns, and appears to represent the copula or body of the 2nd arch. It may be separated from the body of the hyoid by a foramen evidently for the passage of a remnant of the thyro-glossal duct. It will be seen later that the basal or pharyngeal part of the tongue arises from the floor of the pharynx in the field between the 2Dd and 3rd arches. The skeletal bases of their ventral parts come to form the bone of the tongue. The skeletal part of the hyoid arch suspends the tongue. There may be a process of bone from the concavity of the body representing the hyolingual of lower vertebrates (Parsons).

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Fig. 255. Showing what becomes of the Cartilages of the Visceral Arches.


The great horn of the hyoid represents the cartilage of the 3rd arch (Fig. 255). In the lowest mammals the cartilaginous bases of the 4th and 5th arches unite to form the thyroid cartilage, but in higher mammals, including man, this cartilage is made up entirely by the 4th arch.[3] The cartilages of the ultimate arches (5th and 6th) are probably represented by the cricoid, arytenoid and rings of the trachea (see also Fig. 375, p. 352).


Even in mammals the cartilages of the three last branchial arches remain subservient to the purposes of respiration, just as in vertebrate animals in which these arches carry gills.

Nerves of the Visceral Arches

(see Figs. 256, 257, 93). — The 3rd division of the 5th nerve is, as has been already seen, the principal nerve of the first or mandibular arch. The nerve for the second or hyoid arch is represented by the 6th or facial. The nerve of the 3rd arch is the glossopharyngeal, that for the 4th is the superior laryngeal branch of the vagus, and for the 5th and 6th the inferior laryngeal (Fig. 257).


Each nerve of a visceral arch supplies (1) the muscles of the arch, (2) the pharyngeal lining and cleft recess in front of the arch. The chorda tympani and great superficial petrosal nerves represent the sensory branches of the facial to the first cleft.

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Fig. 256. The Visceral Arches and their Nerves and Ganglia in a Human Embryo of the 5th week. (Professor Streeter.)

The relationship of the nerves to the visceral arches is shown in Fig. 256, in a human embryo of five weeks. The position of these nerves in the adult is diagrammatically represented in Fig. 257. The Vth nerve and Gasserian ganglion are seen to lie at the base of the mandibular process. The ganglion of the Vllth and Vlllth nerves lies at the base of the hyoid (second) arch, in front of the otic vesicle, the fibres of the facial having already entered the arch. The glosso-pharyngeal and its ganglia lie behind the otic vesicle and at the base of fche third arch. The large ganglionic mass of the vagus lies over the bases of the fourth, fifth and sixth arches — or rather the tissue representing these arches. At this stage — the 5th week — ^the ganglion of the vagus and its issuing fibres rest on the dorsal wall of the pericardium, the heart being quite close to the source of its nerve fibres.

Epibranchial Placodes

When the ganglia of the Vllth, IXth and Xth nerves begin to difierentiate in the 5th week, they are in contact with the upper ends of their respective gill depressions — the 1st, 2nd and 3rd. An area of ectoderm at the upper end of each cleft depression becomes modified to form an epibranchial placode representing sense organs which are now lost in higher vertebrates. During the 5th week these placodes are in contact with the ganglia just mentioned and the ganglion of the trunk of the vagus (ganglion nodosum) and of the trunk of the glossopharyngeal (ganglion petrosum) receive additions from cells which are produced in, and migrate from, the placodes.

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Fig. 257. Showing what becomes of the Nerves of the Visceral Arches.

Aortic Arches — the Arteries of the Visceral Arches

In Fig. 248 is given the foetal arrangement of the aortic arches, and in Fig. 258 the vessels in the adult which are formed from them. The primitive aorta in the embryo divides into two trunks, which run forwards along the floor of the pharynx, one on each side, lying between the ventral ends of the visceral arches. These may be termed the right and left ventral aortic stems. From these stems arteries (aortic arches) pass upwards, one in each visceral arch, to terminate in the right and left dorsal aortae, which run backwards and become fused to form the descending thoracic aorta. The aortic arches are formed at a very early date. At the beginning of the 4th week the first or mandibular aortic arch is already present ; the second (liyoid), third, fourth, fifth and sixth appear in succession, but by the 6th week, when the 6th or pulmonary arch[4] has appeared, the first and second are in a process of atrophy. Only for a brief period towards the end of the 5th week, when the embryo is about 5 mm. long, are all the arches open, and even then the 1st is atrophic while the 6th or pulmonary is developing. The 5th arch has only a transient existence. The aortic arches are formed by the union of a network of blood spaces which are developed within each visceral arch.


The 1st and 2nd aortic arches disappear ; the 3rd remains as the first part of the internal carotid, the 4th forms the 1st and 2nd stages of the right subclavian. On the left side the 4th aortic arch forms that part of the arch of the aorta between the origin of the left carotid and entrance of the ductus arteriosus (Fig. 258). The right and left 5th arch, or, to be more accurate, the 6th, for a transient arch appears between it and the 4th, give off vessels to the lungs which are developed in close connection with these arches. This arch on the left side persists as part of the right pulmonary artery and ductus arteriosus (Fig. 260). On the right side the dorsal part disappears, the remaining segment joining in the formation of the right puhiionary artery. When it is remembered that the 6th or puhiionary arch lies at the level of the larynx in the 5th week, and that, owing to the development of the neck, it has almost reached its final position in the 7th week, the rapid transformation of the parts in the region of the pharynx in the second month will be realized. It is in this period that the hinder gill arches are buried and the cervical sinus formed and obliterated.

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Fig. 258. Showing what becomes of the Aortic Arches in the Adult. Only the shaded parts persist. The position of the 1st and 2nd aortic arches should be indicated above and below the position of the external auditory meatus.


Subclavian Arteries

The visceral arches with their arteries are well developed before the limb buds appear. When, at the end of the 4th week, these buds grow out to form the upper extremities, the artery which supplies each bud springs from the dorsal aorta and represents a dorsal segmental branch of that vessel. The embryonic or primitive subclavian is the artery of the 7th cervical segment, being situated at a considerable distance behind the 6th aortic arch. As the aortic arch-system is elongated to form the great vessels of the neck during the 6th and 7th weeks, the origin of the subclavian comes to lie opposite the dth arch (Fig. 259). This artery forms the entire subclavian on the left side, but only that part beyond the origin of the vertebral on the right side (Fig. 259).

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Fig. 259. The condition of the Right and Left Dorsal Aortae in a 7th week Human Foetus. (After His.) The right arch and right dorsal aorta disappear beyond the origin of the right subclavian ; a constriction may appear at the corresponding point on the left side.


Aortic Arch on the Right Side

In birds it is the 4th right arch which forms the aortic arch, and this occasionally happens in man. In amphibians both the right and left 4th arches persist. The two dorsal aortae in which they end, unite together, as they do in the human embryo, to form the descending thoracic aorta. The primitive subclavian arteries spring from the dorsal aortae above the point where these two vessels fuse together. In the latter part of the second month the short part of the right dorsal aorta, between the origin of the right siibclavian artery and point of aortic fusion, disappears, and then the subclavian artery appears as if it arose from the 4th right arch (Fig. 259). The communicating arterial twig, which is often seen uniting the superior intercostal artery of the right side with the artery of the lower spaces, is formed by a secondary anastomoses, and does not represent the right dorsal aorta.[5]


Not unfrequently the right subclavian arises, not from the innominate, which represents the right ventral aortic stem, but as the last of the great branches which arise from the arch of the aorta (Fig. 260). In such cases two things have happened : (1) the 4th right aortic arch has been .obliterated, (2) the right dorsal aorta has persisted.


Cases occur in which the permanent aorta is very much constricted at or near the point of entrance of the ductus arteriosus (see Fig. 259). It will be noticed that the corresponding part of the right dorsal aorta is obliterated. Such a constriction on the left side is to be regarded as corresponding to that on the right side, and indicates an attempt to produce a right aortic arch.

Dorsal Aortae

It will be noticed that the parts of the dorsal aortae between the 3rd and ith arches disappear (Fig. 260). The ventral aortae persist as the innominate, the common carotid and external carotid arteries. With the marked elongation of the cervical region and the development of the lungs in the second month, the primitive position of the aortic arches is greatly disturbed. The heart, being the pump of the lungs, must accompany these organs. The ventral aortae become elongated into the common carotid and innominate arteries (Figs. 258, 260). The 4th aortic arch, which should lie opposite the upper part of the thyroid cartilage, comes to rest at the level of the 1st rib on the right side and within the thorax on the left, while the last aortic arch dragging the nerve of its segment in front of it (the recurrent laryngeal) comes to be situated within the thorax.

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Fig. 260. Diagram showing tiie manner in which the Right Subclavian may arise as the last branch of the Arch of the Aorta. The parts of the aortic arch system which become obliterated are stippled.

Muscles of the Visceral Arches

Within each visceral arch a muscle plate is formed[6] — recalling in mode of appearance the muscle plates which develop in connection with each vertebral somite. The muscles arising in each arch are supplied by the nerve of that arch ; hence from the nerve supply alone one could infer the derivation of the musculature of the pharyngeal region. The muscles become differentiated in the latter part of the second month. All the muscles supplied by the facial nerve — the platysma, muscles of expression, the stapedius, stylo-hyoid, posterior belly of the digastric, etc. — are derived from the muscle plate of the 2nd or hyoid arch. The muscles of mastication, with the tensors of the palate and tympanum, the anterior belly of the digastric and mylohyoid, are derived from the muscle segment of the mandibular arch. The stylo pharyngeus is derived from the 3rd arch. The musculature of the soft palate and the constrictors of the pharynx are derived from the third and fourth arches. The musculature of the larynx comes from the fifth and sixth arches.

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Fig. 261. The expansion of the Platysma Sheet in a Human Foetus of 7 weeks. (Futamura.)

The Platysma and Muscles of the Face and Scalp

The platysma myoides[7], the muscles of the face, scalp and external ear, are derived from the muscle plate of the second or hyoid arch. They are supplied by the facial, the nerve of this arch. The muscle bud, from which the whole platysma sheet is developed, is still confined to the area of the hyoid arch until the 7th week of development when the bud spreads out and forms a continuous muscular hood over the head and neck. To this hood or sheet, which is composed of two layers, a deep and superficial, the name of platysma sheet may be given. It is developed in the superficial fascia. During its expansion or migration the platysma sheet separates into three main divisions — a part for the neck — platysma colli ; for the ear and occiput —the occipito-auricular ; and the facial division — for mouth, nose, orbits and forehead (Fig. 261). The muscles become differentiated during the 3rd month.


In man, the platysma sheet has undergone marked retrograde changes in the neck, scalp and external ear, but over the face it has become more highly specialized and differentiated than in any other animal. From this sheet are derived the epicranial aponeurosis, the occipitalis and frontalis muscles. On the face the platysma sheet forms the muscles round the orbit, nose and mouth. The buccinator and levator anguli oris represent parts of the deeper layer of the sheet. The transversus nuchae, fibres occasionally seen in man passing from the middle line of the neck behind, towards the ear and cheek, represent fibres constantly developed in lower primates, and better still in rodents and carnivora as the sphincter colli and sterno-facialis.


The muscles supplied by the facial nerve are peculiar in that they are the physical basis into which many mental states are reflected and in which they are realized. Through them mental conditions are manifested. It is found that the differentiation of this sheet into well-marked and separate muscles proceeds pari passu with the development of the brain. The more highly convoluted the brain of any primate, the more highly specialized are its facial muscles. It is remarkable that the sheet should arise from a visceral arch, which originally was closely connected with the function of respiration. To some extent the platysma does come into play during forced respiration even in man.

The Neck

If the reader will turn to Fig. 43 it will be seen that the head becomes demarcated from the trunk and a neck comes into existence in the human embryo during the 7th and 8th weeks of development. It is during these weeks that the fish-like organization of the embryonic pharynx becomes replaced by one which is mammalian. Although the seven cervical somites are demarcated early in the 4th week of development, the head is so flexed upon the trunk that the mandible is in contact with the pericardium. The neck comes into existence by the production and growth of tissues between the mandibular arch and pericardium, this growth in the ventral aspect of the cervical region being accompanied by an extension or elevation of the head. The heart itself is anchored to the roots of the developing lungs ; all the tissues — nerves, vessels, muscles, air and food passages — passing from the head to the region of the thorax are elongated during this movement.



  1. For an account of the various developmental anomalies of the pharyngeal region see Keith, Brit. Med. Journ. 1909, Aug. 7th, 14th, 21st. For a description of its development see J. Ernest Frazer, Journ. Anat. and Physiol. 1910, vol. 44, p. 156 ; H. Fox, Amer. Jo^irn. Anat. 1908, vol. 8, p. 187 ; B. Kingsbury, Amer, .Journ. A7iat. 1915, vol, 18, p. 329.
  2. F. G. Parsons, Jouni. Anat. and Physiol. 1909, vol. 43, p. 279.
  3. F. H. Edgeworth, Quart, Journ. Mic. Sc. 1916, vol. 61, p. 383.
  4. For literature on 6th arch and 5th cleft recess see Frank Reagan, Amer. Journ, Amt. 1911, vol. 12, p. 493 ; J. Tandler, Aiiat, Hefte, 1909, vol, 38, p. 393.
  5. See E. Pearce Gould, Journ. Anat. and Physiol, 1909, vol. 43, p. 329.
  6. F. H. Edgeworth, Journ. Anat. 1920, vol. 54, pp. 79, 124.
  7. R. Futamura, Anat. Hefte, 1907, vol. 32, p. 479 ; 1906, vol. 30, p. 433.


Historic Disclaimer - information about historic embryology pages 
Mark Hill.jpg
Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Human Embryology and Morphology: 1 Early Ovum and Embryo | 2 Connection between Foetus and Uterus | 3 Primitive Streak Notochord and Somites | 4 Age Changes | 5 Spinal Column and Back | 6 Body Segmentation | 7 Spinal Cord | 8 Mid- and Hind-Brains | 9 Fore-Brain | 10 Fore-Brain Cerebral Vesicles | 11 Cranium | 12 Face | 13 Teeth and Mastication | 14 Nasal and Olfactory | 15 Sense OF Sight | 16 Hearing | 17 Pharynx and Neck | 18 Tongue, Thyroid and Pharynx | 19 Organs of Digestion | 20 Circulatory System | 21 Circulatory System (continued) | 22 Respiratory System | 23 Urogenital System | 24 Urogenital System (Continued) | 25 Body Wall and Pelvic Floor | 26 Limb Buds | 27 Limbs | 28 Skin and Appendages | Figures