Book - Human Embryology and Morphology 1
|Embryology - 11 Apr 2021 Expand to Translate|
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Chapter I. Development of the Face
Processes which form the Face.— About the middle of the first month of foetal life, five processes begin to spring from the base of the primitive cerebral capsule, which, by the end of the second month, have completely united together to form the facial part of the head. In figure 1, a diagrammatic representation is given of the condition of these five processes about the end of the first month. Of the five, one, the nasal or fronto-nasal, composed of symmetrical right and left halves, is median, and projects beneath the fore-brain; the others are lateral, two on each side, the mandibular and maxillary. The cavity which these five processes surround is the stomodaeum (Fig. 1). It ultimately forms the nasal and part of the buccal cavities. The part of the adult face formed by each process is shown in figure 2.
|Fig. 1. Showing the formation of the face by the Nasal, Maxillary, and Mandibular processes in an embryo of the 4th week. (After His.) .||Fig. 2. Showing the parts of the face formed from the Nasal, Maxillary and Mandibular processes.|
Malformations of the Face
These processes may fail to unite in the second month and in this manner malformations of the face are produced. The most common anomaly is a partial failure of the nasal and maxillary processes to fuse, various degrees of hare lip and cleft palate being thus caused. In hare lip, the cleft appears in the upper lip between the middle part formed by the middle nasal processes and the lateral parts formed by the maxillary processes (Fig. 2). In cleft palate, the failure of union occurs between the deep parts of the nasal and maxillary processes (Fig. 8). There is a mesial cleft in the upper lip of hares and rabbits, but it occurs between the two maxillary processes, the labial part of the middle nasal process being undeveloped. Macrostoma is due to a partial failure of the mandibular to unite with the maxillary element. Any of these processes may be under or over developed ; over-development of the nasal and under-development of the mandibular (micrognathia) are of common occurrence.
The Nasal Process
The nasal process at a very early stage is seen to be divided into two lateral processes and two mesial, the latter having globular enlargements as tips (Fig. 1). It must be remembered that the lateral and mesial nasal processes are really vertical septa springing from the basis of the primitive capsule of the fore-brain, and the parts seen on the face are the anterior extremities of these septa (see Fig. 8).
Fig. 3. Showing the structures formed in the Mesial Nasal Processes, .
What become of the Mesial Nasal Processes. — From the mesial nasal processes, which fuse together, and may enclose epithelial remnants between them, are formed the whole septum of the nose (Fig. 3), the premaxillary part of the upper jaw and the middle third of the upper lip (Fig. 2). It contains a skeletal basis of cartilage, formed by the trabeculae cranii (Figs. 135, 136, p. 168).
Structures formed in the Mesial Nasal Processes. — The mesial nasal processes fuse together; in their anterior inferio angles are formed the premaxillae. The remainder forms th septum of the nose.
In the mesial nasal processes a laminar plate of cartilage 1 developed, which is continuous with, and forms part of, th trabeculae cranii (Fig. 136). Part of this cartilage remains a the septal cartilage of the nose (Fig. 3). From the septal carti lage, just over the naso-palatine foramina, a small scroll-lik or turbinate process is thrown out on each side to form a hoo< for an isolated piece of olfactory epithelium — the organ of Jaeobson They form the cartilages of Jacobson. The cartilages and orgai are vestigial in man. In the mesial nasal processes are developed also, the mesial or septal limbs of the alar cartilages of the nosi (Fig. 3).
The Vomer is developed in the membrane (perichondrium) which covers the primitive septal cartilage. A centre of ossification appears in the 3rd month at each side of the cartilage ; they fust together under the palatal margin of the cartilage. Thus th( vomer forms at first a shallow trough in which the cartilage oi the septum appears to be implanted (Fig. 4).
Fig. 4. Showing the trough-shaped Vomer of the newly born.
The Vertical Plate of the Ethmoid is formed by a direct ossification of the primitive cartilage of the septum. Ossification begins in the 4th month. The crista galli, the intra-cranial pari of the septum, is formed in part by the ossification proceeding into the attachment of the falx cerebri.
The two premaxillary bones form the sockets of the upper incisor teeth. In the human foetus at birtr the suture between the pre-maxilla and maxilla can be seen or the hard palate; it runs on each side from the naso-palatine foramen to the alveolus between the lateral incisor and canine (Fig. 9). On the facial aspect, the premaxilla fuses with the superior maxilla in the 3rd month of foetal life, the maxillae overlapping and almost completely excluding them from the face. The nasal spine is formed by the premaxillae.
In mammals generally the premaxillae are highly developed and form the snout part of the face. In the higher Primates the face becomes less elongated, less prognathous and the premaxillae less developed. In the orang, for instance, the premaxillae are distinctly seen on the face at birth '(Fig- 5), but as the permanent canines begin to develop they fuse with the maxillae.
Fig. 5. Showing the suture on the face between the premaxilla and maxilla in the skull of a young orang.
In man each premaxilla is usually ossified by two centres, placed side by side. Hence it sometimes happens in cleft palate that the fissure appears, not between the canine and lateral incisor, but between the lateral and middle incisor. In such cases the two centres of the premaxilla have failed to unite and the cleft occurs between them. The two premaxillae unite together in the first year after birth. Their vestigial character in man is due to the small size of his masticatory apparatus and consequent retrogression in the development of the facial part of his skull.
The naso-palatine foramina are formed where the mesial nasal and two maxillary processes unite to form the palate (Fig. 9). In animals with well-developed premaxillae the two naso-palatine (anterior palatine) foramina are large, and through each passes the naso-palatine duct, which allows a communication between the buccal and nasal cavities. The odour of the food within the mouth thus reaches the organ of Jacobson. In man the upper ends of the ducts remain open ; they terminate blindly below, behind the mesial incisor teeth, in the naso-palatine or incisive papilla (see Figs. 9 and 19).
The lachrymal sac and nasal duct, through which tears pass from the eye to the inferior meatus of the nasal cavity, are formed between the lateral nasal and maxillary processes (Figs. 2 and 7). The epithelium of the skin (epiblast) enclosed between the processes, forms at first a solid cord ; it afterwards becomes caniculised to form the duct.
Fig. 6. Showing the structures formed in the Lateral Nasal Processes.
Structures Formed in the Lateral Nasal Process
In each lateral nasal process a laminar plate of cartilage is developed it is continuous with, and forms part of, the trabeculae cranii (Fig. 1 3 6, p. 1 6 8). Its inner or attached margin is continuous with the septal cartilage of the mesial nasal process; it forms on each side the roof and lateral wall of the nasal cavities (Ficr 7) What becomes of the Cartilage of the "Lateral Nasal Process (Fig. 6).
It forms on each side :
- The cribriform plate, around the olfactory nerves as they issue from the olfactory bulb; .
- The lateral mass of the ethmoid, at first merely a plate of cartilage ; the superior and middle turbinate processes are developed from the plate (Fig. 7) ; ossific centres appear in the cartilage of the lateral mass and turbinate processes during the fourth month of foetal life ;
- The inferior turbinate bone (Fig. 7) (maxilld-turbinal). The body of the superior maxilla is developed on its outer side in the maxillary process (Fig. 7) ;
- The lateral and part of the alar cartilages of the nose ;
- In the membrane over the cartilage, between the ethmoid behind and the cartilages of the nares in front, are developed the lachrymal and nasal bones, and the ascending process of the superior maxilla. The cartilage beneath these bones disappears after birth (Fig. 6).
Fig. 7. Coronal section of the skull of a 7th month human foetus to show the cartilages of the Lateral and Mesial Nasal Processes and the bones formed round them.
Arteries and Nerves of the Nasal Processes
- Mesial Nasal Process. The chief artery and nerve of this process are the naso-palatine, but branches also come from the nasal nerve and its accompanying artery, the anterior ethmoidal.
- Lateral Nasal Process. The lateral nasal nerves are derived from Meckel's ganglion and the descending palatine nerve. Vessels accompany the nerves from the descending palatine. The nasal nerve and anterior ethmoidal artery supply the process in Iron ??? It will thus be seen that the chief nerves and arteries ol botn processes are derived from structures in the spheno-maxillary fossa.
The Parts formed from each Maxillary Process.— The maxillary process springs from the base of the mandibular arch and sweeping forwards below the eye separates that structure from the mouth (Figs. 1 and 2). In front it comes in contact and fuses with the lateral nasal process which forms the outer wall of the nasal cavity and with the globular process of the mesial nasal which forms the premaxillary part of the palate and the middle part of the upper lip. The part of the face formed by the maxillary process is shown in figure 2. The hard palate (with the exception of the premaxillary part), the soft palate and its muscles, with the uvula, are formed by a horizontal plate which grows inwards from the maxillary process and fuses with the plate of the opposite side beneath the septum of the nose, with which the horizontal plates also unite (Figs. 8 and 9). The palatal processes separate the buccal from the nasal cavities, forming the roof of the one and the floor of the other (Fig. 7). The horizontal palatal plates meet first in front; the process of fusion spreads backwards, and by the end of the second month it is complete.
Fig. 8. Showing the ingrowth of the palatal plates of the two maxillary processes early in the 2nd month. (After Julius Kollmann (1834 - 1918)) .
Fig. 9. Showing the Hard Palate at birth. The premaxillary part is formed from the Mesial Nasal Processes ; the remainder by the Palatal Plates of the Maxillary Processes.
The condition of cleft palate is due to a partial or sometimes a complete failure of the process of fusion.
Bones formed in each Maxillary Process
The zygomatic process of the temporal, the malar, and the greater part of the superior maxillary are formed directly from the connective tissue within the process. They are membrane-formed bones. - Pterygo-palatine Bar. — Two other bones formed in this process have quite a different history. The internal pterygoid plate, which is originally a separate bone, and the palate, are developed over cartilage. When the maxillary process grows forwards from the base of the mandibular arch, it carries with it a prolongation of the cartilaginous bar which forms the skeletal basis of that arch. The cartilaginous bar is known as the pterygo-palatine, and in the membrane over this bar the pterygoid (internal pterygoid process) and palatal bones are developed (Fig. 10 A, B, C). From the posterior end of this bar is developed the incus in mammals and the quadrate bone in birds and reptiles. In birds and reptiles the lower jaw articulates with the quadrate bone (Fig. 10 B), and on the quadrate the superior maxilla is supported by the pterygoid and palate bones. In amphibians the quadrate, pterygoid and palate form a continuous bar of cartilage (palato-quadrate). In fishes the Palato-auadrate bar forms part of the mandibular arch (Fig. 10 A). In mammals the quadrate is completely separated from the pterygoid and, instead of acting as a supporting bone for the lower jaw, as in birds is subservient to hearing, and known as the incus.
|Fig. 10, a, b, c. Showing what become of the skeletons of the Mandibular Arch (Meckel's Cartilage) and Maxillary Process (Palato-quadrate Cartilage). The numerals indicate corresponding parts.
A. In Fishes and Amphibians.
B. In Reptiles and Birds.
C. In Mammals.
It is difficult to understand, as Dr. Hans Gadow has pointed out, how a bone such as the quadrate, constantly engaged with the mandible, could have become subservient to hearing, and he has produced good evidence to show that the quadrate does not correspond to the incus but to the tympanic plate, with which the mammalian jaw is still in contact. The incus he believes to be derived from the upper segment of the hyoid arch (see Fig. 10 D).
Fig. 10 D. Illustrating Gadow's view of the origin of the Auditory Ossicles and Tympanic Plate.
Nerves and Arteries of the Maxillary Process
A knowledge of the manner in which the maxillary process is developed explains the distribution and course of its arteries and nerves. The second division of the 5th, represented by the infra-orbital, descending palatine, pterygo-palatine, and Vidian nerves, forms its nerve supply. Its main artery is the internal maxillary. The muscles of the palate are developed in the horizontal palatal processes.
Formation of Foramina and Canals in Bone
The development of canals and foramina in the bones of the maxillary process illustrates the manner in which these are formed in the skull generally. Many foramina and canals occur originally between separate elements (see page 1*70). The Vidian nerve lies between the internal pterygoid plate (a separate bone) and the external pterygoid, a plate which grows into the maxillary process as a cartilaginous prolongation of the great wing of the sphenoid. The pterygo-palatine canal is situated between the pterygoid and palate part of the pterygo-palatine bar. The descending palatine nerves lie between the palate bone and superior maxilla. These are canals formed between different elements. The mfra-orbitat nerve at first passes forwards in a groove on the orbital aspect of the superior maxilla, but in the later months of foetal life, upgrowths from the malar and nasal centres of ossification of the maxilla meet over the nerve and convert the groove into a canal.
The foramen rotundum and foramen ovale are at first notches on the edge of the great wing of the sphenoid, but in the course of foetal growth the notches are converted into foramina. Hence wherever a nerve foramen or canal is found one may conclude that it marks the junction of two elements, originally distinct, or is originally a groove or notch on the edge of the bone (Bland and Sutton). The malar nerves issue between the two centres of ossification of the malar. The two malar centres may fail to unite; the bone is then divided by a suture passing from the orbit to the temporal fossa. It occurs rather more frequently in Japanese and Mongolian skulls, hence the name of Os Japonicum.
In all classes of mammals the mucous membrane on the hard palate is ridged transversely ; three or four of these tranverse ridges are seen on each side of the palate of the newly born child ; they tend to disappear in the adult. Food is triturated between them and the rough papillae on the palatal -aspect of the tongue. Their disappearance in man is probably due to the soft nature of his food.
The Antrum of Highmore
It will be seen from figure 7 that the maxillary process is at first a thin plate, lying between the orbit and mouth, containing the tooth buds. It rests on the outer aspect and covers the cartilaginous basis of the lateral nasal process which forms the outer wall of the nasal cavity (Fig. 7). About the third month of foetal life the mucous membrane in the middle meatus begins to bud outwards, presses before it and bursts through the lateral nasal plate of cartilage and begins tc distend the maxillary process. At birth the antrum is only a shallow recess on the outer wall of the middle meatus. It 'Continues to grow until the 25th year, and is the only one ol the air sinuses developed from the nasal cavity, which is present at the time of birth. In the years of adolescence the antrun reaches out until it inflates the maxillary part of the malar anc as it grows backwards, presses downward the posterior border of the maxilla and thus brings the permanent molar teeth into position (Fig. 11). If the process of growth is arrested, the last molar (wisdom) tooth is left on the posterior border of the maxilla, where it may ultimately be the cause of an abscess.
Fig. 11. Showing the manner in which the development of the Maxillary Antrum affects the size of the palate and position of the molar teeth.
Mandibular Processes and Arch
The two mandibular processes unite in the middle line and form the mandibular or first visceral arch. The arch forms the lower or hinder boundary of the stomodaeum (Fig 1).
Parts formed from the Mandibular Arch
Besides the lower jaw, there are formed from this arch the soft parts over and under the jaw, the lower lip, the muscles of mastication, the internal lateral ligament and the malleus. The anterior twothirds of the tongue, the sublingual and submaxillary glands are formed from the floor of the primitive pharynx between the mandibular and the second or hyoid arch. These parts are supplied from the nerve of the mandibular arch, and are therefore probably derived, in part at least, from the substance of the arch.
The Mandibular Arch bounds the stomodaeum behind, and is the foremost of the five visceral arches which encircle and form the walls of the primitive pharynx. Meckel's cartilage forms its skeletal basis (Figs. 10 C and 12). The 3rd division of the 5 th is its nerve, and its artery is the first aortic arch from which the inferior dental, facial and lingual arteries afterwards arise.
The structures formed from Meckel's cartilage are shown in Figs. 10 C, 10 D, and 12.
Development and Ossification of the Lower Jaw.— In some animals, such as the kangaroo, the two halves of the lower jaw, each developed in its own mandibular process, never unite. In man ossific union takes place early in the second year. In figure 12 are shown the manner of formation and ossification ot the lower jaw, with the changes that take place with age.
Fig. 12. Showing the Centres of Ossification and age changes in the Lower Jaw.
The part of the lower jaw (A, Fig. 12) is developed directly out of Meckel's cartilage. The dentary centre (2?) appears in the membrane on the outer side of Meckel's cartilage, and forms the body of the jaw from the mental foramen almost to the angle; the splenial centre (C) appears on the inner aspect of the cartilage and between it and the dentary, the canal for the dental nerves and vessels is formed. The ascending ramus, developed in mammals only, is formed from the coronoid and condylar centres which appear in membrane. By the condylar process the lower jaw comes to articulate directly with the skull (squamosal bone).
The growth of the antrum of Highmore, by pushing downwards the body of the lower jaw, leads to an elongation of the ascending ramus, and to its assuming a more vertical position to the body of the jaw (Figs. 11 and 12). In old age, when the teeth drop out and the alveolar margins are absorbed, the ascending ramus again becomes oblique, to allow the lower jaw to come in contact with the upper during mastication. The mental eminence is produced after birth, and is a human characteristic. It gives attachment to the depressores labii inferioris.
The ascending ramus is peculiar to mammals. In other vertebrates Meckel's cartilage forms the skeletal basis of the lower jaw and articulates with the quadrate bone (incus) by its upper articular extremity, the os articulare (malleus) (Fig. 10 A), The part of Meckel's cartilage between the malleus and body of the lower jaw forms the internal lateral ligament, and possibly also the interarticnlar cartilage of the temporo-maxillary articulation.
Fig. 13. The chief types of the Temporo-Maxillary Articulation.
A. Carnivorous Type.
B. Omnivorous Type.
C. Herbivorous Type.
Gadow regards the angle of the jaw as the representative of the os articulare.
The Temporo-maxillary Articulation
Two types of this joint are found in mammals, one (see figure 13,4), exemplified in the carnivora, in which only a hinge action is permitted, and hence the jaws act like scissor blades; the second (see figure 13C),in which a gliding movement is allowed, the teeth being thus able to act as grinders. The second type occurs in all vegetable feeders. The human articulation combines the characters of both types (Fi». 12>B), the gliding action taking place between the interarticular cartilage and the skull, the hinge action between the cartilage and the condyle. In rodents the glenoid cavity is a narrow gutter in which the plate-like condyloid process glides backwards and forwards. The interarticular cartilage is developed in all the Mammalia except the monotremes, and one or two marsupials (Parsons). 1 It is probably a derivative of Meckel's cartilage (see Fig. 10Z>).
Fig. 14. Showing the Chief Changes after birth in the form of the TemporoMaxillary Articulation. A. At Birth. B. At Two Years. C. In the Adult.
Development of the Tympanic Plate and Articular Eminence
If the chin be depressed the condyle of the jaw moves on to the articular eminence (Fig. 13); if over- depressed it springs over the eminence, and a dislocation is produced. This is impossible in the early years of life, for at birth there is no eminence and no glenoid cavity (see Fig. 14 A). At birth the membrana tympani lies exposed on the surface of the skull behind the condyle, supported in a fine osseous hoop, the tympanic ring. The ring is imperfect above, and there the flaccid part of the membrane occurs. By the second year the ring has grown into a plate by sending out two processes, which, as they grow out, unite and leave a gap between (Fig. 14 B). This, as a rule, is soon filled up. By the 20th year the tympanic plate is three-quarters of an inch long, forming the bony floor of the external meatus and the posterior wall of the glenoid fossa, which in man is remarkably deep. It protects the meatus from the condyle; every year until the 20th the bony meatus gets longer, while the fibro-cartilaginous part becomes relatively shorter. In the adult the bony part forms two-thirds of the meatus. As the tympanic plate grows outwards, the membrana becomes less easily accessible to the surgeon (Fig. 14 G). The plate also grows inwards to form the floor of the bony part of the Eustachian tube and downwards to form the vaginal process, to which the upper end of the carotid sheath is attached (Fig. 40). Gadow regards the tympanic plate as the representative of the quadrate bone of birds and reptiles.
The stomodaeum or primitive buccal cavity is the depression or narrow pocket formed between the fore-brain above and the mandibular arch below. It is bounded laterally by the or fundus being formed by the oral plate, which separates it from the primitive pharynx (Fig. 15. A). The mesial nasal and palatal plates of the maxillary processes divide it into an upper part — the nasal cavities — and a lower, which forms part of the permanent buccal cavity (see Fig. 15 B). The tongue is developed in the floor of the pharynx and the tonsils in the pharyngeal wall, but the lips, teeth, and gums are formed in the walls of the stomodaeum. In the 3rd week of foetal life the oral plate breaks down and the stomodaeum then communicates with the pharynx.
1 " Joints of Mammals," Journ. of Anat. and Physio., Vol. XXXIV.
The Origin of the Pituitary Body
The lining epithelium (epiblast) of the stomodaeum becomes pouched out against the floor of the fore-brain and forms the buccal element of the pituitary (Fig. 15^4 and Fig. 22, p. 30). A process from the floor of the hinder part of the fore-brain (thalamencephalon) meets it and forms the neural part of the pituitary. The buccal evagination is sometimes called Eathke's pocket. With the development of the base of the skull, the stalk of the buccal evagination disappears. A canal may occasionally be seen passing upwards between the basi- and pre-sphenoid, and opening at the olivary eminence, marking the position occupied by the pocket in the foetus (canalis cranio-pharyngeus, Fig. 3). Gaskell, who regards the neural or cerebro- spinal canal as the homologue of the invertebrate alimentary canal, homologises the pituitary evagination of the buccal epiblast with the invertebrate mouth and gullet.
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Human Embryology and Morphology (1902): Development or the Face | The Nasal Cavities and Olfactory Structures | Development of the Pharynx and Neck | Development of the Organ of Hearing | Development and Morphology of the Teeth | The Skin and its Appendages | The Development of the Ovum of the Foetus from the Ovum of the Mother | The Manner in which a Connection is Established between the Foetus and Uterus | The Uro-genital System | Formation of the Pubo-femoral Region, Pelvic Floor and Fascia | The Spinal Column and Back | The Segmentation of the Body | The Cranium | Development of the Structures concerned in the Sense of Sight | The Brain and Spinal Cord | Development of the Circulatory System | The Respiratory System | The Organs of Digestion | The Body Wall, Ribs, and Sternum | The Limbs | Figures | Embryology History
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