Human Embryology and Morphology 11

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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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Chapter XI. The Cranium

Natural Divisions of the Skull

The human skull is the product of many long epochs, during which it has undergone great changes, but we have every reason for supposing that its general functions have remained much the same since the vertebrate form of animal was evolved. In the first place it has to form a brain-case — a neuro-cranium. Man's brain has reached a degree of development which rendered great changes necessary in this part of the skull. In the second place, the skull has to shelter and protect the special organs of sense — the ear (temporal bone), the eyes (orbits), the olfactory area (nasal region), and taste (bucco-pharyngeal region). In the third place, the skull forms an essential part of the structures concerned in mastication ; the facial part of the skull is in reality a scaffolding for the palate and teeth. In the main the facial part of the skull is visceral or splanchnic in function, and hence is sometimes spoken of as the splanchno-cranium. The outstanding characters of the human skull are the great size of the neuro-cranium and the smaU size of the splanchno-cranium.

Certain Phases in the Evolution of the Skull

The skull has also been closely related to the function of respiration. In fishes the visceral skeleton of the skull forms the arches which carry the gills. We have seen that the representatives of these arches make a temporary appearance in the head of the human embryo. When a pulmonary replaced a branchial system a nasal airway was separated from the mouth by the formation of a primitive palate, such a palate as is seen in amphibians, reptiles and birds. With the evolution of chewing teeth in the mammalian stock the complete palate was formed, thus allowing the mammalian young to suck, and the adult to chew and breathe freely at the same time. We see all of these stages manifested in the development of the human skull.^ ^ For recent research on development of skull see : Ed. Fawcett (Chondrocranium of water-rat), Journ. Anat. 1917, vol. 51, p. 309 ; (Chondrocranium of hedgehog), Journ. Anat. 1918, vol. 52, p. 211 ; (Chondrocranium of seal), Journ. Anat. 1918, vol. 52, p. 412 ; (Skull of Miniopterus), Journ. Anat. 1919, vol. 53, p. 315 : C. R. Bardeen, Keibel and Mall's Textbook of Embryology, vol. 1, 1910 ; Warren H. Lewis, Contrib. Embryology, 1920, vol. 9, p. 299 ; John Kernan (Chondrocranium of 20 mm. embryo), Amer. Journ. Anat. 1916, vol. 17, p. 605 ; Chas. C. Macklin (Cranium of 40 mm. foetus), Amer. Journ. Anat. 1914, vol. 16, p. 317 ; R. J. Terry (Chondrocranium of cat), Journ. Morph. 1917, vol. 29, p. 281 ; Eliz. A. Fraser (Trichosurus), Proc. Zool. Soc. Lond.

1915, p. 299 ; Phihppa C. Esdaile (Chondrocranium of perameles), Phil. Trans. (B) 1916, vol. 207, p. 439 ; D. M. S. Watson (Duckbill), Phil. Trans. (B) 1916, vol. 207,

Cartilaginous Skull

In trying to interpret the meaning of many of the developmental processes which we see taking place in the human embryo, it is often profitable to seek light from comparative anatomy, and no group of the lower vertebrates can help us in this respect so well as the selachians — the group of cartilaginous fishes to which sharks, rays and dog-fish belong. This is particularly true of the chondrocranium — the cartilaginous skull, seen in the human foetus during the 2nd month of development. In Fig. 130 is represented the cartilaginous cranial wall which encloses the brain of a shark ; we see at once that the base is made up of two parts — a chordal in which the remains of the anterior part of the notochord are embedded and a prechordal lying in front of the notochordal part. The fossa for the pituitary body occupies the posterior part of the prechordal base. The chordal part represents a continuation forwards of the vertebral column, only the cartilage never becomes segmented but remains as a continuous plate, and thus gives solidity to the part. Signs of segmentation are seen in the series of foramina by which the roots of the hypoglossal nerve escape. The sujDra-chordal part of the cranial cavity is occupied by the hind- and mid-brains, which also show traces of a segmental origin. In the lateral wall of this part of the skull is also placed the otic vesicle — the vestibular or balancing apparatus. It will be remembered that it was the attachment of this organ to the hind-brain which occasioned the development of the cerebellum ; it also gives rise to a disturbance of the skull, for the cartilaginous capsule which is developed round the otic vesicle is thrust into the cranial wall and pushes backwards the representatives of the neural arches of the chordal cranium (Fig. 130). The prechordal part of the skull serves as the capsule of the fore-brain. At no time is there a segmentation of the fore-brain or of its cranial capsule ; we are here dealing with a part of the skull which lies in front of the ancient vertebral region, and has arisen, as has the fore-brain itself, in connection with two organs of sense — the nose and eye. The olfactory organ is enclosed in a capsule of cartilage which is placed like the watchman of a ship, on the prow of the primitive skull of all aquatic vertebrates. The capsule of the optic vesicle never forms part of the cranial wall, but becomes differentiated to form the sheath of the optic nerve and of the eyeball.

Fig. 130 The Chondrocranium of a Shark laid open by a mesial sagittal section.

Fig. 130. The Chondrocranium of a Shark laid open by a mesial sagittal section. (After Gegenbaur.)

p. 311 ; (Amphibia), Phil. Trans. (B) 1919, vol. 209, p. 1 ; E. S. Goodrich (Cranium of dog-fish), Quart. Journ. Mic. Sc. 1918, vol. 63, p. 1 ; Graham Kerr, Textbook of Embryology, vol. 2, 1919.

If we examine the chondrocranium of a human foetus in the 8th week of development (Fig. 131) we note the same divisions as are shown in Fig. 130. The base shows chordal and prechordal parts. In the chordal part we note the cartilaginous otic capsule thrusting backwards the combined occipital elements in the lateral wall ; we see the prechordal part passing forwards as a rostral beam to support the nasal or ethmoidal capsule. But of the cartilaginous roof only mere remnants are present. There is : (1) the tectal plate, or parietal plate as it is sometimes named ; it is attached, along the lower border, to the auditory capsule and occipital element ; (2) there are two small processes springing from the sides of the prechordal base (Figs. 131, 132) — the ala temporalis — the fundament from which the great wing of the sphenoid will be developed and the orhito- sphenoid — the j)late from which the small wing will be fashioned. These three cartilaginous plates are all that appear in the human embryo to represent the cartilaginous roof of the primitive skull.

Fig. 131 Sagittal Mesial Section of the Chondrocranium of a Human Foetus 20 mm

Fig. 131. Sagittal Mesial Section of the Chondrocranium of a Human Foetus 20 mm. long and in the 8th week of development. (Warren Lewis.)

If we turn to Fig. 132 we can see why the primitive cartilaginous skull of the human embryo has become so profoundly modified and reduced. It is a result of the large mass attained by the mammalian central nervous system at an early state of development. When a builder is to erect a great edifice he does not begin by repeating the evolutionary history of house building, but marks out from the beginning the extent of his foundations. It is so in laying down the human brain ; it is laid down on big lines almost from the first ; the ancient roof has become altogether inadequate ; we see the tectal plate growing up and covering the roof of the 4th ventricle ; it meets with its fellow of the opposite side and forma that part of the occipital bone which completes the posterior fossa of the skull and encloses the hind-brain. But all the rest of the roof is formed by a membranous capsule in which cartilage never develops. A glance at Fig. 132 will show why the roof must be fashioned from plastic material, for during the 3rd, 4th and 5th months the cerebral vesicles, lying over the prechordal region of the base, expand upwards and backwards until their occipital poles reach the tectal plates.

There are certain other features seen in the lateral aspect of the foetal chondrocranimn which call for comment here. The auditory capsule, the auditory ossicles and the region of the tympanum, save for their covering of soft parts, lie exposed on the surface of the skull. If we turn to the lateral aspect of the cartilaginous skull of a shark we obtain an evolutionary explanation of this arrangement. At the anterior end (Fig. 133) is seen the nasal or ethmoidal region ; the hind end is formed by the occipital area — compounded from occipital vertebrae. Between these two extreme areas lies a large intermediate part which is definitely demarcated into two regions — orbital and otic (Fig. 133). Lying on the otic area and attached to it are the primitive maxillary apparatus — the tympano-hyal (Fig. 133) which corresponds to the stapes, the quadrate part of the palatoquadrate^ which has been shaped in mammals to form the incus, and the upper end of the primitive mandible which gives rise to the malleus — all lying exposed just as in the human embryo. The cartilaginous prominence — named post-orbital in Fig. 133, because it bounds posteriorly the orbital region of the primitive skull — is worthy of note because it represents the point at which a new mandibular joint — the temporo-mandibular — becomes evolved in mammals, and thus sets free the old maxillary parts for the service of the ear. The post-orbital process of the primitive skull becomes the site of the articular eminence in the mammalian skull, while the pre-orbital is represented by the internal angular process of the mammalian orbit. Thus, out of the primitive orbital region is fashioned, not only the orbit, but the whole floor of the temporal fossa, the malar bone and zygomatic arch being later formations evolved out of membranous skeletal elements. Similarly in the skull of the human embryo, as in that of the shark, there are no cartilaginous representatives of the maxilla or premaxilla.

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Fig. 132. Chondrocranium of a Human Embryo in the 8th week of development, seen from the side. (Warren Lewis.)

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Fig. 133. Lateral Aspect of Skull of Shark. (After Gegenbaur.)

Growth of the Cranial Cavity

The neuro-cranium is framed by the disposition of its bones and sutures, so as to allow a free and easy expansion of the brain. By a mechanism we do not fully understand the bones entering into the formation of the cranial cavity grow as demand is made on them by the brain ; at least, this is so in early life. AVTien the cranial bones begin to form in the latter part of the second month, the brain (cerebral vesicles) is only half an inch long — from frontal to occipital pole ; in the adult the length is fourteen times as much and its volume fifteen hundred times larger. As the cerebral vesicles expand the developing bones alter in shape. By the 7th month of the foetal life the relative proportions become approximately fixed. During the first four years of life, brain and cranial growth go on rapidly. At birth the brain has attained from 20 to 22 per cent, of its size ; by the 4th year over 80 per cent. of the volume is already present. There is a steady increase until the 18th or 20th year, when the maximum is obtained (about 1500 cubic centimetres in Englishmen) ; after then there is a decline in the capacity of the cranium. The changes in the cranial walls are secondary to those in the brain.

From Fig. 134 it will be apparent that the walls of the cranium are made up of two very different parts — basilar and capsular. The basilar part is thick and developed in a cartilaginous basis. Growth proceeds as in a long bone ; the lines between the basi-occipital and basi-sphenoid, the basiand pre-sphenoid, and between the pre-sphenoid and ethmoid are growth or epiphyseal lines. The growth of the base of the skull is determined as much by the needs of the splanchno-cranium as by those of the neurocranium. The capsular part — occipital, parietal, frontal and temporal bones — on the other hand, respond easily to the expansion of the brain. They grow at their edges ; the sutures are growth lines. Growth at the coronal and lambdoid sutures adds to the calvarial length ; growth at the sagittal and squamous sutures increases the calvarial breadth. At the same time there is also a constant deposition or growth on the outer table and an absorption on the inner. In this manner the bones are modelled, and growth of cranial cavity and brain are co-ordinated. Only those bones which enter into the formation of the cranial cavity and help to form the brain chamber are dealt with in this chapter. These bones are the frontal, parietal, occipital, temporal, ethmoid and sphenoid.

Is the Skull made up of Segments? — We have just seen that the body is made up of 33 or more segments. Is the skull made up of a series of segments ? The theory supported by Owen and many others that the cranium is really composed of four modified vertebrae is now no longer tenable. On the other hand the arrangement of the nerves and muscles, the evidence of development and comparative anatomy, indicate that it is composed of a number of segments, probably nine in number. The four posterior, which form the occipital region of the skull, are recognizable at an early stage of development, but at no period in the development of the embryo has cranial segmentation been seen anterior to the otic vesicle.

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Fig. 134. Median Sagittal Section of the Skull of a Foetus of the ninth month.

The Primitive Membranous Skull

The brain is developed in the same manner as the spinal cord from the medullary plates of the neural groove. In the same manner the mesoderm grows under and over the cephalic part of the neural canal, and forms for it a mesenchymal or membranous covering. The covering of mesoderm thus formed is the primitive Anlage of the skull in the embryo.

Membrane and Cartilage Bones

Only the base of the human skull is developed in cartilage, the rest is developed in membrane. How has such a condition arisen ? The brain of amphioxus, if it can be said to possess one, is wrapped in a membranous covering. In fishes with cartilaginous skeletons this embryonic mesodermal capsule becomes chondrified — plates of cartilage develop in it. As in the spinal column, the process of chondrification begins at the base and spreads slowly round to the crown or dorsum of the head. The cartilaginous cranium is an advance on the membranous stage. In many fishes a further most important element is added. The dermal bony plates, to which the placoid scales are fixed, are applied to the cartilage over the sides and dorsum of the skull. Thus to the cartilaginous element of the skull is added a third element — bone formed in membrane. Now, in the mammalian skull, and especially in that of man, the cerebral vesicles grow so quickly that long before the process of chondrification has had time to spread in the membranous capsule from the base to the crown, the dermal bones have formed, and thus supplant the cartilage on the calvarium. Hence, in the human skull, while the process of chondrification occurs in the base, and afterwards undergoes ossification, the roof and sides (calvaria) of the skull are formed by bones which, historically, are dermal bones, and hence are formed directly in membrane. The dermal bones of the human skull are : (1) the frontal, (2) the parietal, (3) the inter-parietal part of the occipital (the part above the superior curved lines), (4) the squamous part of the temporal.

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Fig. 135. The Centres of Ossification for the Dermal Bones of the Skull, which are formed in cartilage are stippled.

Thus the calvarial part of the skull passes directly from the membranous to the bony stage, while the base of the skull, like the spinal column, passes through three stages : (1) membranous, (2) cartilaginous, (3) bony. It will be thus seen that the base of the skull, developed in cartilage, is the most ancient part, while the dermal bones, which form the calvaria, represent a later addition.

Development of the Calvarial (membranous or dermal part) of the Skull

In the 8th week of foetal life — the foetus being then about 25 mm. (1 in.) long — there appear on each side of the membranous cranial capsule four centres of ossification : (1) For the frontal bone, at a point which becomes afterwards the frontal eminence (Fig. 135) ; (2) For the parietal, at the position of the parietal eminence ; this centre is double or even trijDle in nature, but the separate points are placed closely and soon fuse together ; (3) For the squamosal, at the base of the zygoma (Fig. 135) ; (4) For the membranous part of supra-occipital (part above superior curved line). Maggi and Hepburn ^ have shown that there may be four centres (two on each side) in the membranous supra-occipital (Fig. 137).

The two or four occipital centres fuse early into one at the position of the external occipital protuberance, but occasionally these centres may form two, three or four separate bones. The two frontal ossifications fuse about the end of the first year ; the metopic suture,^ which separates them, disappearing then. This suture occasionally persists. One or both parietals may be divided by a suture or by a complex of sutures.^ The centres of ossification in these cases have not fused. The parietal bones ossify together, at the sagittal suture, late in life, commonly between the 35th and 45th year, when the growth of the skull has entered a retrograde phase. The squamosal partly covers the petro-mastoid cartilaginous element and fuses with it in the first year, the temporal bone being thus formed. These bones, as they are laid down, accurately follow the contour of the brain. That organ forms a relatively small sphere when ossification commences. Hence the convexities or eminences at the regions of earliest formation.

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Fig. 136. The Occipital Bone at the 4th month, showing pre-interparietal Wormian Bones. (After Sappey.)

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Fig. 137. The Supra-occipital from a Foetus of 3 months, showing four Centres of Ossification for the Membranous Supra-occipital. (After Maggi.)

The Manner in which these Bones are Developed

In Fig. 138 a vertical section of the skull of a foetus 5 months old is represented. The coverings of the brain are seen to be then (1) scalp, (2) a stout white fibrous capsule, (3) a fine membrane lining it — the inner layer of the dura mater, (4) the arachnoid covering the brain (not shown in figure). Ossifying fibres which form the parietal are seen developing within the capsule and radiating out from the centre of ossification. The ossific fibres, as they spread outwards from a common centre, unite by branches, thus forming an irregular network with osteoblasts and growing vessels within its meshes. Lower down are seen the ossifying fibres of the squamosal. The base of the skull is formed of cartilage which is covered, or ensheathed, by a perichondrium continuous with the membranous capsule. In the cartilage appear the centres of ossification for the sphenoid.

^ Professor D. Hepburn, Journ. Anat. and Physiol. 1907, vol. 42, p. 88.

2 Professor T. H. Bryce, Journ. Anat. 1917, vol. 51, p. 153 ; Dr. A. H. Schultz, Amer. Journ. Anat. 1918, vol. 23, p. 259.

^ Professor Patten, Zeitschrift fiir Morph. und Anthrop. 1912, vol. 14, p. 527 ; Professor R. J. A. Berry, Journ. Anat. and Physiol. 1910, vol. 44, p. 73.

As the bony fibres of the parietal spread out, they divide the primitive cranial capsule into an outer layer — the pericranium — and an inner — the periosteal layer of the dura mater. At the periphery of the bone and in the sutures the continuity of these two layers persists. The growth of the fibroblasts and osteoblasts in the sutural lines between the bones keeps time with the growing brain which expands the capsule, but there is, at each corner of the parietal bone, until the end of the first year, a part of the primitive cranial capsule left unossified. These imossified parts of the membranous capsule are the fontanelles.

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Fig. 138. A Coronal Section of the Skull of a Foetus, 5 months old.

The Fontanelles

There are five fontanelles connected with each parietal bone, one at each of its rounded angles, and one, the sagittal (Fig. 135), which occurs between the radiating fibres of the parietal near the posterior end of the sagittal suture. The parietal foramen marks its position in the adult. In about 15 % of children this fontanelle is unclosed at birth ; a large parietal foramen may permanently mark its situation. The posterior inferior fontanelle, situated at the asterion (Fig. 135), the anterior inferior at the pterion, and the posterior superior at the lambda, close before or about the time of birth. Separate ossifications, which become Wormian bones, are often developed in the primitive capsule of the skuU at those three fontanelles and thus close them. The anterior superior fontanelle, at the bregma, cannot be distinctly felt during life after the first year (Warner), but it is not completely closed until the second year is nearly over. This fontanelle is lozenge-shaped, being bounded by four bones, viz. the two parietals and two frontals. The bregmatic or anterior superior and lambdoid or posterior superior fontanelles are median and common to both parietals.

The membrane-formed bones consist at first of a thin lamella of osseous fibres radiating out from the point at which ossification commenced. The osteoblasts beneath the pericranium on the outer surface of the lamella and the dura mater on the inner surface, deposit bone, and by the 5th year an outer and an inner table, with diploic tissue between, are developed. Into the diploe of the frontal bone protrude the growing buds of the two frontal sinuses. As the brain expands new bone is formed at the sutures to increase the capacity of the skull, but the operation of craniotomy to allow the expansion of a confined brain, by the formation of a new suture, is founded on the assumption that the arrest of brain-growth in microcephalic idiots is due to the closure of the sutures, whereas it is probably due to an inherent defect in the growth of the brain. We frequently see skulls where one or more sutures have been prenaaturely closed, but in such cases there has been compensatory growth at other sutures, giving rise to a peculiarity in cranial form. Growth of the cranial cavity could take place by a deposit of bone on the outer table and an absorption from the inner ; for this manner of growth, sutures are unnecessary. The synostosis of the sutures does not necessarily prevent growth ; synostosis of the skull bones occurs only when the brain has ceased to expand. If the brain of the infant is arrested in its growth, premature ossification of the sutures occurs, the condition of microcephaly resulting therefrom. In hydrocephaly, when the ventricles become enormously dilated, the membranous capsule of the cranium expands so quickly that the process of ossification cannot keep up with its rapid growth. Hence in hydrocephaly the fontanelles are enormous. The growing points of ossific fibres are detached and form Wormian bones. The cartilaginous part of the skull is scarcely afiected in this disease. The membrane-formed part of the skull is liable to diseases which do not affect the cartilage-formed part. The dura mater is very adherent to the bones formed in cartilage.

Development of Bones formed in Cartilage

1 The Occipital Bone

The occipital bone is developed from the parachordal cartilages. Two cartilaginous bars, although appearing separately in the development of fishes, are united from their first appearance in the human embryo, forming a basilar plate (Robinson). The plate is formed in the mesenchymal sheath of the notochord, its centre of chondrification — the first in the base of the skull- — appearing at the end of the 1st month of development. The basal plate may be regarded as a continuation of the vertebral bodies, while the lateral processes (Fig. 140) which are perforated at their bases by the foramen or foramina for the hypoglossal nerve and which separate the jugular foramen in front from the foramen magnum behind, may be regarded as a continuation of the neural arch series.^ Fused to the lateral process and also to the otic capsule is the roof plate already mentioned — the tectal plate (Fig. 132). While the lateral processes never reach the posterior or dorsal margin of the foramen magnum, it is quite otherwise with the right and left tectal plates ; they extend round the hind-brain until they meet and unite, thus forming the posterior margin of the foramen magnum and the supra-occipital plate of cartilage. Thus the cartilaginous basis of the occipital bone is formed out of three elements on each side — the basal j^late representing the centre and hypochordal arches of cervical vertebrae, the lateral processes, corresponding to the neural vertebral arches and an extra element — the tectal plate.

^ For the variations in the manifestation of partly separated occipital vertebrae see Gladstone and Powell, Journ. Anat. 1915, vol. 49, p. 190 ; Elliot Smith, Brit. Med. Journ. 1908, II. p. 594. See also references p. 56.

In Fig. 140 the condition of the occipital region is shown in a 5th-month foetus. Four centres of ossification appear in the tectum (Fig. 137), and quickly fuse to form the cartilaginous j)art of the supra-occipital. A suture between the membranous and cartilaginous parts is clearly visible — especially near the fontanelle at the asterion. The membranous and cartilaginous parts of the supra-occipital become completely fused soon after birth. It will be observed that the process of fusion between the lateral parts of the cartilaginous supra-occipital is not complete in the 5th month (Fig. 140). The occipital fontanelle projects upwards between them from the foramen magnum. This fontanelle is filled by a continuation of the posterior atlanto-occipital ligament, and becomes closed soon after birth. It is the most common site of a cerebral meningocele — ^a saccular protrusion of the membranes of the brain which contains cerebro-spinal fluid, and usually a part of the occipital lobes distended by a dilatation of the posterior horns of the lateral ventricles.

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Fig. 139. Cranial Aspect of the Basal Plate and Occipital Parts of the Chondrocranium of a Human Foetus in the 8th week of development. (Warren Lewis.)

Separate centres of ossification appear in the occipital cartilages to form (1) the basi-occipital, (2) the two exoccipitals, and (3) the supra-occipital.^ The occipital consists of four pieces until the fourth year, when synostosis occurs. The occipital condyles are formed from the exoccipitals and basi-occipital, the exoccipital element constituting in the adult by far the larger part, but when the condyles first appear they are continuous at the anterior border of the foramen magnum, forming a single or median condyle as in reptiles, birds, and lower mammals. The foramen for the hypoglossal nerve, which may be subdivided into two or even three compartments, is formed between the two elements and thus corresponds to the inter-vertebral series. The occipital protuberance is formed by both membranous and cartilaginous parts of the supra-occipital.

^ For a very complete account of the dates at which all centres of ossification appear in the skeleton see Mall, A7ner. Jouni. of Anat. 1906, vol. 5, p. 433.

2 The Petro-mastoid forms part of the base of the skull

We shall see that the petrous bone (p. 224) is primarily developed as an independent cartilaginous capsule round the inner ear, but at an early date (6th week) it fuses at certain points with the parachordal basis of the occipital bone, parietal lambda while an extension from the mastoid part of the capsule enters into the formation of the tectum. Even as late as the thirtieth year remnants of the tectal cartilage may be found between the petro-mastoid and occipital bones, especially between the jugular process of the occipital and the mastoid. The fibro-cartilage in the foramen lacerum medium and perhaps Eustachian cartilage, which is continuous with it, are remnants of the periotic cartilaginous capsule.

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Fig. 140. The Occipital Eegion, in a Foetus of 5 months.

3 Trabeculae Cranii

The basilar plate, containing the notochord and fashioned out of the parachordal cartilages terminates in the dorsum sellae, in the hind wall of the pituitary fossa. The prechordal part of the base of the skull, in the lowest vertebrates, appears first as two irregular plates of cartilage — the trabeculae cranii (Fig. 141). Even in the mammalian skull the trabeculae can still be traced in the pituitary region (Fawcett). Their posterior extremities fuse round the anterior termination of the notochord with the basilar plate. The buccal part of the pituitary grows into the cranial cavity in front of the notochord and keeps the two cartilages apart ; but in front of the pituitary the two bars fuse in the middle line. The mesial fused parts of the trabeculae grow into the embryological basis of the nasal septum (Fig. 142). The posterior part of the median fused bars forms the cartilaginous basis of the pre-sphenoid and basi-sphenoid (Fig. 142).

Development of the Sphenoid

Recently Professor Fawcett[1] has examined the manner in which the cartilaginous basis of the sphenoid is formed in the human embryo. The mesodermal or mesenchymatous basis of the sphenoid becomes chondrified during the second month — right and left centres representing the original trabeculae. While the cartilage, in which the centres for ossification of the basi- and pre-sphenoids appear, is formed out of the trab ecular or prechordal plate, the great and small wings have a separate origin. We have already seen (p. 137) that on each side of the prechordal plate there are formed two plates of cartilage, rudiments of the lateral wall and roof of the primitive cartilaginous cranium (Fig. 133). The anterior of these — the orbito-sphenoids — form the cartilaginous basis of the lesser wings. In the 8th week of development (Fig. 142) each is a sickle-shaped plate lying over the stalk of the optic vesicle, sending one process under the optic nerve to join the cartilaginous prominence — the processus hypochiasmata — from which the muscles of the eyeball take origin. The other process of the orbitosphenoid fuses with its fellow above the prechordal plate and thus completes the optic foramina (Fig. 143). The great wing or alisphenoid arises in a rather complicated manner. In

[[File:Keith1921 fig141.jpg|600px|alt=Fig. 141

Fig. 141. Diagram of the Trabeculae Cranii, Parachordal Cartilages, and Periotic Capsules.

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Fig. 142. The Prechordal Base of the Chondrocranium in the 8th week of development. (Warren Lewis.)

manner the 8th week it is represented by two small nodular masses of cartilage (Fig. 142), the alar process attached to the prechordal plate and the temporal wing. The internal carotid artery lies on the mesial side of the alar process, which is represented merely by the Lingular process of the fullvdeveloped bone. The temporal wing lies under the Gasserian ganglion and separates the 2nd from the 3rd division of the nerve. The mesodermal tissues round the temj^oral wing undergo a secondary chondrification, and it is from this new formation that the greater part of the ali-sphenoid is formed ; ■ as it extends it encloses the 2nd and 3rd divisions of the fifth nerve, the round and oval foramina being thus formed. A gap remains between the orbito-sphenoid and ali-sphenoid to form the sphenoidal fissure. The dorsum sellae may have a separate centre of chondrification. At birth the sphenoid bone consists of three parts, the great wings being separated from the rest of the bone. The sphenoidal turbinate bones, afterwards inflated by the development of the sphenoidal air sinuses, are then nodules of bone, surrounded by cartilage. They also are separate and are derived from the lateral ethmoidal cartilaginous plates which represent the olfactory capsule. The internal pterygoid plates are also separate ossifications laid down in the membrane over a plate of cartilage, representing part of the palato-quadrate bar of lower vertebrates (Fig. 133). Only its hamular process is formed in cartilage (Fawcett). The internal becomes adherent to the external plate during the fourth month of foetal life. The external plate is developed as a membranous outgrowth from the ali-sphenoids or great wings. The pre-sphenoid unites with the basi-sphenoid in the 8th month ; the great wings unite with the basisphenoid soon after birth. The lingula (alar proc. Fig. 143, B) which bounds the outer side of carotid groove is ossified from a centre which appears during the 4th month of foetal life.

File:Keith1921 fig143.jpg

Fig. 143, A. Left half of the Cartilaginous Basis of the Skull in a Foetus of 3.5 months. (After Kollmann.) B. Right half of the Cartilaginous Basis of the Skull in a Foetus of 2i months. (After Fawcett.)

the wings of the splienoid develop in the orbital region of the primitive skull (Fig. 133). The enormous expansion of the cerebral vesicles and the evolution of a new system of mastication have worked a revolution in the primitive orbital region ; the temporal lobes, as it were, have burst the ancient cartilaginous wall. The ala-temporalis appears first in the embryo as a process from which the muscles of mastication take origin (Fawcett).

The Pituitary Body is developed between the trabeculae cranii ; the pre-sphenoid is formed in front of it and the basi-sphenoid behind it. A canal may remain in the foetal or even adult bone to mark the point of ingress of the buccal part of the pituitary.[2] The wings of the vomer cover the opening of the pituitary canal on the pharyngeal aspect of the skull, if it be present. On the cerebral aspect it opens at the olivary eminence which also marks the union of the pre- and the basi-sphenoids. The writer has seen a child, in which the trabecular cartilages had remained apart, leaving a wide gap through which the pituitary projected within the septum of the nose. The pre-sphenoid and afterwards the basi-sj^henoid are much altered by the growth of the sphenoidal sinuses which commence to expand rapidly about the 7th year.[3] The great wings support the temporal poles of the brain, their size depending on the development of that part of the brain. They are much larger in man than in any other mammal, owing to the great size of the human temporal lobes. The small wings project within the vallecula Sylvii. In the early foetus the dorsum sellae is enormously developed, and fills the deep and sharp angle between the mid-brain and fore-brain (Fig. 85).

File:Keith1921 fig144.jpg

Fig. 144. The Sphenoid in a Foetus of 4 months. The Centres of Ossification are deeply shaded. (After Sappey.)

The Ethmoid

The cartilaginous basis of the skull is completed in front by the ethmoid ; on its upper surface rest the olfactory bulbs. In the primitive skull (Figs. 130, 133) the olfactory capsule, out of which the cartilaginous ethmoid has been evolved, is far in front of the space which contains the fore-brain. It has been brought within the floor of the cranial cavity by a double process — by a shortening of that part of the trabecular plate which unites the sphenoid to the olfactory or ethmoidal capsule, and by the forward extension of the cerebral vesicles which have pushed their way into the forehead until they project beyond the olfactory region. The cribriform plate is formed in the 4th month ; up till then a gap separates the lateral mass from the septal or trabecular plate (Fig. 143). Formation of Foramina in Bone. — The foramina of the skull are formed in one of three ways (Bland-Sutton) :

  1. By the union of two bones ; examples of this form are the jugular foramen, sphenoidal fissure, Glaserian fissure, etc.
  2. By the union of two elements of one bone ; the anterior condyloid foramina, optic foramina, the foramen magnum, aqueductus Fallopii, etc.
  3. By the enclosure of a notch on the edge of a bone of which the foramen ovale is the best example. This foramen is at first a notch in the posterior border of the great wing of the sphenoid (Fig. 144) ; it remains in this condition in all mammals except man. In him the margins of the bone on each side grow out and fuse, and thus convert the notch into a foramen. Other examples are the foramen spinosum, the foramen rotundum, parietal foramen, mastoid, etc.

Wormian Bones

In the six fontanelles which occur at the parietal angles ossific centres frequently appear. Fontanelle ossifications form Wormian bones. They occur most frequently at the posterior angles of the parietal (Lambda and Asterion) ; they are also common at the Pterion (epipteric Wormian) but rare at the Bregma. The Wormian at the lastmentioned point receives the name of os anti-epilepticum. Much confusion has been caused by naming a large Wormian, which may occur in the lambdoidal (posterior-superior) fontanelle, the inter-parietal bone. Wormian or sutural bones are particularly numerous in the skulls of infants who have been the subjects of hydrocephaly. It is possible that, during the rapid expansion of the skull, the tips of ossifying fibres become detached, thus forming separate centres of ossification in the sutures and fontanelles.

The Inter-parietal Bone

It has already been shown that the part of the supra-occipital above the superior curved lines is developed from membrane by four centres of ossification, and is at first, and almost until birth, nearly separated from the lower part developed from cartilage (Figs. 137, 140). The membranous part of the supra-occipital represents the inter-parietal bone. In marsupials, ruminants and ungulates, the inter-parietals fuse with the parietals, and not with the occipital. In rodents they fuse with both occipitals and parietals. In primates and carnivora, as in man, they fuse with the occipital. It is extremely rare to find the whole inter-parietal as a separate bone in man, but a large Wormian, partly replacing the inter-parietal, is very frequent. Such a Wormian bone, if large, is named variously, os epactal, os Incae, os triquetrum, or pre-interparietal.

The Post-frontal does not occur in mammals as a separate bone ; in them it has fused with the frontal, and forms that part of the bone which articulates with the great wing of the sphenoid and malar. A Wormian bone — the epipteric— which is occasionally developed in the fontanelle at the pterion, may be mistaken for it. Traces of a true post-frontal, partly separated from the frontal, rarely occur in man,

The Cephalic Index

Anthropologists have employed the shape of the head as a character in classifying the races of mankind. The cephalic index is used to express the shaj)e of the head. It states the proportion that the breadth bears to the length of the skull (Figs. 145, A, B). The length or long diameter of the skull is usually measured from the glabella to the most projecting point of the occiput^commonly situated over the occipital poles of the brain ; the breadth or widest diameter is measured between the widest points — usually some distance below the parietal eminences. If the length of a skull is 100 mm. and the breadth 75, the cephalic index of that skull is 75, i.e. the breadth is 75 % of the length. Human races, on an average, are either Dolichocephalic (long-headed), the breadth being 75 % or less of the length ; Brachycephalic, in which the breadth is 80 % or more of length ; or Mesaticephalic, in which the breadth is between 75 % and 80 % of the length. Various methods are employed in estimating the height of the skull, but the best is that which takes the upper margin of the external auditory meatuses as representing the basal plane. The height is measured from this plane to the highest point in the sagittal suture, when the skull is oriented so that the lower border of the orbit and the middle of the meatus are in one plane (see Duckworth, Morphology and Anthropology).

File:Keith1921 fig145.jpg

Fig. 145, A. Diagram of a Long-head (Dolichocephalic). B. — Diagram of a Short-head (Brachycephalic).

The English people have an average cephalic index of 78, the South Germans 83, but it must be remembered the individuals of every race show a wide range of variation. It will be seen that the topography of the brain, worked out by German surgeons, cannot be applied to the longer English heads without modification.

Factors which determine the Shape of Head

The shape of the skull depends (1) on the size and shape of the brain ; (2) on the size and strength of the muscles which arise from it — the muscles of mastication, or are inserted to it — the muscles of the neck. Brain growth is by far the most important factor, but we do not know the conditions which flatten the brain from side to side in dolichocephalic races, or shorten it frora frontal pole to occipital pole in brachyceplialic races. Muscular action can only exercise a minor effect. Professor Arthur Thomson[4] has shown that there is a correlationship between dolichocephaly and the size of the temporal muscles — which are relatively large in long-headed races — and the shape and mechanism of the mandible. It is to be remembered that (1) the muscles of mastication and of the neck undergo their greatest development between the 12th and 28th years ; (2) before that time the brain has almost completely attained its adult size and shape ; (3) all the evidence obtained from measurements in the living indicates that the changes in cranial form which take place then affect its external contour, leaving the shape of the cranial cavity unaffected.

File:Keith1921 fig146.jpg

Fig. 146. Outlines of Abnormal Skulls, showing a contrast in shape.

Abnormal Crania

It is possible that light will be thrown on the factors which determine head-form by the study of certain pathological conditions.[5] In the disease known as Acromegaly, where there is always a great enlargement of the pituitary gland, the skull undergoes peculiar growth changes. The supra-orbital ridges become greatly developed, the face elongates, the temporal lines from which the temporal muscles arise, grow upwards on the side of the skull, thus increasing the area of the temporal muscles. At the same time the lines which mark the attachment of the muscles of the neck — the mastoid processes, superior curved lines and external occi2:)ital protuberance — also increase greatly in size. In achondroplasia and in rickets the skull assumes characteristic forms due to a disturbance in the growth of the base of the skull. To a certain degree the growth of the cranial bones is regulated by internal secretions. In Fig. 146 two common types of abnormal skull forms are shown. They are contrasted types ; in one — Acrocephaly or steeple-skull — -the base is abnormally short, owing to an arrest of growth at the junction of the presphenoid and ethmoid. Compensation is obtained by an upward growth of the brain, thus heightening the roof. In severe cases the optic nerves may be pressed on, and blindness thus caused. In the second type — Scaphocephaly, or boat-shaped skull — the cranium is very narrow from side to side, while the calvarial arc — from nasion to opisthion (posterior border of foramen magnum) — is greatly elongated. In scaphocephaly there is an arrest of growth — often a synostosis — along the sagittal suture. In acrocephaly the coronal suture is closed. In these two, and in allied conditions, there is a certain amount of evidence which points to a disturbance in the function of the glands of internal secretion.

File:Keith1921 fig147.jpg

Fig. 147, A. The Facial Angle as estimated by two lines drawn from the Nasion to the Basion and to the Prosthion (incisor alveolus). B. Method of estimating the degree of flexion and extension of the cranial axis, a, anterior border of cribriform plate ; b, on olivary groove in front of olivary eminence, a, b, trabecular axis ; b — basion = the chordal axis. The angle of flexion is contained by the two lines meeting at b.

The Facial Angle

The Facial Angle[6] is the angle at which the face projects from the axis of the skull (Figs. 147, 148). The skull consists in man, as in all mammals, of two parts — the facial part (splanchnocranium), which carries the teeth and is developed according to their size, and the brain capsule (neurocranium), which depends on the size of the brain. The smaller the brain and the larger the face, the more does the face project in front of the skull, and, therefore, the greater is the facial angle, and vice versa. It will thus be seen that the facial angle is to a certain degree an index of brain development. It is smallest in the most highly developed races of man ; it is larger in the lower races, and larger still in the anthropoids ; it increases in size with the advent of the permanent teeth and the necessary increase in the size of the face. It is, therefore, greater in the adult than in the newly born.

Flexion of the Cranial Axis

In Figs. 147, A, and 148 the axis of the cranial base is represented by a line drawn from basion to nasion, but it is quite apparent that this line does not represent the axis accurately. The truth is that there are two parts in the cranial axis which are functionally as well as morphologically distinct, the chordal and prechordal parts (p. 136). In the higher primates — especially in man — the prechordal part is bent downwards — or flexed — on the chordal. The manner in which the degree or angle of flexion may be measured is shown in Fig'. 147, B ; it is a much openej" angle in anthropoids than in man. The degree of flexion is most variable in man ; in cases where the flexion is great the forehead is projecting and the face receding, the facial angle being apparently small. If there is a great degree of extension of the axis, then the forehead is receding, and the lower part of the face projecting or prognathous. Thus the facial angle is not a safe guide to the degree of prognathism or face projection, because it may be exaggerated or masked by the extension or flexion of the cranial base.

File:Keith1921 fig148.jpg

Fig. 148. Profile of the Cranium of an Immature Chimpanzee, showing the ascent of the Temporal Ridges, the formation of Occipital Crests and the lines of the Facial Angle.

The Para-occipital Process is sometimes present in man, and projects downwards from the jugular process of the occipital bone. The rectus capitis lateralis is inserted to it. The process represents the para-occipital process, which is so highly developed in four-footed mammals. The paramastoid process projects from the temporal bone lateral to the para-occipital (Parsons).

Upgrowth of the Temporal and Occipital Ridges or Curved Lines.— In lower animals, such as the ape or dog, a great increase in the development of the temporal and nuchal muscles takes place with the eruption of the permanent teeth, the area of their origin from the skull being necessarily enlarged. The ridges of bone which mark the limit of attachment of these muscles, the temporal and occipital ridges, ascend on the skull as waves of bone before the growing muscles. The ridges may meet, as in apes, along the sagittal and lambdoidal sutures and form great crest-like upgrowths. In Fig. 148 the position of the temporal lines in a juvenile chimpanzee is shown ; they are approaching the sagittal suture. They have extended backwards, and met with the occipital lines, which are ascending above the attachment of the growing muscles of the neck. The temporal and occipital lines are seen to be fused together to form a temporooccipital crest. At the same time the temporal lines spread forwards on the frontal region, the frontal extension being accompanied by a marked growth of the supra-orbital ridges and of the zygomatic arches. Thus the skull is modified by the growth of the muscles of mastication and of the neck. In man these changes also take place, but to a less extent than in anthropoids. At birth the temporal lines are just above the lower border of the parietal bones. During the second year the mastoid part of the ridge for the attachment of the neck muscles grows downwards into a pyramidal process — the mastoid — which is peculiar to the human species. In Neanderthal man, the mastoid process is shaped as in anthropoids.[7]

File:Keith1921 fig149.jpg

Fig. 149. Scheme of a Segmental Head Cavity and of the various parts formed from it.

Segmentation Theory of the Skull[8]

It is inferred from investigations made on the developing heads of fishes and amphibians that each primitive cephalic segment contains a cavity comparable to that seen in each body segment (p. 67), from the wall of which are developed (see Fig. 149) : (1) a sclerotome, (2) muscle plate, (3) skin plate, (4) modified nephrotome, (5) a ventral part of the walls join in the formation of the coelom. A part of each segment, on the lateral aspect of the fore-gut, is modified to form a visceral arch (Fig. 149). The sclerotome of each segment forms (1) a cartilaginous sheath for the notochord, (2) a cartilaginous roof for the neural tube, (3) a process which runs into the branchial part of the segment. The number of segments in the mammalian head is by no means settled ; on the evidence of the cranial nerves the number appears to be seven (p. 98), but certain considerations, specially relating to the facial and branchial structures, which we proceed to examine in the next chapter, lead us to suspect that the number is nine — the number of neuromeres which are marked out on the hind-brain.

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Fig. 150. A schematic diagram of the segmental elements of the Skull. The numbers refer to the Cartilaginous Bars of the various Visceral Arches. The 4th and 5th are combined in the Hyoid Bone, the 6th and 7th in the Thyroid Cartilage, the 8th (and 9th ?) in the Arytenoid, Cricoid, and Tracheal Cartilages.

In Fig. 150 a diagrammatic representation is given of one of the many segmental theories of the skull. The parachordal plate represents the unseparated centra of the nine segments. The primitive neural arches have been disturbed by (1) the enormous enlargement of the neural tube, but especially by the expansion of that tube in front of the notochord and parachordal plate to form the cerebrum and basal ganglia. In amphioxus the neural tube does not extend beyond the notochord. All that remains of the neural arches of the nine primitive segments are the lateral occipital cartilaginous processes (Fig. 139). Of the cartilaginous processes of the nine segments the 1st form the trabeculae cranii (Huxley, Howes) ; with the forward protrusion of the neural tube these come to form part of the base of the skull ; the 2nd form the palato-quadrate bars. Both of these processes are preoral. The 3rd forms the mandibular bar, the 4th the hyoid bar, the 5th, 6th, 7th, 8th form the cartilaginous bars in the 1st, 2nd, 3rd and 4th branchial arches. The reader will see that if the first and last cartilages are rejected as having no segmental significance, the theory put forward here is identical with that formulated in connection with the cranial nerves. We are at least justified in assuming that the parachordal part of the skull is the oldest, and is therefore known as the palaeocranium ; whereas the prechordal part is more recent and is for this reason known as the neocranium. Further details relating to the facial and pharyngeal parts of the head will be given in the following chapters.

Gaskell's Theory

Gaskell[9] regarded the trabecular or prechordal part of the vertebrate head as a derivative of the prosoma, and the parachordal part from the mesosoma of an invertebrate form such as is now exemplified by the Kingcrab (Limulus). The prosoma carries 7 pairs of appendages which surround the mouth. The last of these represents the mandible, the first, the nasal processes ; the intermediate appendages are combined in the maxillary j)rocesses. The mesosoma carries processes which serve for respiration and locomotion. In vertebrates these are modified to form branchial arches.

  1. <pubmed>17232851</pubmed> See also references, p. 135.
  2. H. Wrai, Anat. Hefte, 1907, vol. 33, p. 411 (Cranio-pharyngeal Canal).
  3. V. Z. Cope, "Ossific. of Sphenoid," Journ. Anat, 1917, vol. 51, p. 127,
  4. Arthur Thomson, Man's Cranial Form, Oxford, 1903.
  5. For skull in achondroplasia see Dr. Murk Jansen, Achondroplasia, Leyden, 1912 ; A. Keith, Journ. Anat. and Physiol. 1913, vol. 47, p. 189. For AcromegaUc changes: Keith, Lancet, 1911, vol. 1, p. 993.
  6. For a description of the various methods of estimating the facial angle see Duckworth's Morphology and Anthropology, 2nd Edition, 1915.
  7. See Keith, Journ. Anat. and Physiol. 1910, vol. 44, p. 251.
  8. Some researches on the morphology and segmentation of the skull are : W. H. Gaskell, Origin of Vertebrates, London, 1910 ; E. S. Goodrich, Proc. Zool. Soc. Lond. 1911, p. 101 ; W. E. Agar, Proc. Roy. Soc. Edin. 1907, Feb. 4th; Schumacher, .4 ?ia«. Anz. 1907, vol. 31, p. 145; Gaupp, Verhand. Anat. Oesellsch. 1907, p. 129; Greil, ibid. p. 59 ; F. H. Edgeworth, Quart. Journ. Mic. 8c. 1911, vol. 56, p. 167, Journ. Anat. and Physiol. 1903, vol. 37, p. 73 ; J. W. van Wijhe, Petrxis Camper. 1906, vol. 4, p. 1 ; A. Meek, Journ. of Anat. and Physiol. 1911, vol. 45, p. 357 (Dev. Skull of Crocodile) ; W. Wright, Lancet, 1909, vol. 1, p: 669 (Morphology and Variations of Skull). See also references on p. 135.
  9. See Origin of Vertebrates, London, 1910.

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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