Book - Human Embryology and Morphology 13
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Chapter XIII. The Cranium
Development of the Skull. — The facial parts of the skull have already been dealt with (Chap. I.). Only those bones which enter into the formation of the cranial cavity and help to form the brain chamber are dealt with here. 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 4 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 recognisable at an early stage of development, but at no period in the development of the embryo have the anterior five segments been seen to be demarcated.
The Primitive Membranous Skull. — The brain is developed in the same manner as the spinal cord from the medullary plates of the neural groove (Fig. 69, p. 90). In the same manner the mesoblast grows under and over the cephalic part of the neural canal, and forms for it a membranous covering. The covering of mesoblast thus formed is the primitive basis of the skull in the embryo.
The Evolution of the Mammalian Cranium. — It is not possible to understand the manner in which the bones of the human cranial cavity are developed without some reference to comparative anatomy. Only the base of the human skull is developed in cartilage, the rest is developed in membrane. How has that come to be ? 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 mesoblastic 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 whole calvarium and sides 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.
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 calvarium, represent a later addition.
Development of the Roof (membranous or dermal part) of the Skull. — In the 7th week of foetal life there appear on each side of the membranous cranial capsule four centres of ossification :
- For the frontal bone at a point which becomes afterwards the frontal eminence (Fig. 130);
- For the parietal, at the position of the parietal eminence ;
- For the squamosal — at the base of the zygoma (Fig. 130) ;
- For the membranous part of supra-occipital (part above superior curved line).
Fig. 130. The Centres of Ossification for the Dermal Bones of the Skull. The Bones which are formed in Cartilage are stippled.
The two occipital centres fuse early into one at the position of the external occipital protuberance. The two frontal ossifications fuse about the end of the first year ; the metopic suture which separates them disappearing then. This suture occasionally persists. The parietal bones fuse together, at the sagittal suture, late in life, commonly between the 35 th and 45 th year. The squamosal partly covers the petro-mastoid cartilaginous element and fuses with it in the first year, the temporal bone being thus formed.
The Manner in which these Bones are Developed. — In Fig. 131 a vertical section of the skull of a foetus 4.5 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). Spicules of bone which form the parietal are seen developing within the fibrous capsule and radiating out from the centre of ossification. 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.
Fig. 131. A coronal section of the Skull of a Foetus, 4.5 months old.
As the bony spicules 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 spicules of bone 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 unossified parts of the membranous capsule are the fontanelles.
There are five fontanelles connected with each parietal bone, one at each of its rounded angles, and one, the sagittal (Fig. 130) 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. 130), 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 skull 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 anteriorsuperior 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 inner table, with deploic 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 a wrong principle. Expansion of the cranial cavity takes place principally by a deposit 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 hydrocephalus, 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 hydrocephalus the fontanelles are enormous. The cartilaginous part of the skull is scarcely affected in this disease. The membrane-formed part of the skull is liable to diseases which do not affect the cartilageformed part. The dura mater is very adherent to the bones formed in cartilage.
Development of the Cartilaginous part of the Skull
The Occipital Bone
The occipital bone is developed from the parachordal cartilages, two cartilaginous bars which partly surround the cranial part of the notochord (Figs. 133 and 121). The parachordal cartilages represent in the skull the cartilaginous sheath of the notochord out of which, in the spinal column, the bodies of the vertebrae are developed. Each cartilage throws out a wing (Fig. 134); these meet over the hind brain and form the exoccipitals and cartilaginous part of the supra-occipital, and thus enclose the foramen magnum. In Fig. 132 the condition of the occipital region is shown in a 5 th -month foetus. The supra-occipital parts of the parachordal cartilages have fused. 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 at the 5th month (Fig. 132). The occipital fontanelle (Sutton) projects upwards between them from the foramen magnum (Fig. 132). 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 possibly also a part of the brain.
Fig. 132. The Occipital Region in a Foetus of 5 months.
Separate centres of ossification appear in the parachordal cartilages to form (1) the basi-occipital, (2) the two exoccipitals, and (3) the supra-occipital. The occipital consists of these four pieces until the fourth year, when synostosis occurs. The occipital condyles are formed from the exoccipitals and basioccipital, the exoccipital element constituting by far the larger part. The anterior condylar foramen is formed between these two parts. The occipital protuberance is formed by both membranous and cartilaginous parts of the supra-occipital.
Fig. 133. The Parachordal Cartilages out of which the Cartilaginous Parts of the Occipital Bone are formed.
Fig. 134. The expansion backwards of the Parachordal Cartilages to enclose the Foramen Magnum and form the Supra-occipital.
The Petro-mastoid forms part of the base of the skull
We have already seen that the petro-mastoid part of the temporal bone is developed out of the cartilage which forms the periotic capsule (Figs. 35, p. 50, and 135). The periotic cartilages fuse at points with the parachordal, which form the basis of the occipital bone. Even to a late stage (30th year or later) remnants of these cartilages may be found between the petromastoid and occipital bones, especially between the jugular process of the occipital and the mastoid (Fig. 136). The fibrocartilage in the foramen lacerum medium is a remnant of the periotic cartilaginous capsule. (See also p. 58.) .
(Figs. 135 and 136). — The notochord terminates behind the pituitary body and sella Turicae ; the parachordal cartilages develop above and at each side of it (Fig. 135). Two bars of cartilage — the trabeculae cranii — develop in the membranous basis of the embryonic brain capsule in front of the notochord and on each side of the pituitary body. In Fig. 136 is shown what become of these two cartilaginous bars. Their posterior extremities fuse round the anterior termination of the notochord with the parachordal cartilages. 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 mesial nasal processes of which they form the skeletal basis and become transformed into the primitive cartilaginous septum of the nasal cavities (Figs. 136 and 3, p. 3). The posterior segment of the median fused bars forms the cartilaginous basis of the pre-sphenoid and basi-sphenoid (Fig. 136). From the trabeculae four lateral processes or wings grow out on each side (Fig. 136). The posterior, which is small at first, forms the great wing of the sphenoid and external pterygoid plate; the second is originally large, and forms the small wings (orbito-sphenoids) ; the third and fourth outgrowths are closely joined, — they form the lateral masses of the ethmoid and alar cartilages of the nose (Fig. 6, page 6). The nasal bones, the lachrymal and ascending nasal processes of the superior maxilla, develop in the membrane over the lateral nasal wings of the trabeculae, in the same way as the vomer develops over the cartilage of the septum.
Fig. 135. Diagram of the Trabeculae Cranii, Parachordal Cartilages, and Periotio Capsules.
Fig. 136. Diagram of the structures formed from the Trabeculae Cranii.
Development of the Sphenoid
(Fig. 137). — At birth the sphenoid bone, which is developed by ossification of the posterior parts of the trabeculae cranii, 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 form part of the lateral ethmoidal cartilaginous plates. The internal pterygoid plates, formed from the pterygo-palatine bar, become adherent to the external pterygoid plates, which are developed as outgrowths from the ali-sphenoids or great wings. The pre-sphenoid unites with the basi-sphenoid in the 7th month ; the great wings unite with the basi-sphenoid soon after birth. The lingula which bounds the outer side of the carotid groove (Fig. 137) is ossified from a centre which appears during the 4th month of foetal life. The orbito-sphenoids unite over the pre-sphenoid and cover its cranial aspect.
Fig. 137. The Sphenoid in a foetus of 4 months. The Centres of Ossification are deeply shaded. (After Sappey.)
The Pituitary Body is developed between the trabeculae cranii ; the pre-sphenoid is formed in front of it and the basi-sphenoid behind it (p. 19). A canal may remain in the foetal or even adult bone to mark the point of ingress of the buccal part of the Pituitary, Fig. 167, p. 203. 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 pre-sphenoid and afterwards the basi-sphenoid are much altered by the growth of the sphenoidal sinuses which commence about the 7th year. 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. 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. 167).
Formation of Foramina in Bone
The foramina of the skull are formed in one of three ways (Sutton) :
- By the union of two bones; examples of this form are the jugular foramen, sphenoidal fissure, Glaserian fissure, etc.
- By the union of two elements of one bone ; the anterior condyloid foramina, optic foramina, the foramen magnum, aqueductus Fallopii, etc.
- 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. 137); 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.
In the six fontanelles which occur at the parietal angles separate ossific centres frequently appear and close them. The fontanelle ossifications form "Wormian bones". They occur most frequently at the posterior angles of the parietal (Lambda and Asterion) but they are also common at the Pterion (epipteric Wormian) and rare at the Bregma. The wormian at the last-mentioned 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.
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 two centres of ossification and is at first and almost until birth nearly separated from the lower part developed from cartilage (Pig. 132). 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 separate 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 suture round a wormian bone may be mistaken for a fissure or fracture when exposed by the trephine.
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 shape of the head. It states the proportion that the breadth bears to the length of the skull (Figs. 138 and 139). The length or long diameter of the skull is measured from the glabella to the inion (external occipital protuberance) ; the breadth or widest diameter is measured between the widest points' — 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; Brachycephalie, in which the breadth is 80°/ o or more of length; or Meso-cephalic, in which the breadth is between 75°/ D and 80°/ o of the length.
Fig. 138. Diagram of a Long-head (Dolichocephalic).
Fig. 139. Diagram of a Short-head (Brachycephalie).
The English people have an average index of 75, the South Germans 83, but it must be remembered the individuals vary widely. 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.
The Facial Angle is the angle at which the face projects from the base of the skull (Fig. 140). The skull consists in man, as in all mammals, of two parts — the facial part, which carries the teeth and is developed according to their size, and the brain capsule, 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.
Fig. 140. The Facial Angle of alEuropean contrasted with that of an Anthropoid.
It will thus be seen that the facial angle is a good 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.
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 para-mastoid process projects from the temporal at its outer side (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 crests, like that on a fireman's helmet. In man the temporal ridges and superior curved lines of the occipital bone also ascend with the eruption of the permanent teeth, but only to a slight extent. Man retains in the adult the condition seen in young apes.
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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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