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DEVELOPMENT OF THE CHONDRO-CRANIUM AND MORPHOLOGY OF THE SKULL.
As has been already stated, the chorda dorsalis or notochord extends headwards to a point immediately beneath the anterior end of the mid-brain. In front of this the head takes a bend so that the large fore-brain overlaps the anterior extremity of the notochord. At this stage of development the cerebral vesicles are enclosed in a membranous covering derived from the mesen
Superior orbital fissure
internus Subarcuate fossa
Orbital portion of orbito-sphenoid
Superior semicircular canal
FIG. 282. VIEW OF THE CHONDRO-CRANIUM OF A HUMAN FOETUS 5 CM. IN LENGTH FROM VERTEX TO Coccyx (about the middle of the third month); the cartilage is coloured blue. The line to the right of the drawing shows the actual size.
chyme surrounding the notochord; this differentiated mesodermal layer is called the primordial membranous cranium. From it the meninges which invest the brain are derived. In lower vertebrates this membranous capsule becomes converted into a thick-walled cartilaginous envelope, the primordial cartilaginous cranium. In mammals, however, only the basal part of this capsule becomes chondrified, the roof and part of the sides remaining membranous. In considering the chondrification of the skull in mammals, it must be noted that part only of the base is traversed by the notochord, viz., that portion which extends from the foramen magnum to the dorsum sellæ of the sphenoid. It is, therefore, conveniently divided into two parts-one posterior, surrounding the notochord, and hence called chordal, and one in front, into which the notochord does not extend, and hence termed prechordal. These correspond respectively to the vertebral and evertebral regions of Gegenbauer. In the generalised type, a pair of elongated cartilages called the parachordal cartilages appear on either side of the chorda in the chordal region, similarly in the prechordal region two curved strips of cartilage named the prechordal cartilages, or the trabeculæ cranii of Rathke, develop on either side of the cranio-pharyngeal canal. In the human embryo, however, this symmetrical arrangement has not hitherto been observed. In man, chondrification of the cranial base commences early in the second month and attains its maximum development about the end of the third month, at which time the chordal part of the chondrocranium consists of a ring of cartilage, the ventral part of which is formed by the fusion of two parachordal cartilages, so forming around the chorda dorsalis, a central axial part,
which comprises the basilar portion of the occipital bone. From this there pass extensions which form the lateral parts of the occipital bone, and serve to unite the occipital plate, as this part of the cartilaginous base is sometimes called, to the cartilaginous auditory capsules on either side. These latter are formed by the chondrification of the cochlear and canalicular parts of the labyrinth, which do not develop at the same rate, so that the part around the semicircular canals is completed much sooner than the cochlear portion; in consequence, at the end of the second month, the facial nerve and the genicular ganglion lie in a groove, to be subsequently converted into a canal, on the vestibular part of the capsule.
The dorsal part of the ring consists of a thin cartilaginous plate, the tectum posterius, from which is developed the only part (i.e. the inferior part of the occipital squama) of the cranial vault preformed in cartilage. In the membranous tissue from which this plate is developed chondrification at first begins, on either side, by an extension from the posterior aspect of the pars lateralis of the occipital; growing rapidly forwards this ultimately unites with the posterior and dorsal borders of the cartilaginous auditory capsule, from which it is for some time separated by a narrow membranous interval. At a later stage the cartilages of either side unite, dorsal to the foramen magnum, to form the tectum posterius or the tectum synoticum (Keibel and Mall). To the axial part of this portion of the chondrified base the chorda dorsalis has the following
FIG. 283.-OSSIFICATION ON BASE AND LATERAL WALLS OF SKULL OF FOUR AND A HALF MONTHS' FETUS (Schultze's method). Cartilage, blue; cartilage-bone, black; membrane-bone, red.
relations in front of the foramen magnum it runs for a short distance in a groove on the dorsal surface of the occipital plate, then pierces the cartilage so as to lie ventral to it in the retropharyngeal tissue, again enters the chondro-cranium by passing dorsalwards in the suture between the occipital plate and sphenoidal cartilage and ends dorsal to the latter cartilage.
The prechordal portion of the cartilaginous basis cranii in man displays the following features: at the third month it is irregularly diamond-shaped in outline, its posterior angle is wedged in between the two auditory capsules and is related to the anterior part of the axial portion of the occipital plate. The anterior angle forms the ventral end of the nasal capsule, whilst the lateral angles extend over the orbital cavities and correspond to the tips of the ale orbitales of the sphenoid.
Within this area chondrification takes place as follows (Bardeen). In the region of the posterior angle, above referred to, a cartilaginous nodule appears anterior and ventral to the end of the chorda dorsalis; from this arises the cartilaginous body of the sphenoid, the further development of which is associated with its union with the anterior end of the median portion of the occipital plate and the formation there, by the appearance of an independent transverse strip of cartilage, of the dorsum sellæ of the sphenoid; whilst from its anterior and superior surface on either side there extend forwards strips of cartilage which surround the hypophyseal pocket, and unite in front of it to form the anterior part of the body of the sphenoid, thereby enclosing the hypophyseal canal, which, at first wide, is gradually closed by the chondrification of its walls. It may, however, remain open.
The region occupied by the ala temporalis is slow to chondrify. According to Fawcett, the only part of it which is preformed in cartilage is that which corresponds to the root of the two pterygoid laminæ in the adult: this is, perforated by the maxillary division of the trigeminal nerve. According to the same authority, the whole of the lateral pterygoid lamina and that part of the ala temporalis projected into the orbital and temporal fossæ are ossified in membrane. So, too, are the foramen ovale and foramen spinosum.
The ala orbitalis, at first much larger than the ala temporalis, is described as chondrifying in the following way. The process begins by the appearance of cartilage posterior to the position of the optic foramen; medially this fuses with the lateral aspect of the anterior part of the body of the sphenoid, laterally it extends into the orbital plate, with the independent cartilaginous centre of which it unites. The foramen opticum is completed by the extension of the cartilage from the side of the anterior extremity of the body of the sphenoid, in front of the nerve, to reach the orbital plate. These three centres fuse to form a single piece of cartilage during the third month.
Anterior to the orbito-sphenoids, the base of the skull is intimately associated with the nasal capsule, and is the last part of the chondro-cranium to become cartilaginous, this change not being effected till the third month. The roof of the capsule is formed by the coalescence of cartilaginous elements appearing, first in the nasal septum by an extension of the cartilage from the ventral surface of the body of the sphenoid and secondly by an independent centre in each lateral wall of the capsule. At first the nasal capsule is open dorsally on either side of the nasal septum in correspondence with the olfactory bulbs, but during the third month the wall of the capsule corresponding to the cribriform plate commences to chondrify around the perforating nerve-fibres, and so the lamina cribrosa is preformed in cartilage. Laterally strips of cartilage (cartilago ethmosphenoidalis) pass backwards from the lateral edges of the cribriform plate to unite it with the anterior edges of the alæ orbitales of the sphenoid.
By the third month the nasal capsule has become cartilaginous. As has been stated above, the nasal septum chondrifies by an extension forwards of the ventral part of the body of the cartilaginous sphenoid. On either side of the ventral margin of this septum anteriorly are developed the paraseptal cartilages, which in man persist till after birth. These are connected posteriorly by means of a connective tissue bridge with small pieces of cartilage-the posterior paraseptal cartilages, which are in turn associated with the paranasal cartilages posteriorly, and there in part form the floor of the recessus terminalis or cupola of the cartilaginous nasal capsule (Fawcett). In man, owing to the deficiency of the lamina transversalis anterior, the fenestra narina and the fenestra basalis which pierce the floor of the nasal capsule, on either side of the septum, become confluent and form the fissura rostroventralis of Gaupp. Meanwhile the lateral walls of the nasal capsule are chondrifying independently, forming the paranasal cartilages. These become subsequently united anteriorly with the nasal septum to form the tectum nasi or roof of the nose. At first this is open posteriorly where it is in relation with the olfactory bulb, but later, as has been already described, the tissue around the nerve filaments chondrifies to form the cartilaginous lamina cribrosa. The inferior concha is derived from the cartilage of the lower and lateral part of the nasal capsule, from which, however, it becomes isolated about the seventh month. Above and behind this the middle and superior concha, the ethmoidal turbinals, become chondrified, as well as the cartilaginous rudiments which subsequently form the agger nasi, the bulla ethmoidalis, and the concha sphenoidale or ossiculum Bertini.
Throughout life certain parts of the cartilaginous nasal capsule persist as the cartilaginous nasal septum and the cartilages of the ale of the nose, whilst other parts are absorbed and are replaced by surrounding bones of membranous origin.
The various foramina met with in the cranial base are formed either as clefts in the line of union of the several cartilaginous elements, or through inclusion by means of bridging processes derived from these same elements.
From the ventral surface of this cartilaginous platform-formed, as described, by the union of the trabeculæ, parachordal cartilages, and cartilaginous auditory capsules-is suspended the cartilaginous framework of the visceral arches, which play so important a part in the development of the face, an account of which is elsewhere given.
A consideration of the facts of comparative anatomy and embryology appears to justify the assumption that the mammalian skull is of twofold origin-that, in fact, it is composed of two envelopes, an outer and an inner, primarily distinct, but which in the process of evolution have become intimately fused together. The inner, called the primordial skull, is that which has just been described, and consists of the chondro-cranium and the branchial skeleton. The outer, which is of dermic origin, includes the bones of the cranial vault and face which are developed in membrane. This secondary skull, which first appears in higher fishes as ossified dermal plates overlying the primary skull, acquires a great importance in the mammalia, as owing to the expansion of the brain and the progressive reduction of the chondro-cranium, these derinal bones become engrafted on and incorporated with the primordial skull, and act as covering bones to the cavities of the cranium and face; for it may be well to point out that these dermal or membrane bones are not necessarily external in position, as over the cranial vault, but also develop in the tissues underlying the mucous membrane of the cavities of the face.
Advantage is taken of this difference in the mode of development of the bones of the skull to classify them according to their origin into cartilage or primordial bones, and membrane or secondary bones. These differences in the growth of the bone must not be too much insisted on in determining the homologies of the bones of the skull, as it is now generally recognised that
all bone is of membranous origin, and that whilst in some cases cartilage may become calcified, it never undergoes conversion into true bone, but is replaced by ossific deposit derived from a membranous source. In the subsequent growth of the skull, parts of the cartilaginous cranium persist as the septal and alar cartilages of the nose, whilst for a considerable period the basisphenoid and basi-occipital are still united by cartilage. The cartilage also which blocks the foramen lacerum may be regarded as a remnant of the chondro-cranium.
Whilst in many instances the primordial and secondary bones remain distinct in the fullydeveloped condition, they sometimes fuse to form complex bones, such as the temporal and sphenoid.
Various theories have been advanced to account for the mode of formation of the skull. The earliest of these was called the vertebrate theory, which assumed that the cranium was built up of a series of modified vertebræ, the bodies of which corresponded to the basi-cranial axis, whilst the vertebral arches were represented by the covering bones of the cranium. In view of the more recent researches regarding the composite origin of the skull above referred to, this theory was necessarily abandoned. It gave way to the suggestion of Gegenbauer that the primordial cranium has arisen by the fusion of several segments equivalent to vertebræ, the number of which he determined by noting the metameric arrangement of the cerebral nerves, of which he concluded there were nine pairs, arranged much like spinal nerves, both as to their origin and distribution. The olfactory and optic nerves, though frequently referred to as cerebral nerves, are excluded, since from the nature of their development they are to be regarded as metamorphosed parts of the brain itself. Gegenbauer therefore assumed that that portion of the cranial base which is traversed by the nine pairs of segmentally arranged cerebral nerves must be formed by the fusion of nine vertebral segments; and as the region where the nerves escape corresponds to the part of the chondro-cranium traversed by the notochord, he calls it the vertebral portion of the cranial base, in contradistinction to the trabecular or non-vertebral part which lies in front. This latter he regards as a new formation adapted to receive the greatlydeveloped brain and afford protection to the organs of sight and smell.
As has been pointed out by Hertwig, there is an essential difference between the development of the axial cartilaginous skeleton of the trunk and head. The former becomes segmented into distinct vertebræ alternating with intervertebral ligaments; whilst the latter, in order to attain the rigidity necessary in this part of the skeleton, is never so divided. It follows from this that the original segmentation of the head is only expressed in three ways, viz., in the appearance of several primitive segments (myotomes), in the arrangement of the cerebral nerves, and in the fundament of the visceral skeleton (visceral arches). According to Froriep, the mammalian occipital corresponds to the fusion of four vertebræ, and there is some reason for supposing that in some classes of vertebrates the occipital region of the primordial cranium is increased by fusion with the higher cervical segments.
The form of skull characteristic of man is dependent on the large proportionate development of the cranial part, which contains the brain, and the reduction in size of the visceral part (face), which protects the organs of special sense. This leads to a decrease in the mass and projection of the jaws, as well as a reduction in the size of the teeth. Associated with the smaller mandible there is a feebler musculature, with a reduced area of attachment to the sides of the skull. In this way the disappearance of the muscular crests and fossæ, so characteristic of lower forms, is accounted for. At the same time the fact that the skull is poised on the summit of a vertical column, leads to important modifications in its structure. The disposition of parts is such that the occipito-vertebral articulation is so placed that the fore and hind parts of the head nearly balance each other, thus obviating the necessity for a powerful muscular and ligamentous mechanism to hold the head erect.
Another noticeable feature in connexion with man's skeleton is the prolonged period during which growth may occur before maturity is reached; this is associated with a more complete consolidation of the skull, since bones, which in lower forms remain throughout life distinct, are in man fused with each other, as exemplified in the case of the presphenoid and postsphenoid, the occipital and the interparietal, to mention one or two instances among many. It is noteworthy, however, that during ontogeny the morphological significance of these bones is clearly demonstrated by their independent ossification.
The points of exit of the various cerebral nerves remain remarkably constant, and in their primitive condition serve to suggest the segmental arrangement of the cartilaginous chondrocranium already referred to. Owing to the very great modifications which the mammalian skull has undergone in the process of its evolution, it may be pointed out that the passage of the nerves through the dura mater-a derivative, the readers may be reminded, of the primordial membranous cranium (see ante)-alone represents the primitive disposition of the nerves. Their subsequent escape through the bony base is a later and secondary development. In some cases the two, membranous or primary and the osseous or secondary foramina, correspond. In other instances the exit of the nerves through the dura mater does not coincide with the passage through the bone.
Of interest in this connexion it may be pointed out that the foramina and canals which traverse the skull are either situated in the line of suture between adjacent bones or in the line of fusion of the constituent parts of which the bone pierced is made up. For example, the superior orbital fissure is situated between the orbito and alisphenoids; the hypoglossal canal between the basi and exoccipitals; the jugular between the petrous, basi, and exoccipital; the optic between the orbito-sphenoid and the presphenoid; the pterygoid between the alisphenoid, medial pterygoid lamina, and the lingula.
MORPHOLOGY OF THE LIMBS.
Development and Morphology of the Appendicular Skeleton.
FIG. 284.-DIAGRAM TO ILLUSTRATE THE HOMOLOGIES OF
THE BONES OF THE LIMBS.
The two limb buds of an embryo prior to flexion and rotation. The anterior or pre-axial border is coloured red; the posterior or post-axial border, blue. B. After the assumption of the foetal position. Flexion and rotation have now taken place. The red and blue lines indicate the altered position of the pre-axial and post-axial borders. C. The fully developed limbs with the flexor aspects directed towards the reader. The coloured lines indicate the effect of the torsion of the upper segment of the limb through quarters of a circle.
The paired limbs first appear in the human embryo about the third week as small buds on either side of the cephalic and caudal ends of the trunk. That these outgrowths are derived from a large number of trunk segments is assumed on the ground that they are supplied by a corresponding number of segmental nerves, and the circumstance that they are more particularly associated with the ventral offsets of these nerves would point to the conclusion that they belong rather to the ventral than the dorsal surface of the body.
At first the surfaces of these limb buds are so disposed as to be directed ventrally and dorsally, the ventral aspect corresponding to the future flexor surface of the limb, the dorsal to the extensor side. At the same time, the borders are directed headwards (pre-axial), and tailwards (post-axial). As the limbs grow, they soon display evidence of division into segments corresponding to the hand and foot, forearm and leg, upper arm and thigh. Coincident with this (about the second month) the cartilaginous framework of the limb is being differentiated. The disposition of these cartilages furnishes a clue to their homologies. In the fore-limb the radius and thumb lie along the pre-axial borders, and correspond to the tibia and great toe, which are similarly disposed in the hind-limb; whilst the ulna and fifth finger are homologous with the fibula and fifth toe, which are in like manner arranged in relation to the posterior (post-axial) border of their respective limbs. Up to this time the limbs are directed obliquely ventralwards. During the third month a change in the position of the limbs takes place, associated with the assumption of the foetal position. Owing to the elongation of the limbs, they become necessarily bent at the elbow and knee, the upper arm inclining downward along the thoracic wall, whilst the thigh is directed upwards in contact with the abdominal parietes. At the same time a rotation of each of these segments of the limb takes place in an inverse direction, so that the pre-axial border of the humerus is turned faterally, whilst the pre-axial border of the femur is turned medially. Assuming that these borders are homologous, it results from this, that the lateral condyle of the humerus corresponds to the medial condyle of the femur. This torsion of the limb is in part effected at the shoulder and hip joints, and to some extent also in the shafts of the bones. Some anatomists hold that this rotation is not confined to the limb, but involves the dorsal part of the limb girdles. Others maintain that there is no evidence that such takes place. In the upper limb, owing to a certain amount of pronation, the