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but faintly marked. The anterior extremity of the rib is thickened and often expanded for the reception of its costal cartilage, which is not infrequently ossified. Here, on the upper surface, are attached the costo-clavicular ligament and the subclavius muscle. The inferior surface of the rib is smooth and is covered with pleura. The outer convex border, thin in front, is usually thick and rough behind the subclavian groove, where it has attached to it the fibres of the first digitation of the serratus anterior. Along this edge, also, are attached the external and internal intercostal muscles of the first intercostal space. The inner concave border is thin, and has connected with it the aponeurotic expansion known as Sibson's fascia.
The second rib may be distinguished by the sharpness of its curve; the absence of any twist on its body, so that it can be laid flat on the table; the oblique direction of the surfaces of its body, the outer being directed upwards and outwards, whilst the inner is turned downwards and inwards; and the presence of a well-marked, rough, oval area about the middle of its external surface and lower border for part of the first, and the whole of the second digitation of the serratus anterior muscle. The head has two facets, and the angle is close to the tubercle posteriorly.
The tenth rib has usually only a single articular facet on the head, and may or may not have a facet on the tubercle.
The eleventh and twelfth ribs are recognised by their length. The head of each is usually large in proportion to the body; it supports a single facet for articulation with the eleventh or twelfth thoracic vertebræ. The tubercles are illdeveloped and have no articular facets. The angle is faintly marked on the
eleventh, scarcely perceptible on the twelfth. The anterior extremities of both are narrow and pointed and tipped with cartilage. The costal groove is absent in the twelfth, and but slightly seen in the eleventh. The twelfth is considerably shorter than the eleventh rib.
Ossification. Ossification begins in the cartilaginous ribs about the sixth week, and rapidly extends along the body, so that by the end of the third month it has reached the permanent costal cartilage. The sixth and seventh ribs are the earliest to ossify; the first rib being the last (Lambertz). At puberty, or before, secondary centres appear. One for the head. In the first rib there is one epiphysis for the tubercle. In the second to the sixth ribs inclusive there are two epiphyses for the tubercle, one for the ar ticular part and one for
the non-articular part. In the remaining ribs which have articular tubercles there is only one epiphysis (Fawcett). By the twenty-fifth year fusion between the epiphyses and the body is complete.
THE COSTAL CARTILAGES.
The costal cartilages, of which there are twelve pairs, are bars of hyaline cartilage united to the anterior extremities of the ribs, into which they are recessed and held in position by the periosteum. Through these cartilages the first seven ribs are connected directly with the sternum by means of synovial joints corresponding to the notches along the margins of the breast bone. To this there is an exception in the case of the first rib, the cartilage of which is directly blended with the manubrium sterni. The eighth, ninth, and tenth are connected indirectly with the sternum by their union with each other, and their articulation, through the medium of the eighth, with the seventh rib cartilage, whilst the eleventh and twelfth cartilages tip the ribs to which they belong, and lie free in the muscles of the flank. The costal cartilages increase in length from the first to the seventh, below which they become shorter. The first inclines obliquely downwards and medially to unite with the superior angle of the manubrium. The second lies more or less horizontally. The third to the seventh gradually become more and more curved, inclining downwards from the extremities of their respective ribs, and then turning upwards to reach the sternum. The tenth cartilage articulates by means of a synovial joint with the ninth, the ninth with the eighth, and the eighth with the seventh. There are also surfaces for the articulation of the seventh with the sixth, and sometimes for the sixth with the fifth.
THE THORAX AS A
The bony and cartilaginous thorax is barrel-shaped, being narrower above than below, and compressed from before backwards. Its posterior wall is longer than its anterior, and its transverse width, which reaches its maximum opposite the eighth or ninth rib, is much in excess of its sagittal diameter. This is largely be owing to the forward projection of the thoracic part of the vertebral column into the thoracic cavity.
The anterior wall is formed by the ribs and rib cartilages, together with the sternum. The posterior wall comprises the thoracic part of the vertebral column and the ribs as far as their angles. Owing to the posterior curve of the ribs, and the projection forwards of the vertebral bodies, the antero-posterior diameter of the thoracic cavity is considerably greater on each side of the median plane than in the median plane, thus allowing for the lodgment of the rounded
FIG. 127. THE THORAX AS SEEN FROM THE RIGHT SIDE.
posterior parts of the lungs. For the same reason the furrow on each side of the spinous processes of the thoracic vertebræ is converted into a broad groove (vertebral groove), the floor of which is in part formed by the ribs as far as their angles. The grooves so formed are each occupied by the fleshy mass of the sacrospinalis muscle.
The side walls are formed by the costal arches. The ribs, which run obliquely from above downwards and forwards, do not lie parallel to each other, but spread somewhat, so that the intervals between them (intercostal spaces) are wider in front than behind.
The superior aperture or inlet, formed by the body of the first thoracic vertebra behind, the arch of the first rib on either side, and the upper border of the manubrium sterni in front, is contracted and of reniform shape, measuring on an average from 10 to 12 cm. transversely and 5 cm. in an antero-posterior direction. The plane of the inlet is oblique from behind downwards and forwards, so that in expiration the superior border of the sternum lies on a level with the fibro-cartilage between the second and third thoracic vertebræ.
The inferior aperture, of large size, is bounded in the median plane behind by the twelfth thoracic vertebra; passing thence the twelfth ribs slope laterally, downwards and forwards. A line carried horizontally forwards from the tip of the twelfth rib touches the end of the eleventh rib, and then curving slightly upwards reaches the cartilage of the tenth rib. Thence it follows the confluent margins of the cartilages of the tenth, ninth, eighth, and seventh ribs, finally reaching the xiphoid process, where it forms, with the costal margin of the opposite side, the infrasternal angle, the summit of which coincides with the xiphi-sternal articulation; in expiration this joint usually lies on a level with the intervertebral fibro-cartilage between the ninth and tenth thoracic vertebræ, and corresponds with the surface depression familiarly known as the pit of the stomach. The inferior aperture of the thorax is occupied by the vault of the diaphragm.
In the foetal condition the form of the thorax differs from that of the adult. It is compressed from side to side-in this respect resembling the simian type. Its anteroposterior diameter is relatively greater than in the adult. At birth, changes in form take place dependent on the expansion of the lungs; during subsequent growth, the further expansion of the thoracic cavity in a transverse direction is correlated with the assumption of the erect posture, and the use of the fore-limbs as prehensile organs.
Sexual Differences.-The thorax of the female is usually described as being proportionately shorter and rounder than the male. It also tends to narrowness in the lower segment. It is hardly necessary to point out that the natural form is often modified by the use of tight or ill-fitting corsets.
The Thoracic Index
Transverse diameter x 100
taken at the level of the junction of the xiphoid process with the body expresses the proportions of these diameters. That of the female is on an average lower than the male, indicating a more rounded form.
(In view of the vast amount of accurate knowledge the medical student is now called upon to acquire, it is, in the opinion of the writer of this article, desirable that less stress should be laid upon the details of the disarticulated bones of the skull and more emphasis placed on the study of the skull as a whole.
It has hitherto been the custom to disarticulate the bones of the skull, imposing on the student the task of again reconstructing it, much after the manner of a Chinese puzzle. In this way a minute acquaintance with the forms and articulations of the individual bones became necessary, and the student's memory was burdened with a mass of detail of little or no practical or scientific value, for in regard to the latter aspect of the subject the points of phylogenetic and ontogenetic interest are best illustrated by a consideration of the details of the evolution of the skull and the development and ossification of its parts. With possibly the exception of the temporal bones and the mandible, the author holds that most of the useful information relating to the skull can best be studied in the complete cranium, or in sections of it made in different planes. By this method the student acquires a more intimate knowledge of its structure and topography, and is consequently better equipped to deal with the regions he may have to explore in the living.
With this object in view, the writer of this article has given more space to the description of the skull as a whole and in section than is usually the case. Such a plan has doubtless given rise to some repetition; at the same time it renders more complete, and, it is hoped, also more useful from a practical standpoint, the account supplied.
It must, however, be borne in mind that a text-book of Anatomy serves the double purpose of a "Manual" of instruction and a work of reference. In view of this, the author has furnished a detailed account of the disarticulated cranial bones, such as has been hitherto supplied in works of a like kind.
The student, however, must not assume on this account that this section of the article should be neglected. He will find most of the more important details described in the article on the skull as a whole; but he would do well to supplement his knowledge by a reference to the more detailed account for information regarding the development, ossification, and variations of the individual bones.
(The Bones of the Skull.)}
The term skull (cranium) is commonly employed to signify the entire skeleton of the head. This comprises the bony envelope which surrounds the brain (cranium cerebrale), and the osseous structures which support the face (ossa faciei).
The cranium cerebrale is composed of the occipital, the sphenoid, the ethmoid, the frontal, the two parietals, and the two temporals, the inferior nasal conchæ (O.T. inferior turbinated bones), the lacrimals, the nasal, and the vomer-fifteen. bones in all.
The bones of the face (cranium viscerale, ossa faciei) include the following:One single, viz., the mandible, and six bones, arranged in pairs, viz., the maxillæ, zygomatic (O.T. malar), palate-seven bones in all.
The hyoid bone is usually described along with the skull. If, in addition, the bones of the middle ear, three on each side (malleus, incus, and stapes), are included, the skeleton of the head consists of twenty-nine bones.
THE SEPARATE BONES OF THE SKULL.
The frontal bone, situated in the anterior part of the cranium, is a single bone formed by the fusion in early life of two symmetrical halves. It consists of a frontal part, which corresponds to the region of the forehead; an orbital part, which enters into the structure of the roof of the orbits; and a nasal part, which assists in forming the roof of the nasal cavities.
Pars Frontalis.-The frontal part is the shell-like portion of the bone which rises upwards above the orbital arches. Its external surface is rounded from side to side and from above downwards. This convexity is most pronounced about 1 inches above the orbital arches on either side of the median plane, constituting what are known as the frontal tuberosities. These mark the original sites of the centres from which the bone ossifies. The inferior margin of this part is formed on either side of the median plane by the curved supraorbital margin, the lateral and medial extremities of which constitute the zygomatic process (O.T. external angular) and the medial angular process, respectively. The latter, which descends to a lower level than the former, articulates with the lacrimal bone, and is separated from its fellow by a rough articular surface -the nasal notch-for the nasal and maxillary bones. The curve of the supraorbital margin varies in different individuals and races; towards its medial third
In catalogues of craniological collections the terms used are as follows:-
Calvaria that part of the skull which remains after the bones of the face have been removed
it is crossed by a groove, often (25 per cent., Krause) converted into a foramen-the supraorbital notch or foramen. Through this there pass the supraorbital nerve and artery. Sometimes (16 per cent., Loja) a series of grooves, radiating upwards and laterally, indicate the course of the nerve (Dixon). Above the supraorbital margin the character of the bone displays marked differences in the two sexes: in the male, above the interval between the two medial angular processes, there is usually a well-marked prominence, called the glabella; from this the fulness extends laterally above the supraorbital margin, varying in degree and extent, and forming the elevations known as the arcus superciliares (superciliary arches). The prominence of these naturally reacts on the character of the supraorbital margins, which are thicker and more rounded in the male than in the female. Passing upwards over the glabella, the remains of the suture which originally separated the two halves of the frontal bone can usually be seen; above this point all trace of the suture is generally obliterated. Extending from the zygomatic process is a well- marked ridge, which
curves upwards and slightly medially, then, turning backwards, it arches across the lateral aspect of the bone. This is the linea temporalis, which serves to separate the anterior surface of the frontal portion of the bone from its temporal aspect. The latter (facies temporalis) forms the floor of the upper and anterior part of the temporal fossa, and serves for the attachinent of the temporal muscle.
Pars Orbitalis.-The orbital part of the bone consists of two transversely curved plates, each having the form of a sextant; their medial edges, which are irregular and formed of cellular bone, lie parallel to each other, and are separated in their posterior half by the incisura ethmoidalis (ethmoidal notch), in which the ethmoid bone is lodged. The edges of the notch on either side are grooved in front and behind by the anterior and posterior ethmoidal foramina, which are completed when the ethmoid is in situ. The anterior transmits the anterior ethmoidal branch of the naso-ciliary nerve and the anterior ethmoidal vessels; the posterior, the posterior ethmoidal vessels and nerve. Anterior to the ethmoidal notch is the nasal notch, from the centre of which the nasal process projects downwards and forwards to terminate in the frontal spine, which lies between, and articulates with the nasal bones and perpendicular part of the