Part 3

Passing craniad from the fourth thoracic vertebra to the third cervical there is a gradual transition. The centra of the cervical vertebræ are broader and thinner than those of the thoracic vertebræ, while the vertebral arches and vertebral canal are larger (Fig. 11). The caudal end of each centrum is concave and looks dorsocaudad when the centrum is held with its long axis horizontal. The cranial end of the centrum is convex and looks ventrocraniad when the centrum is horizontal. These peculiarities are more marked in the third vertebra than in the seventh. The spinous processes grow rapidly shorter as we pass craniad; the fifth, sixth, and seventh are directed dorsocraniad, the third and fourth dorsad.

[Illustration: FIG. 10.--CERVICAL VERTEBRÆ, SIDE VIEW.

_a_, spinous processes; _b_, cranial articular processes; _c_, caudal articular facet; _d_, intervertebral foramina; _e_, transverse process proper; _f_, processus costarius; _g_, wing of the atlas; _h_, dorsal arch of the atlas; _i_, atlantal foramen.]

The caudal articular processes are situated at the junction of the radices and laminæ; their facets (Fig. 10, _c_) look ventrocaudolaterad. The cranial articular processes also become more prominent than is the rule in the thoracic vertebræ; they are borne at the junction of radix and lamina and have their facets (Fig. 11, _b_) directed dorsomediad. The cranial and caudal articular processes of each side are joined by a prominent ridge which is most pronounced in the third, fourth, and fifth vertebræ.

The characteristic feature of the cervical vertebræ is their transverse process, so called. In each of them it arises by two roots, one from the centrum and one from the arch. These two roots, which are broad and thin, converge and unite so as to enclose a canal or foramen, the =foramen transversarium= (Fig. 11, _g_), for the vertebral artery. Laterad of the foramen the two parts of the process are, in the third cervical, almost completely united, the dorsal part being, however, distinguishable as a tubercle at the caudolateral angle of the thin plate formed by the process as a whole. This dorsal component is the =transverse process proper= (Figs. 10 and 11, _e_), while the ventral portion represents a rib, and is hence known as the =processus costarius= (_f_). The expanded plate formed by the union of these two processes is directed nearly ventrad and somewhat craniad in the third, fourth, and fifth vertebræ. The two components of the process gradually separate as we pass caudad; in the fourth and fifth vertebræ the part which represents the transverse process proper forms a very prominent tubercle at the caudolateral angle of the plate formed by the processus costarius. In the sixth (Fig. 11) the two parts are almost completely separated; the dorsal part forms (_e_) a slender knobbed process, while the processus costarius is divided into two portions (_f_ and _f′_) by a broad lateral notch. In the seventh the ventral part (processus costarius) is usually quite lacking, though sometimes represented by a slender spicule of bone. In the former case the foramen transversarium is of course likewise lacking.

[Illustration: FIG. 11.--SIXTH CERVICAL VERTEBRA, CRANIAL END.

FIG. 12.--ATLAS, VENTRAL VIEW.

Fig. 11.--_a_, spinous process; _b_, cranial articular facet; _c_, lamina; _d_, radix or pedicle; _e_, transverse process proper; _f_, _f′_, processus costarius; _g_, foramen transversarium; _h_, centrum; _i_, vertebral canal.

Fig. 12.--_a_, ventral arch; _b_, tuberculum anterius; _c_, lateral masses; _d_, transverse processes; _e_, cranial articular facets; _f_, groove connecting the foramen transversarium with the atlantal foramen; _g_, atlantal foramen; _h_, caudal articular facets.]

=The Atlas= (Fig. 10, 1; Fig. 12).--The first cervical vertebra or atlas has somewhat the form of a seal ring. The centrum is absent; it has united with the second vertebra to form the odontoid process or dens. Its place is taken in the atlas by a narrow flat arch of bone, narrower at the ends than in the middle, the =ventral arch= (Fig. 12, _a_) of the atlas. This connects the lateral, thicker portions of the ring ventrally and bears on its caudal margin a blunt tubercle (=tuberculum anterius=, Fig. 12, _b_). Laterally the ring is thickened, forming thus the =lateral masses= (_c_) which are continued into the broad thin transverse processes (Fig. 10, _g_; Fig. 12, _d_). Each lateral mass bears at its cranial end on its medial surface a concave, pear-shaped facet, cranial (or superior) articular facet, (Fig. 12, _e_) for articulation with the condyles of the skull. These facets look craniomediad. Dorsad of each is a foramen, the atlantal foramen (Fig. 10, _i_; Fig. 12, _g_), which pierces the dorsal arch at its junction with the lateral mass. Caudal to the facet, on the medial face of each lateral mass, within the vertebral canal, is a tubercle. To the two tubercles are attached the transverse ligament (Fig. 14, _b_) which holds in place the odontoid process (dens) of the axis.

That part of the lateral mass which bears the articular facet projects craniad of the dorsal arch and is separated by a deep triangular notch from the transverse process. Along the bottom of this notch runs a groove (Fig. 12, _f_), convex craniad, which connects the cranial end of the foramen transversarium and the atlantal foramen. The vertebral artery passes along it. The foramen transversarium is circular. It is bounded laterally by the lateral masses, and dorsally by the dorsal arch.

The dorsal arch (Fig. 10, _h_) is two to three times as broad as the ventral, has a thick convex cranial border with a median notch, and a thin concave caudal border.

[Illustration: FIG. 13.--AXIS OR EPISTROPHEUS, SIDE VIEW.

_a_, odontoid process or dens; _b_, cranial articular facets; _c_, spinous process; _d_, caudal articular facet; _e_, transverse process; _f_, foramen transversarium.]

The caudal articular facets (Fig. 12, _h_) are borne by the caudal ends of the lateral masses. They are slightly concave, triangular, and look caudomediad, so that their dorsal borders form with the caudal border of the dorsal arch nearly a semicircle. The transverse processes are flat and directed laterad. The attached margin of each is about two-thirds the length of the thinner free margin. The somewhat thicker caudal end of the transverse process projects further caudad than any other part of the vertebra and is separated by a slight notch from the caudal articular facet. From the bottom of this notch the foramen transversarium extends craniad and opens at the middle of the ventral face of the transverse process.

=Epistropheus= or =Axis= (Fig. 10, 2; Fig. 13).--The second cervical vertebra (epistropheus or axis) is not so wide as the atlas but is much longer. Craniad the centrum is continued into a slender conical, toothlike projection, the =dens= or =odontoid process= (Fig. 13, _a_) which represents the centrum of the atlas. The dens is smooth below for articulation with the ventral arch of the atlas. It is rougher above. Laterad of the dens the centrum bears a pair of large cranial articular facets (_b_) which look craniolaterad. These have each the form of a right-angled triangle with rounded angles, one side of the triangle being nearly horizontal. Each is separated from the articular face of the dens by a roughened groove. The spinous process (_c_) runs the length of the vertebral arch. It extends craniad of the vertebral arch nearly as far as the dens, as a flat rounded projection. Caudad of the vertebral arch it projects for a short distance as a stout triangular spine. The caudal articular facets (_d_) are borne on thickenings of the caudolateral portions of the arch; they face almost directly ventrad. The transverse process (_e_) is slender and triangular and directed nearly caudad. Its apex reaches no farther than the caudal or articular face of the centrum. Its base is traversed by the foramen transversarium (_f_).

=Differential Characters of the Cervical Vertebræ.=--It is possible to identify each of the cervical vertebræ:

The first by the absence of the centrum.

The second by the dens or odontoid process.

The third by the small spinous process and slightly marked tubercle of the transverse process, and by a median tubercle on the cranial border of the vertebral arch.

The fourth by the spinous process directed dorsad, and the short thick tubercle of the transverse process not trifid.

The fifth by the spinous process directed craniad, and the more slender spine-like tubercle of the transverse process not trifid.

The sixth by the trifid transverse process.

The seventh by the long spinous process and the slender simple transverse process, and by the usual absence of the foramen transversarium.

LIGAMENTS OF THE VERTEBRAL COLUMN.

=Fibro-cartilagines intervertebrales.=--The separate vertebræ (except the atlas and axis) are united by the disk-shaped =intervertebral fibro-cartilages=, which are situated between the centra of the vertebræ. Each consists of a central pulpy portion and a fibrous outer portion, covered by strong intercrossing tendinous fibers which unite with the periosteum of the vertebræ.

=Ligamentum longitudinale anterius.=--On the ventral face of the centra of the vertebræ, from the atlas to the sacrum, lies a longitudinal ligament, the =anterior longitudinal ligament=. It is very small, almost rudimentary, in the cervical region: large and strong in the thoracic and lumbar regions.

=Ligamentum longitudinale posterius= (Fig. 14, _a_).--A corresponding ligament (=posterior longitudinal ligament=) lies on the dorsal surface of the centra (therefore within the vertebral canal). It is enlarged between each pair of vertebræ and closely united to the intervertebral fibro-cartilages.

=Ligamentum supraspinale.=--Between the tips of the spinous processes of the thoracic and lumbar vertebræ extend ligamentous fibers. They are not united to form a distinct band, and can hardly be distinguished from the numerous tendinous fibers of the supraspinous muscles. Together they represent the =supraspinous ligament=. From the tip of the spinous process of the first thoracic vertebra to the caudal end of the spine of the axis extends a slender strand representing the =ligamentum nuchæ= or =cervical supraspinous ligament=. It is imbedded in the superficial muscles of this region, some of which take origin from it.

Ligamentous fibers are also present between the spinous processes of the vertebræ (=ligamenta interspinalia=): between the transverse processes (=ligamenta intertransversaria=), and between the vertebral arches (=ligamenta flava=).

=Capsulæ articulares.=--The joints between the articular processes are furnished with =articular capsules= attached about the edges of the articular surfaces. These are larger and looser in the cervical region.

=Atlanto-occipital Articulation.=--The joint between the atlas and the occipital condyles has a single articular capsule, which is attached about the borders of the articular surfaces of the two bones. This capsule is of course widest laterally, forming indeed two partially separated sacs, which are, however, continuous by a narrow portion across the ventral middle line. This capsule communicates with that which covers the articular surface of the dens, and through this with the capsule between the atlas and axis. That portion of the capsule which covers the space between the ventral arch of the atlas and the occipital bone represents the =anterior atlanto-occipital membrane=; it is strengthened by a slender median ligamentous strand. The =posterior atlanto-occipital= membrane covers in the same way the space between the dorsal arch of the atlas and the dorsal edge of the foramen magnum. In it a number of different sets of fibers, with regard to direction and to degree of development, may be distinguished; these have sometimes been considered separate ligaments.

The =lateral ligaments= of the atlas begin at the lateral angle of the cranial margin of the atlas, at about the junction of its dorsal and ventral arches, and pass cranioventrad to the jugular processes.

=Articulation between the Axis and Atlas.=--The =articular capsule= is large and loose, being attached to dorsal and ventral borders of the atlas, about the articular surfaces of the axis, and to the cranial projection of the spine of the atlas. It also passes craniad along the ventral side of the dens and communicates here with the capsule of the atlanto-occipital articulation. In the dorsal part of the capsule a short strong ligamentous strand is developed, connecting the caudal border of the dorsal arch of the atlas with the tip of the cranial projection of the spinous process of the axis.

[Illustration: FIG. 14.--LIGAMENTS OF THE ODONTOID PROCESS OR DENS.

First three cervical vertebræ and base of the skull, with dorsal surface removed. _a_, ligamentum longitudinale posterius; _b_, transverse ligament of the atlas; _c_, ligamenta alaria; _d_, odontoid process; _e_, occipital condyles; 1, 2, 3, the first three cervical vertebræ; 4, basal portion of the occipital bone.]

The dens or odontoid process is held in place by the =transverse ligament= (Fig. 14, _b_) of the atlas, which passes across the process as it lies within the vertebral canal of the atlas. The transverse ligament is attached at its two ends to the medial surface of the sides of the atlas at about the region where the dorsal and ventral arches of the atlas unite.

From the cranial end of the odontoid process the two =ligamenta alaria= (Fig. 14, _c_) diverge craniolaterad to the rough ventromedial angle of the condyles of the occipital bone.

=II. RIBS. COSTÆ= (Figs. 1 and 15.)

The cat has thirteen pairs of ribs. One of the fifth pair (Fig. 15) may be taken as typical. It is a curved flattened rod of bone attached at its dorsal end to the vertebral column, and at its ventral end to a cartilage (=costal cartilage=, Fig. 15, _f_) which serves to unite it to the sternum.

The most convex portion of the bone is known as the =angle= (_e_). Each rib presents a convex lateral and a concave medial surface, a cranial and a caudal border. The borders are broad dorsad and narrow ventrad, while the surfaces are narrow dorsad and broad ventrad. The rib has thus the appearance of having been twisted.

The rib ends dorsad in a globular head or =capitulum= (_a_), by which it articulates with the costal demifacets of two contiguous thoracic vertebræ. Between the capitulum and angle on the lateral surface is an elevated area, the =tubercle=, marked by the smooth tubercular facet (_c_) for articulation with the transverse process of a vertebra. The constricted portion between the head and tubercle is known as the =neck= (=collum=) (_d_). The angle is marked by a projecting process (_e_) (=angular process=) on its lateral border, for attachment of a ligament.

[Illustration: FIG. 15.--FIFTH RIB OF LEFT SIDE, CRANIAL VIEW.

_a_, head; _b_, tubercle; _c_, tubercular facet; _d_, neck; _e_, angle, with angular process; _f_, cartilage.]

The ribs increase in length to the ninth (the ninth and tenth are of the same length) and then decrease to the last. They decrease in breadth behind the fifth. The first is nearly in a dorsoventral plane, while the others have their dorsal ends inclined slightly craniad.

[Illustration: FIG. 16.--STERNUM, VENTRAL VIEW.

_a_, manubrium; _b_, the separate pieces forming the body; _c_, bony part of the xiphoid process (the expanded cartilaginous portion not being shown); _d_, facet for attachment of first rib.]

The tubercles become less prominent as we pass caudad and are absent on the last two or three ribs, which do not articulate with the transverse process.

The first nine ribs (=true ribs= or =costæ veræ=) are attached separately to the sternum by their costal cartilages. The last four (false ribs or costæ spuriæ) are not attached separately to the sternum. The costal cartilages of the tenth, eleventh, and twelfth are united to one another at their sternal ends. They may be united also to the ninth costal cartilage or to the sternum by a common cartilage of insertion, or they may be quite free from the sternum. The thirteenth costal cartilages are free (floating ribs).

=Ligaments of the Ribs.=--The articular surfaces between the head of the rib and the centra, and between the tubercle and the transverse process of the vertebra, have each an articular capsule. There are also a number of small ligamentous bands from the tuberosity and the neck of the rib to the transverse process of the vertebra.

=III. STERNUM.= (Fig. 16.)

The sternum consists of three portions, a cranial piece or =manubrium= (_a_), a caudal piece or =xiphoid= process (_c_), and a middle portion or =body= (=corpus=), which is divided into a number of segments (_b_).

To the sternum are united the ventral ends of the first nine ribs. It thus forms the median ventral boundary of the thorax. Since the thorax decreases in dorsoventral measurement craniad, the long axis of the sternum is inclined from its caudal end dorsocraniad, and if continued would strike the vertebral column in the region of the first cervical vertebra.

The manubrium (_a_) makes up about one-fifth the whole length of the sternum and projects craniad of the first rib. It has the form of a dagger and presents a dorsal surface and two lateral surfaces, the latter uniting ventrad to form a sharp angle. In the middle of the lateral surface near the dorsal margin is an oval articular surface (_d_) borne on a triangular projection. It looks caudodorsad and is for the first costal cartilage.

The caudal end articulates with the body by a synchondrosis and presents a slightly marked oval facet on each side for the second costal cartilage.

The body consists of six cylindrical pieces (_b_) enlarged at their ends and movably united by synchondroses. They increase in breadth from the first, and decrease slightly in length and thickness. At the caudal end of each near its ventral border there is a pair of facets looking caudolaterad. They are for the costal cartilages.

The xiphoid process (_c_) is a broad thin plate of cartilage at its caudal end; bony and cylindrical at its cranial end. It is attached by its base to the last segment of the body by a considerable cartilaginous interval, while the opposite end is free and directed caudoventrad.

The cartilage of the ninth rib is attached to the lateral face of the cartilage between the xiphoid and the body, and just caudad of this the common cartilage of insertion of the tenth, eleventh, and twelfth costal cartilages is attached, if present.

IV. THE SKULL.

The bones of the head consist of the =skull proper= together with a number of separate bones forming part of the =visceral skeleton=; these are the lower jaw, the hyoid, and the ear-bones.

The skull proper is considered as divided into =cranial= and =facial= portions. The former includes all the bones which take part in bounding the cranial cavity or cavity of the brain; the latter includes the bones which support the face.

The cranial portion of the skull includes all that part enclosing the large cavity which contains the brain. For convenience this portion may be considered as made up of three segments, each of which forms a ring surrounding a part of the cranial cavity. The first or caudal segment or ring consists of the occipital bone (with the interparietal) surrounding the foramen magnum. The second segment consists of the sphenoid ventrad, the parietals laterad and dorsad. Between the first and second segments are intercalated laterally the temporal bones containing the auditory organ. The third segment or ring consists of the presphenoid ventrad, of the frontals laterad and dorsad. The cranial opening of this ring is closed by the lamina cribrosa of the ethmoid.

The cranial portion of the skull therefore contains eleven separate bones: one occipital, one interparietal, two temporals, one sphenoid, two parietals, one presphenoid, two frontals, and one ethmoid.

The facial portion of the skull is much smaller than the cranial, and lies craniad of the latter; it encloses the nasal cavity. It contains the following thirteen bones: two palatines, one vomer, two maxillaries, two lachrymals, two premaxillaries, two nasals, two malar or zygomatic bones. The two halves of the mandible or lower jaw are frequently included in the facial portion of the skull, making in all fifteen separate bones in this part of the skull.

In the following the bones of the skull are first described separately, then an account is given of the skull as a whole.

=Occipital Bone.= =Os occipitale= (Figs. 17 and 18).--The occipital bone forms the most caudal portion of the cranium, entering into the formation of its caudal wall and of its base. It connects the cranium with the vertebral column and surrounds a large opening, the =foramen magnum= (_d_), by means of which the cranial cavity communicates with the vertebral canal.

[Illustration: FIG. 17.--OCCIPITAL BONE, CAUDAL OR OUTER SURFACE.

FIG. 18.--OCCIPITAL BONE, INNER SURFACE.

_a_, basilar portion; _b_, lateral portions; _c_, squamous portion; _d_, foramen magnum; _e_, occipital condyles; _f_, jugular processes; _g_, jugular notch; _h_, lambdoidal ridge; _i_, external occipital crest; _j_, external occipital protuberance.]

In young kittens four portions may be distinguished in this bone. These are, a basal portion (the =basioccipital bone=), two lateral portions (the =exoccipital bones=), and a dorsal portion (the =supraoccipital bone=). These four bones remain separate through life in many lower vertebrates. In the adult cat they are completely united into a single bone, but it is convenient to describe this bone as made up of four parts: a basal portion (_a_) (=pars basilare=), corresponding to the basioccipital, two lateral portions (_b_) (=partes laterales=), corresponding to the exoccipitals, and a squamous portion (_c_) (=squama occipitalis=) corresponding to the supraoccipital.

The basilar portion (_a_) as seen from the dorsal or ventral surface is oblong and flattened. It is broadest at the junction of its middle and last thirds, and tapers toward both ends. It presents a cranial end and a caudal end, a dorsal, a ventral, and two lateral surfaces. Its caudal end, which helps to form the ventral boundary of the foramen magnum, is concave from side to side. The cranial end is transversely elongate, about five times as broad as high, pointed laterally and roughened for attachment to the caudal end of the body of the sphenoid, which has a corresponding form. The joint is a synchondrosis.

The dorsal surface is concave from side to side, forming a longitudinal groove in which rest the pons and medulla. The concavity is more pronounced caudad, where the bone is thinner at its middle.

The ventral surface is marked by three parallel longitudinal ridges. One of them is median and expands caudad into a smooth triangular elevated area which extends to the foramen magnum. The other two ridges run near the lateral edges of the bone. Between them and the median ridge the surface is smooth and is depressed caudad. Laterad of each lateral ridge is a rough triangular surface overlaid in the natural state by the medial edge of the tympanic bulla.

The lateral surfaces are smooth and sharp and abut against the petrous portion of the temporal bone. They pass into the lateral portions of the occipital caudad.

The =lateral portions= (_b_) of the occipital arise from the caudal margin of the basilar portion in the transverse plane; a short distance laterad of the line of junction they turn dorsad at an angle of nearly ninety degrees. They form the lateral boundaries of the foramen magnum and pass dorsally into the squamous portion.

The external surface of each presents an elongated elevated spirally curved surface, the =occipital condyle= (_c_), for articulation with the atlas. A small part of each condyle is formed from the basal portion. The two condyles are separated from one another ventrally by a narrow notch, and each extends (laterad) along the border of the foramen magnum to a point slightly dorsad of the transverse diameter of the latter.

Laterad of each condyle the bone is elevated into a blunt triangular projection, the =jugular process= (_f_), which covers the caudal end of the tympanic bulla. Between the jugular process and the condyle is a deep depression.