Part 3

On the dorsal face of each lamina at its cranial border is a smooth oval area, the =cranial articular facet= (superior articular facet of human anatomy) (Figs. 2 and 3, _h_). Its long axis is oblique and it looks dorsolaterad. The slight projections of the cranial edge of the laminæ on which the facets are situated are the inconspicuous =cranial articular processes= (prezygapophyses).

On the ventral surface of each lamina at the caudal border, near the middle line is a similar area, the =caudal articular= =facet= (inferior articular facet of human anatomy) (_i_); these occupy the ventral surfaces of two projections which form the =caudal= (inferior) =articular processes= (postzygapophyses) (_j_). These are separated by a median notch. When the vertebræ are in their natural position the caudal articular facets lie dorsad of the cranial facets and fit against them. They thus strengthen the joint between contiguous vertebræ, while permitting slight rotary motion.

=Differential Characters of the Thoracic Vertebræ= (Fig. 4).--Following the thoracic vertebræ caudad there is to be seen a gradual increase in the size of the centra brought about by an increase in their craniocaudal and transverse measurements. The dorsoventral measurements remain nearly the same. The costal facets (Fig. 4, _e_) shift caudad so that on the eleventh, twelfth, and thirteenth thoracic vertebræ each lies entirely on the cranial end of its centrum, while the caudal end of the centrum immediately preceding is not marked by any part of it. In the eleventh thoracic vertebra each costal facet is usually still confluent with the smooth cranial end of the centrum. In the twelfth vertebra the facets are separated by smooth ridges from the cranial end of the vertebra, while in the thirteenth vertebra they are separated by rough ridges.

The spinous processes (_a_) of the first four are of about the same length. They then decrease in length to the twelfth, while the twelfth and thirteenth are slightly longer than the eleventh. The first ten slope more or less caudad, while the spinous process of the tenth (=anticlinal=) vertebra is vertical and those of the eleventh, twelfth, and thirteenth point craniad.

Each of the transverse processes of the seventh thoracic vertebra shows a tendency to divide into three tubercles; one of these is directed craniad, the mammillary process (or metapophysis), one caudad, the accessory process (or anapophysis), while the third (transverse process proper) looks ventrad and bears the transverse costal facet. This division becomes more prominent in the succeeding vertebræ, being most marked in the ninth and tenth. In the eleventh, twelfth, and thirteenth vertebræ the mammillary (_g_) and accessory (_f_) processes are very pronounced, while the transverse costal facet and that part of the transverse process which bears it have disappeared. The ribs of the eleventh, twelfth, and thirteenth vertebræ are thus attached to their respective centra by their heads alone.

The cranial articular processes (_b_) are prominent on the first two thoracic vertebræ; back of these they are very small as far as the eleventh, so that the articular facets seem to be borne merely upon the dorsal surface of the cranial edge of the laminæ. In the eleventh, twelfth, and thirteenth the cranial articular processes are large, bearing the articular facets on their medial surfaces, while the mammillary processes appear as tubercles on the lateral surfaces of the articular processes. The caudal articular processes (_h_) are prominent in the first thoracic, then smaller until the tenth is reached; in the tenth, eleventh, twelfth, and thirteenth they are large and their facets are borne laterally, so as to face the corresponding cranial facets. Thus from the tenth to the thirteenth thoracic vertebra rotary motion is very limited, owing to the interlocking of the articular processes.

=The Lumbar Vertebræ.= =Vertebræ lumbales= (Fig. 5).--The last thoracic vertebræ form the transition to the typical lumbar vertebræ. These are larger than the thoracic vertebræ. The centra are of the form of the centra of the thoracic vertebræ, and increase in length to the sixth, but the seventh is about the length of the first. They increase in breadth to the last.

The cranial articular processes (Fig. 5, _a_) are prominent and directed craniodorsad; they have the facets on their medial surfaces, while their dorsolateral surfaces bear the mammillary processes (_b_) as prominent tubercles. The caudal articular processes (_c_) are likewise large; their facets look laterad. When the vertebræ are articulated they are received between the medially directed cranial processes.

The accessory processes (_d_) are well developed on the first vertebra, diminish in size to the fifth or sixth, and are absent on the seventh and sometimes on the sixth.

The transverse processes (more properly pseudo-transverse processes) (_e_) arise from the lateral surface of the centra; are flat and are directed ventrocraniolaterad. The first is small, and they increase in length and breadth from the first to the sixth, those of the last being slightly smaller than in the sixth. The free ends of the last four are curved craniad.

The spinous processes (_f_) are flat and directed craniodorsad. They increase in length to the fifth and then decrease. The first five are knobbed at the end. In a dorsal view the spinous process and cranial articular processes of each vertebra are seen to interlock with the caudal articular processes and accessory processes of the preceding vertebra in such a way as to prevent rotary motion, and this arrangement may be traced craniad as far as the eleventh thoracic vertebra.

=Sacral Vertebræ.= =Vertebræ sacrales= (Figs. 6 and 7).--The three sacral vertebræ are united in the adult into a single bone, the =os sacrum=, or =sacrum=. In a kitten the three vertebræ are separate, while in an animal almost mature the first two are united and the third is still separate. The sacrum lies between the last lumbar and the first caudal vertebræ and articulates laterally with the two innominate bones. It is pyramidal, with the base of the pyramid directed craniad, and is perforated by a depressed longitudinal canal, the =sacral canal=, which is a continuation of the vertebral canal, and by four large foramina dorsally and four ventrally. It may be described as having a cranial end or base and a caudal end or apex, a dorsal, a ventral, and two lateral surfaces.

The =base= is slightly oblique and presents a smooth transversely oval articular facet (the cranial end of the centrum of the first sacral vertebra), for articulation with the centrum of the last lumbar vertebra. Dorsad of this is the sacral canal, more depressed than the vertebral arch craniad of it. It supports a spinous process (Fig. 6, _a_) which is directed dorsad. At the junction of its lamina and radix is seen the prominent cranial articular process (_b_) with sometimes slight indications of a mammillary process on its lateral surface. Laterad of the articular facet is seen the cranial face of the expanded “pseudo-transverse process” (_c_) of the first sacral vertebra. The ventral border of the base is concave ventrad, forming an arc of about 120 degrees. The =apex= shows the caudal end of the last sacral centrum. Dorsad of this are the vertebral arch with a very short spinous process (_a′_), and the caudal articular processes (_d_). Laterad of the centrum appears the laterally directed thin transverse process (_e_).

The ventral or pelvic surface (Fig. 7) is smooth, concave craniad, convex caudad, and crossed by two transverse ridges (_a_) along which are seen the ossified remains of the intervertebral fibro-cartilages. At the ends of the first ridge is a pair of nearly circular =ventral= (or anterior) =sacral foramina= (_f_) for the passage of sacral nerves. At the end of the second ridge is a pair of ventral sacral foramina (_f′_), smaller than the first pair and continued laterocaudad into shallow grooves for the ventral rami of the sacral nerves. That portion of the bone lying laterad of a line joining the medial borders of these two pairs of foramina is known as the =lateral mass= of the sacrum and is composed of the fused transverse processes of the sacral vertebræ. At the caudal margin of the ventral surface there is a notch between the lateral mass and the centrum (_g_). When the caudal vertebræ are articulated, this notch helps to form a foramen for the third sacral nerve.

The dorsal surface (Fig. 6) is narrower at its cranial end than is the ventral surface. Its cranial border bears laterally a pair of cranial articular processes (_b_) with their medially directed facets and between them it is concave, so that a large dorsal opening is left into the vertebral canal between the last lumbar vertebra and the sacrum. Caudad of the articular processes are two pairs of tubercles (_f_). These are the fused cranial and caudal articular processes of the sacral vertebræ. Caudad of them are the caudal articular processes of the last sacral vertebra (_d_). Craniolaterad of the middle and cranial tubercles are =dorsal= (posterior) =sacral foramina= (_g_) for the transmission of the dorsal rami of the sacral nerves. Three spinous processes (_a_) appear between these rows of tubercles. They decrease in height caudad. That part of the surface included between the spinous process and the tubercles is made up of the fused laminæ of the sacral vertebræ. That part between the tubercles and a line joining the lateral margins of the dorsal (posterior) sacral foramina is formed by the fused radices of the sacral vertebræ.

The lateral surface may be divided into two parts. Craniad is a large rough triangular area with equal sides and with one of its angles directed ventrocraniad. It is the lateral face of the pseudo-transverse process of the first sacral vertebra (Fig. 6, _c_). A smooth curved surface (the auricular facet) along its ventral edge articulates with the ilium, while the dorsal portion is rough for attachment of ligaments. Caudad is the narrow longitudinal triangular area of the lateral faces of the fused transverse processes of the second and third sacral vertebræ.

=Caudal Vertebræ.= =Vertebræ caudales= (Fig. 1, _y_, and Figs. 8 and 9).--The caudal vertebræ (21-23 in number) decrease gradually in size to the last one. Caudad they become longer and more slender and lose the character of vertebræ. They become finally reduced to mere centra,--slender rods of bone knobbed or enlarged at their two ends (Fig. 8). The last one is more pointed than the others and bears at its caudal end a small separate conical piece, the rudiment of an additional vertebra.

The parts of a typical vertebra--vertebral arch, transverse processes, cranial and caudal articular processes--may be recognized in the vertebræ as far back as the eighth or ninth. The transverse processes (Fig. 9, _a_) are directed caudad and decrease rapidly in length. They are very small on the ninth vertebra, but may be recognized for a considerable distance back of this. The spinous process disappears at about the fourth caudal vertebra, and the vertebral canal becomes gradually smaller caudad, until on the eighth or ninth vertebra it becomes merely a groove open dorsad.

Caudad of the third vertebra for a considerable distance, each centrum bears on each lateral face at its cranial end a short =anterior transverse= process, and on its ventral face at its cranial end a pair of rounded tubercles, =hæmal processes= (_c_), which articulate with a small pyramidal =chevron= bone (_d_) so as to enclose a canal. These structures disappear caudad.

=Cervical Vertebræ.= =Vertebræ cervicales= (Fig. 10).--The cervical vertebræ number seven. The first two of these are so peculiar as to require a separate description, so that the last five may be first considered.

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.

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.

=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.

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.