Part 7

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.

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.

The internal surface (Fig. 18) is concave dorsoventrally, following the outline of the foramen magnum. It is convex from side to side. It is smooth except at its outer margin, which is rough for articulation with the mastoid portion of the temporal bone. The cranial face of the jugular process (f) shows a rough concavity for the reception of the bulla tympani. Mediad of the jugular process is a notch (=jugular notch=) (g) which when the bones are articulated forms part of the boundary of the jugular foramen. Mediad of this notch is a foramen which forms one end of the =hypoglossal canal=. It passes dorsocaudad into the cranial cavity and transmits the hypoglossal nerve. Dorsad of the hypoglossal canal is the cranial opening of the =condyloid canal=, which passes caudad and opens just craniad of the dorsal end of the condyle. It transmits a vein. The outer border of this portion is rough for articulation with the petrous and mastoid portions of the temporal.

The =squamous portion= (c) has the form of a sector of a circle whose arc is a little more than ninety degrees. The central angle of the sector is truncated and bounds the foramen magnum dorsally. The arc of the sector forms the dorsal margin of the bone, while along the radii it passes into the lateral portions. Its dorsal portion is thick and porous; its ventral portion near the foramen magnum is thin and compact.

The external surface (Fig. 17) is marked by a prominent ridge, the =lambdoidal ridge= (h), parallel with the dorsal border and near to it. The narrow portion of the outer surface that lies dorsad of the ridge forms an angle of about ninety degrees with the remainder of the surface.

A median crest (i) extends ventrad from the middle of the lambdoidal ridge toward the foramen magnum; this is the =external occipital crest=. At its junction with the lambdoidal ridge it is elevated into a tubercle, the =external occipital protuberance= (j).

The inner surface (Fig. 18) presents depressions for the convolutions of the cerebellum.

The dorsal border is thick and rough for articulation with the parietals and interparietal. The ventral border abuts on the foramen magnum and is thin and smooth.