composed of a number of bones, which form the sides and roof of the brain-case These bones are arranged in such a manner that if both cartilage and membrane bones are included they can be divided into three rings or segments. The hinder one of these segments is the occipital, the middle the parietal, and the anterior one the frontal.
The occipital segment is formed of four cartilage bones, the basi-occipital below, two exoccipitals at the sides, and the supra-occipital above. The parietal segment is formed of the basisphenoid below, two alisphenoids at the sides and two membrane bones, the parietals above, and the frontal segment in like manner consists of the presphenoid below, the two orbitosphenoids at the sides, and two membrane bones, the frontals, above. The parietals and frontals, being membrane bones, are not comparable to the supra-occipital, in the way that the presphenoid and basisphenoid are to the basi-occipital.
The cartilage bones of the occipital segments are derived from the parachordals of the embryonic skull, those of the parietal and frontal segments from the trabeculae.
In front of the presphenoid the basicranial axis is continued by the mesethmoid.
(2) The sense capsules.
These enclose and protect the special sense organs.
(a) Auditory capsule.
The basisphenoid is always continuous with the basi-occipital, but the alisphenoid is not continuous with the exoccipital as the periotic or auditory capsule is interposed between them. Each periotic capsule has three principal ossifications; an anterior bone, the pro-otic, a posterior bone, the opisthotic, and a superior bone, the epi-otic.
These bones may severally unite, or instead of uniting with one another they may unite with the neighbouring bones. Thus the epi-otic often unites with the supra-occipital, and the opisthotic with the exoccipital.
Two other bones developed in the walls of the auditory capsule are sometimes added, as in Teleosteans; these are the pterotic and sphenotic.
(b) Optic capsule.
The eye is frequently enclosed in a cartilaginous sclerotic capsule, and in this a number of scale-like bones are often developed.
Several membrane bones are commonly formed around the orbit or cavity for the eye. The most constant of these is the lachrymal which lies in the anterior corner; frequently too, as in Teleosteans, there is a supra-orbital lying in the upper part of the orbit, or as in many Reptiles, a postorbital lying in the posterior part of the orbit.
(c) Nasal capsule.
In relation to the nasal capsules various bones occur.
The basicranial axis in front of the presphenoid is ossified, as the mesethmoid, dorsal to which there sometimes, as in Teleosteans, occur a median ethmoid and a pair of lateral ethmoids[9]. Two pairs of membrane bones very commonly occur in this region, viz. the nasals which lie dorsal to the mesethmoid, and the vomers (sometimes there is only one) which lie ventral to it.
The part of the skull lying immediately in front of the cranial cavity and in relation to the nasal capsules constitutes the ethmoidal region.
There remain certain other membrane bones which are often found connected with the cranium. Of these, one of the largest is the parasphenoid which, in Ichthyopsids, is found underlying the basicranial axis. Prefrontals often, as in most reptiles, occur lying partly at the sides and partly in front of the frontal, and postfrontals similarly occur behind the orbit lying partly behind the frontals and partly at their sides. Lastly a squamosal bone is, as in Mammals, very commonly developed, and lies external and partly dorsal to the auditory capsules.
The Jaws and Visceral Skeleton.
In the most primitive fish these consist of a series of cartilaginous rings or arches placed one behind another and encircling the anterior end of the alimentary canal. Originally they are mainly concerned with branchial respiration.
The first or maxillo-mandibular arch forms the upper jaw and the lower jaw or mandible.
The second or hyoid arch bears gills and often assists in attaching the jaws to the cranium. The remaining arches may bear gills, though the last is commonly without them.
The above condition is only found in fishes, in higher animals the visceral skeleton is greatly reduced and modified.
The first or maxillo-mandibular arch is divisible into a dorsal portion, the palato-pterygo-quadrate bar, which forms the primitive upper jaw and enters into very close relations with the cranium, and a ventral portion, Meckel's cartilage, which forms the primitive lower jaw. The cartilaginous rudiments of both these portions disappear to a greater or less extent and become partly ossified, partly replaced by or enveloped in membrane bone.
The posterior part of the palato-pterygo-quadrate bar becomes ossified to form the quadrate, the anterior part to form the palatine and pterygoid, or the two latter may be formed partially or entirely of periosteal bone, developed round the cartilaginous bar. Two pairs of important membrane bones, the premaxillae and maxillae form the anterior part of the upper jaw, and behind the maxilla lies another membrane bone, the jugal or malar, which is connected with the quadrate by a quadratojugal. The premaxillae have a large share in bounding the external nasal openings or anterior nares.
In lower vertebrates the nasal passage leads directly into the front part of the mouth cavity and opens by the posterior nares. In some higher vertebrates, such as mammals and crocodiles, processes arise from the premaxillae and palatines, and sometimes from the pterygoids, which meet their fellows in the middle line and form the palate, shutting off the nasal passage from the mouth cavity and causing the posterior nares to open far back.
The cartilage of the lower jaw is in all animals with ossified skeletons, except the Mammalia, partly replaced by cartilage bone forming the articular, partly overlain by a series of membrane bones the dentary, splenial, angular, supra-angular and coronoid. In many sharks large paired accessory cartilages occur at the sides of the jaws; and in a few reptiles and some Amphibia, such as the Frog, the ossified representative of the anterior of these structures occurs forming the mento-meckelian bone. In mammals the lower jaw includes but a single bone.
The quadrate in all animals with ossified skeletons, except the Mammalia, forms the suspensorium of the mandible or the skeletal link between the jaw and the cranium; in the Mammalia, however, the mandible articulates with the squamosal, while the quadrate is greatly reduced, and is now generally considered to be represented by the tympanic ring of the ear.
The second visceral or hyoid arch in fishes consists of two pieces of cartilage, a proximal piece the hyomandibular, and a distal[10] piece the cerato-hyal. The cerato-hyals of the two sides are commonly united by a median ventral plate, the basi-hyal. The hyoid arch bears gills on its posterior border, but its most important function in most fishes is to act as the suspensorium. In higher vertebrates the representative of the hyomandibular is much reduced in size, and comes into relation with the ear forming the auditory ossicles; the cerato-hyal looses its attachment to the hyomandibular and becomes directly attached to the cranium, forming a large part of the hyoid apparatus of most higher vertebrates.
Behind the hyoid arch come the branchial arches. They are best developed in fishes, in which they are commonly five in number and bear gills. Their ventral ends are united in pairs by median pieces, the copulae.
In higher vertebrates they become greatly reduced, and all except the first and second completely disappear. In the highest vertebrates, the mammals, the second has disappeared, but in birds and many reptiles