Encyclopaedia Britannica, 11th Edition, "Bible" to "Bisectrix" / Volume 3, Slice 7

by Various

One general feature of the adult bird’s skull is the almost complete disappearance of the sutures between the bones of the cranium proper, whilst another is the great movability of the whole palatal and other suspensorial apparatus. The occipital condyle (fig. 1) is a single knob, being formed almost wholly by the basioccipital, while the lateral occipitals (often perversely called exoccipitals) take but little share in it. Part of the membranous roof between the supra-occipital and parietal bones frequently remains unossified and presents in the macerated skull a pair of fontanelles. The squamosals form the posterior outer margin of the orbits and are frequently continued into two lateral downward processes across the temporal fossa. One of these, the processus orbitatis posterior, often combines with an outgrowth of the alisphenoid, and may be, e.g. in cockatoos, continued forwards to the lacrymal bone, so as to form a complete infraorbital bridge. The posterior, so-called processus Zygomaticus is very variable; in many Galli it encloses a foramen by distally joining the orbital process. The ethmoid frequently appears on the dorsal surface between the frontals. There are three periotic bones (pro-, epi-, opisth-otic). The proötic encloses between it and the lateral occipital the fenestra ovalis, into which fits the columella of the ear. The epiotic is often small, ossifies irregularly, and fuses with the supra-occipital. The opisthotic lies between the epiotic and the lateral occipital with which it ultimately fuses; in some birds, e.g. in Larus, it extends far enough to help to bound the foramen magnum. The basisphenoids are ventrally overlaid, and later on fused with, a pair of membrane bones, the basi-temporals, homologous in part with the parasphenoid of lower vertebrates. They contribute to the formation of the auditory meatus, and of the right and left carotid canals which accompany the eustachian tubes. In many birds the basisphenoids send out a pair of basipterygoid processes by which they articulate with the pterygoids. Dorso-laterally the basisphenoid is joined by the alisphenoid, which forms most of the posterior wall of the orbit. The orbito-sphenoids diverge only posteriorly, otherwise they are practically unpaired and form the median interorbital septum, which is very large in correlation with the extraordinary size of the eyeballs.,

	Fig. 1.—End view of skull of a Chicken three weeks old. Herethe opisthotic bone appears in the occipital region, as in theadult Chelonian. (After W.K. Parker.)
	bo, Basi-occipital. bt, Basi-temporal. eo, Opisthotic. f, Frontal. fm, Foramen magnum. fo, Fontanella. oc, Occipital condyle. op, Opisthotic. p, Parietal. pf, Post-frontal. sc, Sinus canal in supra-occipital. so, Supra-occipital. sq, Squamosal. 8, Exit of vagus nerve.

Fig. 2.—Ripe Chick’s head, 1¼ in. long. (After W.K. Parker.)
as, Alisphenoid. bo, Basi-occipital. bt, Basi-temporal. dpx, Dentary process of premaxilla. eo, Opisthotic. eu, Eustachian tube. f, Frontal. fm, Foramen magnum. j, Jugal. l, Lacrymal. mx, Maxilla. mxp, Maxillo-palatine process. oc, Occipital condyle.	pa, Palatine. pf, Post-frontal. pg, Pterygoid. pn, Prenasal cartilage. ppx, Palatine process of pre-maxillary. prp, Pterygoid process of sphenoid. qj, Quadratojugal. so, Supra-occipital. sq, Squamosal. ty, Tympanic cavity. v, Vomer. 8, Exit of vagus nerve. 9, Exit of hypoglossal nerve.

Fig. 3.—Skull of an old Fowl, upper view. (After W.K. Parker.)
eo, Lateral occipital. eth, Ethmoid. f, Frontal. j, Jugal. l, Lacrymal. n, Nostril. np, Upper process of nasal.	npx, Nasal process of premaxillary. p, Parietal. pf, Post-frontal. px, Premaxilla. qj, Quadratojugal. so, Supra-occipital. sq, Squamosal.

Prefrontal bones are absent; post-frontals are possibly indicated by a frequently occurring separate centre of ossification in the post-orbital process, to which the frontals always contribute. The lacrymal is always present, and perforated by a glandular duct. Attached to it or the neighbouring frontal is often a supraorbital; infraorbitals occur also, attached to the jugal or downward process of the lacrymal. The nasals were used by A.H. Garrod to distinguish the birds as holorhinal (fig. 2) where the anterior margin of the nasal is concave, and schizorhinal where this posterior border of the outer nares is continued backwards into a slit which extends beyond the frontal processes of the premaxilla. Many birds possess a more or less well developed cross-joint in front of the frontals and lacrymals, perhaps best developed in Anseres and Psittaci. Owing to this joint the whole upper beak can be moved up and down with extra facility, according to the shoving forwards or backwards of the palato-pterygo-quadrate apparatus which moves sledge-like upon the cranial basis. The premaxilla is always unpaired, but each half has three long processes directed backwards; one fuses with the maxillary bone, another helps to form the anterior part of the palate, while the third, together with its fellow, forms the “culmen” and extends backwards to the frontals, or rather to the ethmoid which there crops up on the surface. The maxillaries (fig. 3) have besides others, a maxillo-palatine process directed inwards in a transverse horizontal direction. The palatines are long, always fused anteriorly with the premaxilla, and frequently with the maxillo-palatine processes; posteriorly they slide upon the presphenoidal rostrum, and articulate in most birds with the pterygoids; they form the greater part of the palatal roof and border the choanae or inner nares. Between these, resting vertically upon the rostrum, appears the vomer; very variable in shape and size, often reduced to a mere trace, as in the Galli, or even absent, broken up into a pair of tiny splints in Pici.

The taxonomic importance of the configurations of the palate was first pointed out by J. de Cornay. T.H. Huxley, in 1868, divided the carinate birds into Dromaeo-, Schizo-, Desmo-, and Aegithognathae, an arrangement which for many years had a considerable influence upon classification. However, subsequent additions and corrections have detracted much from its value, especially when it became understood that the above sub-orders are by no means natural groups. Dromaeognathae have a struthious palate, with a broad vomer meeting in front the broad maxillo-palatal plates, while behind it reaches the pterygoids. The only representatives are the Tinamous. Schizognathae, e.g. fowls (fig. 4), pigeons, gulls, plovers, rails and penguins, have the vomer pointed in front while the maxillo-palatines are free, leaving a fissure between the vomer and themselves. The schizognathous formation is doubtless the most primitive, and its representatives form a tolerably natural assembly. Desmognathae (fig. 5) were supposed to have the maxillo-palatines united across the middle line, either directly or by the intermediation of ossifications in the nasal septum. This is a hopeless assembly. Parker and Fürbringer have demonstrated that desmognathism has been produced in half a dozen ways, implying numerous cases of convergence without any nearer relationship than that they are all derived from some schizognathous group or other. The Aegithognathae, meant to comprise the passeres, woodpeckers and swifts, &c., are really schizognathous but with a vomer which is broadly truncated in front.

Fig. 4.—Skull of adult Fowl. This skull is unusuallyschizognathous, the vomer (v.) being very small, and themaxillo-palatine process (mxp) much aborted.

bo, Basi-occipital. bt, Basi-temporal. eo, Lateral occipital. eu, Eustachian tube. ic, Internal carotid. j, Jugal. l, Lacrymal. mx, Maxilla. mxp, Maxillo-palatine process. oc, Occipital condyle. pa, Palatine. pf, Post-frontal. pg, Pterygoid. prp, Pterygoid process of sphenoid. px, Premaxilla. q, Quadrate. qj, Quadratojugal. rbs, Rostrum of basisphenoid. so, Supra-occipital. v, Vomer. 8, Exit of vagus nerve. 9, Exit of hypoglossal nerve. (After W.K. Parker.)

Fig. 5.—Skull of a nestling Sparrow-hawk (Accipiternisus), palatal view. The circular space on each side of the basi-temporal(bt.) is the opening of the anterior tympanic recess. The basi-pterygoids(bpg) are mere knobs, and the common eustachian opening is seenbetween them. The maxillo-palatine plates (mxp) are dotted toshow their spongy character.

bt, Basi-temporal. bpg, Basi-pterygoid. eo, Lateral occipital. f, Frontal. fm, Foramen magnum. j, Jugal. l, Lacrymal. mpg, Mesopterygoid process of W.K. Parker. mx, Maxillary. mxp, Maxillo-palatine process. op, Opisthotic. pa, Palatine. pg, Pterygoid. px, Premaxilla. pto, Prootic. q, Quadrate. qj, Quadratojugal. sn, Nasal septum. so, Supra-occipital. ty, Tymapanic cavity. v, Vomer. 8, Exit of vagus nerve. 9, Exit of hypoglossal nerve. (After W.K. Parker.)

The remainder of the appendicular skeleton (fig. 6) of the head requires little description. The maxillaries are connected with the distal anterior corner of the quadrate by the thin, splint-like jugal and quadratojugal. The quadrate is invariably a conspicuous bone and movably articulating with the cranium and by a special process with the pterygoid. The mandible is composed of several bones as in reptiles. The os articulare bears on its inner side the inner mandibular process which serves for the insertion of part of the digastric muscle or opener of the mouth; another portion of this muscle is attached to the os angulare, which frequently forms a posterior mandibular process. The greater part of the under-jaw is formed by the right and left dentaries, which in all recent birds are fused together in front. Supra-angular and coronoid splint-bones serve for the insertion of part of the temporal or masseter muscle. Additional splints rest on the inner side of the jaw. Like the crocodiles, birds possess a siphonium, i.e. a membranous, or ossified, tube which rises from a pneumatic foramen in the os articulare, on the median side of the articulation, and passes upwards between the quadrate and lateral occipital bone, opening into the cavity of the middle ear.

Fig. 6.—Skull of adult Fowl. Here the temporal fossa is bridgedover by the junction of the post-frontal and squamosal processes(pf., sq.). The processes of the mandible (iap, pap) arecharacteristic of this type, and of the anseres.
a, Angular of mandible. ar, Articular. bt, Basi-temporal. d, Dentary. eo, Lateral occipital. eth, Ethmoid. f, Frontal. iap, Interangular process of mandible. ios, Interorbital septum. j, Jugal. l, Lacrymal. mx, Maxillar. n, Nasal. os, Orbito-sphenoid. p, Parietal.	pa, Palatine. pap, Posterior angular process of mandible. pe, Ethmoid. pf, Post-frontal. pg, Pterygoid. ps, Pre-sphenoid. px, Premaxilla. q, Quadrate. qj, Quadratojugal. sa, Supra-angular or coronoid. so, Supra-occipital. sq, Squamosal. ty, Tympanic cavity. v, Vomer. 1, Exit of olfactory nerve.