The opisthotic lies between the epiotic and the lateral occipital with which it ultimately fuses; in some birds, e.g.
Vt', Atlas; Vt2, second vertebrae; a, intercondyloid process of the atlas; b, the articular surfaces for the occipital condyles.
I) is a single knob, being formed almost wholly by the basi-occipital, 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 proOtic encloses between it and the lateral occipital the fenestra ovalis, into which fits the columella of the ear.
Here the opisthotic bone appears in the occipital region, as in the adult Chelonian.
Oc, Occipital condyle.
Oc, Occipital condyle.
Oc, Occipital con dyle.
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.
(2) Procoelous, concave in front; only in the atlas, for the reception of the occipital condyle.
Birds possess an ear-muscle which at least acts as a tensor tympani; it arises near the occipital condyle, passes through a hole into the tympanic cavity, and its tendon is, in various ways, attached to the inside of the membrane and the neighbouring extracolumellar processes.
The apex of the cochlea is turned towards, and almost reaches the anterior wall of the occipital condyle; at most it makes but half a twist or turn; it possesses both Reissner's membrane and the organ of Corti.
The head is large, and the skull elongated, and elevated posteriorly into a transverse occipital crest.
The squamosals form part of the cranial wall, being firmly wedged in between the quadrate, prootic and occipital bones.
- The pterygoids are widely separated from the quadrates, not reaching beyond the level of the occipital condyle.
Thus the rabbit and the dog are not absolutely blinded by removal of the entire cortex, but in man destruction of the occipital cortex produces total blindness, even to the extent that the pupil of the eye does not respond when light is flashed into the eye.
In the equally large Bramatherium and Hydaspitherium of India the horns of the males were complex, those of the former including an occipital pair, while those of the latter arise from a common base.
As regards their distinctive features, the antlers are of a complex type and situated close to the occipital ridge of the skull, and thus far away from the sockets of the eyes, with the brow-tines in adult males palmated, laterally compressed, deflected towards the middle of the face, and often unsymmetrically developed.
Thus the " Celtic " ox (Bos longifrons), from remote ages the common type in the Alpine regions, is characterized by the height of its forehead above the orbits, by its highly-developed occipital region, and its small horns.
A difficulty naturally arises with regard to the fact that in reptiles the occipital condyle by which the skull articulates with the vertebral column is single, although composed of three elements, whereas in amphibians and mammals the articulation is formed by a pair of condyles.
The bones which bear the two occipital condyles have given rise to much discussion, and the definition given by Huxley in the previous edition - "two occipital condyles, the basi-occipital region of the skull either very incompletely or not at all ossified" - requires revision.
On the other hand, some reptiles have the occipital condyle divided into two and produced either by the basi-occipital or by the exoccipitals.
Few have any occipital crest, but several have the face ornamented by the outgrowth of a fleshy lobe or lobes.
The cleido-mastoid generally arises from the basi-occipital, and the pectoralis major is connected with the latissimus dorsi.
The three remaining families of the Hystricoidea, of which one is African while the other two are chiefly South American, are very closely allied and often brigaded in a single family group. In the Capromyidae, which includes only the South American and West Indian hutias, the South American coypu and the African cane-rats, the tympanic bulla of the skull is hollow, the par-occipital process straight, the lachrymal small, and the cheekteeth rooted, with deep enamel-folds; the first front toe Leing occasionally absent.
Some authors have held that the bone on which the occipital condyles have been found most developed in some labyrinthodonts (2) represents a large basi-occipital bearing two knobs for the articulation with the first vertebra, whilst the skull of the batrachians of the present day has lost the basi-occipital, and the condyles are furnished by the exoccipitals.
Fully developed pre oc, Occipital condyle.
From both birds and reptiles the class is distinguished, so far at any rate as existing forms are concerned, by the following features: the absence of a nucleus in the red corpuscles of the blood, which are nearly always circular in outline; the free suspension of the lungs in a thoracic cavity, separated from the abdominal cavity by a muscular partition, or diaphragm, which is the chief agent in inflating the lungs in respiration; the aorta, or main artery, forming but a single arch after leaving the heart, which curves over the left terminal division of the windpipe, or bronchus; the presence of more or fewer hairs on the skin and the absence of feathers; the greater development of the bridge, or commissure, connecting the two halves of the brain, which usually forms a complete corpus callosum, or displays an unusually large size of its anterior portion; the presence of a fully developed larynx at the upper end of the trachea or windpipe, accompanied by the absence of a syrinx, or expansion, near the lower end of the same; the circumstance that each half of the lower jaw (except perhaps at a very early stage of development) consists of a single piece articulating posteriorly with the squamosal element of the skull without the intervention of a separate quadrate bone; the absence of prefrontal bones in the skull; the presence of a pair of lateral knobs, or condyles (in place of a single median one), on the occipital aspect of the skull for articulation with the first vertebra; and, lastly, the very obvious character of the female being provided with milk-glands, by the secretion of which the young (produced, except in the very lowest group, alive and not by means of externally hatched eggs) are nourished for some time after birth.
In man the occipital foramen, through which passes the spinal cord, is placed just behind the centre of the base of the skull, which is thus evenly balanced in the erect posture, whereas the gorilla, which goes habitually on all fours, and whose skull is inclined forward, in accordance with this posture has the foramen farther back.
Of the occipital foramen.
The distinction, therefore, between the movement of the eyeballs, elicited from the occipital (visual) cortex, and that of the hand, elicited from the cortex in the region of the central sulcus (somaesthetic), is not a difference between motor and sensory, for both are sensori-motor in the nature of their reactions; the difference is only a difference between the kind of sense and sense-organ in the two cases, the muscular apparatus in each case being an appanage of the sensual.
The under jaws are hinged on to the quadrate bones, which extend obliquely backwards, and are immovably wedged in between the squamosal and the lateral occipital wings.
The Octodontidae, which are exclusively South American, differ from the preceding family by the tympanic bulla being filled with cellular bony tissue, and by the par-occipital process curving beneath it, while the cheek-teeth are almost or completely rootless and composed of parallel plates.