The squamosal appendage, inseparably coalesced with the other elements composing the temporal bone, commences anteriorly by a thin pedicle (zygomatic process) which articulates with the malar. It assists the tympanic in the formation of the glenoid fossa for the articulation of the lower jaw (frontal haemapophysis); the remainder of the bone then spreads out into a flat scale, smooth externally, where it unites with the ali-sphenoid in front, and overlaps the parietal above; grooved internally, where it supports the ramifications of the middle meningeal artery, and forms part of the brain-case.
If vertical sections of the crania of the sheep, of the monkey, and of man be compared, it will be seen that, in the first, only the petrosal bone, or the auditory sense-capsule, makes its appearance in the cranial cavity, wedged in between the occipital and the parietal vertebrae; in the second, the squamosal bone is intercalated between the petrosal and the parietal, in the third is seen petrosal, squamosal, and mastoid; the latter deeply grooved for the terminal portion of the lateral sinus.
The proper interpretation of the meaning of the cranial bones has led to correcter views than were heretofore entertained of the relative importance of the different parts composing the encephalon. The common description of "the brain" throws no light upon the relations existing between the containing and the contained parts. All that we there learn is, that a mass of nervous matter occupies the brain-case, and overlaps a succession of ganglia, several of which are connected with the organs of special sense. Some of these parts are absolutely misnamed: as, for example, "the olfactory nerves," which in many animals are prolongations of the lateral ventricles, and even in man combine the necessary qualifications for a nervous centre, namely, the co-existence of white and of grey matter. They should be properly regarded as inter-cerebral commissures, which connect the olfactory ganglia with the cerebral hemispheres. The true olfactory nerves are those filaments which, invested in a neurilemma, pass through the cribriform plate of the ethmoid bone. Similar instances of incorrectness might be multiplied. In no department of science has greater light been thrown than in this, by the study of Comparative Anatomy. We learn that each of the four cranial vertebrae contains an encephalic segment similar to that which exists along the whole course of the neural canal, Although masked in man, by the great development of the cerebral hemispheres, each maintains to some extent its typical connections. The cerebellum is contained in the occipital vertebra; the mesencephalon, comprising the pons varolii, the corpora quadrigemina, the pituitary body, and the space called the third ventricle, is surrounded by the parietal vertebra ; the prosencephalon (cerebral hemispheres) lies in the frontal vertebra, and is continued in man both in the forward and backward direction ; over the other encephalic ganglia, the rhinencephalon, or the fourth primary division, is represented by the olfactory ganglia : it rests upon the cribriform lamella of the ethmoid bone, and is connected to the prosencephalon by the chords of white and grey nervous matter, misnamed olfactory nerves. These segments of the encephalon are connected by commissures, both longitudinal and transverse. In man, the cerebral ganglia far exceed all the others in relative size ; but in fish the ganglia are nearly equal, and succeed one another in linear series.