The cavity of the middle ear is formed by an upgrowth from the first visceral pouch ; in Sphenodon and lizards the cavity communicates with the pharynx by a wide opening, in Chelonia by a narrow Eustachian tube. In crocodiles the Eustachian tubes of the two ears meet and form a duct running in a special canal between the basisphenoid and basioccipital to open in the middle line just behind the choanae ; lateral branches from the duct pass in canals between the basisphenoid and the pterygoids up into the supraoccipital and cranial roof, there enlarging into air spaces which again communicate with the tympanic cavities. Finally, a tube rising from each cavity leads air down into the quadrate and lower jaw. The history of this elaborate arrange ment can be made out from fossil materials. In snakes, the tym panic cavity is totally obliterated. The outer wall of the tympanic cavity is the tympanic membrane, which, in crocodiles and most lizards, is a thin sheet sunk below the surface of the head at the lower end of an external auditory meatus. In Sphenodon and Chelonia it lies flush with the surface, and its outer surface is indistinguishable from that of the neck. In snakes and chameleons it is absent.
The tympanic membrane is connected with the fenestra ovalis by a rod, whose inner end is an ossified columella or stapes, whilst the unossified outer end is the extra columella. This is small and simple in Chelonia, absent in snakes, where the end of the stapes articulates with the quadrate, and in Amphisbaenans. In lizards, the extra columella has a dorsal process attached to the end of the paroccipital process, and often detached, a ventral process applied to the quadrate, and a plate for insertion in the tympanic mem brane. The whole structure is of hyoidean origin, and the hyoid articulates with the end of the paroccipital process. In Sphenodon it fuses with the end of the extra columella, and in crocodiles it arises from its shaft to pass down the air canal to the lower jaw and become continuous with Meckel's cartilage.
Most or all reptiles are capable of hearing, but we know nothing of their ability to discriminate musical notes.
The sympathetic system presents an advance over that of most Amphibia in that many of the ganglia in the thoracic region are fused into a single large ganglion, and that the cervical sympa thetic is separated into deep and superficial portions, each running continuously from the ganglion of the vagus to the thorax, the details varying in different orders. This arrangement is derived
from that of Urodeles and leads directly to birds.
The cranial nerves of reptiles differ from those of living Am phibia in the absence of all trace of the lateral line system, except the auditory nerve, in the presence of a spinal accessory nerve XI. and in the fact that the hypoglossal nerve XII. passes out through the exoccipital bone.
In Sphenodon and in Chelonia the whole of the upper surface of the hemisphere, from the hippocampus on the inner surface to the lateral face, is covered by a pallium, devoted to the sense of smell. This layer of cells then turns inward from the surface, and lies on the top of the corpus striatum, forming a hypopallium. In lizards, and especially in crocodiles, the dorsal surface of the hemisphere becomes less and less nervous until, in birds, it is a mere membrane playing no part in the functioning of the brain. In these reptiles the hypopallium becomes broken up by a pene tration of nerve fibres, loses its pallial appearance and becomes assimilated to the corpus striatum. Thus, presumably in these animals, and certainly in birds, behaviour is controlled and memory exercised by a part of the brain quite different from that which fulfils these functions in mammals. In some reptiles, at any rate, the first trace of the neopallium, which is the import ant and developing part of the brain in mammals, is represented by a small cortical area in which alone other senses than smell gain a direct representation.
The mid brain of reptiles has its roof thickened and raised into a pair of optic lobes, which not only receive the endings of the optic nerves from the retina but are the motor area, stimulation of which brings about movements of the body.
The cerebellum of reptiles is always larger and better devel oped than that of Amphibia, though in living forms it is not externally divided into regions, as is that of a bird or mammal.
The brain of the extinct pterodactyls is interesting because, in the reduction of the olfactory lobe, the large size of the cere bellum and the lateral position of the optic lobes, it exactly resembles that of a bird, is indeed more like that of recent birds than is the brain of Archaeopteryx, which is the most primitive member of that class.