OCR Text |
Show 416 DR. J. F. GEMMILL ON [Apr. 10, the olfactory region is destitute of cartilage and there is no tegmen over the third ventricle. In both specimens the supra-orbitals are displaced downwards so as to be ventro-lateral to the brain. The auditory cartilages are displaced similarly but to a slighter degree. Brain.-The cerebral lobes are markedly reduced in size and are fused together, the longitudinal fissure being almost entirely absent. The central cavity is slightly enlarged and extends downwards on the outer sides of the lobes further than in the normal condition. The pineal diverticula are small unstalked pouches. The optic lobes are of considerable size and are normal as regards their dorsal parts, but internally the medial furrow of the central canal is only slightly marked (fig. 5) and there is absence alike of the optic recess, of the hypophysis, and of the hypoaria. Optic tracts and nerves are absent. As in type A, the olfactory nerves are small and closely approximated. Eye.-The single small deeply embedded eyeball has no choroidal fissure, vitreous humour, or optic nerve. The lens and the retina are, however, fairly well developed and there are two choroidal glands. The position of the optic pore is marked inside the eyeball by an interruption of the retina exhibiting a few nerve-fibres, which, however, fail to pierce the hexagonal pigment layer on the sclerotic (fig. 6). Two external and two superior rectus muscles are present. The other eye-muscles are wanting, with the exception of a pair of small superior obliques found in the specimen mentioned as having its supra-orbital bars extending forward into the frontal process. Mouth.-The mouth-opening is represented by a minute canal, beginning at the bottom of the groove between the maxillary flaps and extending backwards above the symphysis of the lower jaw. In one case this canal ends blindly, in another it joins the pharynx. 2. The specimen which had two small eyes closely approximated but ununited shows the following characters:-cerebral lobes well developed, deeply cleft anteriorly, but united posteriorly; pineal diverticula small; third ventricle almost obliterated; fusion of structures in the floor of the optic lobes; rudimentary hypophysis and hypoaria; optic tracts and nerves absent; eyes small, embedded, almost touching one another, without choroidal fissure, vitreous humour, or optic nerve, but with well-developed lens, retina, and retinal pigment-layer; superior obliqui, superior and external rectus present for each eye, inferior recti and obliqui wanting; no mouth, the upper and lower jaws being sealed together; trabecule cranii extremely short, forming a single bar projecting downwards and forwards into wall of pharynx ; olfactory capsules absent; supra-orbital bars ending separately in front, the tip of each giving origin to an obliquus oculi superior ; olfactory pits approximated and supplied by small olfactory nerves. |