42 CYCLOPIA— STRUCTURE 



region is destitute of cartilage and there is no tegmen over the third ventricle. In both specimens 

 the supraorbitals are displaced downwards so as to be ventro-lateral to the brain. The auditory 

 cartilages are displaced similarly but in 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 diverticulum is 

 a small unstalked pouch. The optic lobes are of considerable size and are normal as regards 

 their dorsal parts, but internally the mesial furrow of the central canal is only slightly marked, 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. 



A transverse section at the origin of the olfactory nerves shows an area of brain which is only 

 ■32 of normal size as judged from a specimen of the same age. The proportion to normal in a 

 similar section at the back part of the cerebral lobes is '7, while the middle region of the optic lobes 

 is very distinctly (I "3 times) in excess of normal. Compare with the corresponding figures for the 

 (A) type of cyclops. 



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 fail to pierce the hexagonal pigment layer or the 

 sclerotic (PI. XXIV. fig. 104). Two external and two superior recti muscles are present. The 

 other eye-muscles arc awanting, with the exception of a pair of small superior obliques occurring 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 un-united shows the 

 following characters : cerebral lobes well developed, deeply cleft anteriorly, but united posteriorly ; 

 pineal diverticulum small ; third ventricle almost obliterated ; incomplete 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 as well as superior 

 and external recti present for each eye ; inferior recti and obliqui wanting ; no mouth, the upper and 

 lower jaws being sealed together ; trabeculae 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 ocidi superior ; olfactory pits 

 approximated and supplied by small olfactory nerves. 



Summary. Olfactory Organs. — Olfactory nerves and pits, reduced in size, are present in all 

 my specimens. The olfactory pits lie close together on the inferior aspect of the mesial frontal 

 process. 



Brain. — Fusion of the posterior parts of the cerebral lobes is found in all my specimens. By 

 itself, as in type A, this condition is compatible with the presence of a well-developed cyclopic eye 

 possessing vitreous humour and an optic nerve, as well as with the presence of pineal diverticulum, 

 hypophysis, and hypoaria, and of optic tracts and optic recess. 



Fusion of the basal structures in the mid-brain, as in type B, is associated with greater defects, 

 viz. reduction in the size of the eye-ball ; absence of choroidal fissure, optic nerve and optic tracts ; 

 absence or rudimentary condition of hypophysis and hypoaria. 



The absence of dropsy of the central cavity of the brain is remarkable. 



Eye. — As seen in type A, the eye may be remarkably well-developed, possessing lens, retina, 

 vitreous humour, retinal pigment, and optic nerve. A double set of normal eye-muscles, excepting 

 only the internal recti, may be present. Paired superior and external recti are constant, while the 

 superior obliqui and the inferior recti are variable. The remarkable set of conditions which 



