OPTIC THALAMI. 115 



particles, aggregated into masses and formed of earthy salts (phosphate and 

 carbonate of lime, with a little phosphate of magnesia and ammonia) combined 

 with animal matter. 



The same sandy matter is frequently found on the outside of the pineal body, or 

 deposited upon its peduncles. It is found also in the choroid plexuses ; and in a 

 scattered form occurs in other parts of the membranes of the brain. It occurs at 

 all ages, frequently in young children, and sometimes even in the foetus. It cannot, 

 therefore be regarded as the product of disease. The pineal body is larger in the 

 child and the female than in the adult male (Huschke). In the brains of other 

 mammals it is proportionally larger than in the human subject, and less loaded with 

 brain-sand. 



The pineal body is developed originally as a hollow outgrowth from that part of 

 the embryonic brain which afterwards forms the third ventricle ; the diverticulum 

 becomes subsequently cut off from the ventricle, and undergoes ramification to form 

 tubes which are afterwards separated for the most part into isolated vesicles. 



The pineal body is present in all vertebrates, Amphioxus only excepted. In 

 elasraobranch fishes and in most reptiles, as the researches of de Graaf, Baldwin 

 Spencer, and others, have shown, it is continued into a long tubular prolongation 

 from the third ventricle, which passes through an aperture in the skull (parietal 

 foramen) and ends under the skin in a small vesicle lined with ciliated epithelium. 

 But in some reptiles (e.g., Hatteria, blind-worm, lizard) this vesicle becomes 

 developed into a structure which bears a close resemblance to an invertebrate eye 

 (pineal eye), the part nearest the surface becoming thickened to form a kind of lens, 

 and the part connected with the stalk becoming pigmented and stratified like a 

 retina, whilst the stalk itself becomes solid and has nerve-fibres developed in it. It 

 is doubtful how far this structure serves as an eye in any living reptile, but in 

 certain extinct forms it was probably more completely developed. In birds and 

 mammals the pineal eye is not developed, but the organ is similar in structure to 

 that of man. 



The posterior perforated space (locus perforatus posticus) (fig. 82, x ). 

 lies in a deep fossa (fossa Tarini, His) at the base of the brain, at the bottom of 

 which is greyish matter, connecting the diverging crura. It is perforated by 

 numerous small openings for the passage of blood-vessels ; and some horizontal 

 white striae usually pass out of the grey matter and turn round the peduncles close 

 to the upper border of the pons, entering which they reach eventually the medullary 

 centre of the cerebellum (tcenia pontis). It corresponds posteriorly, as far as a line 

 joining the anterior borders of the third nerves, to the floor of the aqueduct of 

 Sylvius, but in front of those nerves to the posterior part of the floor of the third 

 ventricle. In the grey matter over the space are a few scattered nerve-cells. 



The corpora albicantia or mamillaria (fig. 32, a ; fig. 86) are two round 

 white eminences in front of this space, each about the size of a small pea, connected 

 together across the middle line. Each corpus albicans contains grey matter 

 concealed within its superficial white fibres, the nerve-cells being arranged in two 

 groups, the lateral and mesial (nuclei of the corpus albicans) ; of these the lateral 

 contains larger nerve-cells than the mesial. 



The white matter of the corpora albicantia is formed by the anterior pillars of 

 the fornix : hence they have also been named bulbs ofthefornix; and by the bundle 

 of Vicq d'Azyr, which enters the anterior part of each tubercle at the dorso-mesial 

 aspect. Posteriorly each corpus albicans receives a bundle of nerve-fibres, which 

 is termed its peduncle. This, which in man is concealed within the grey matter of 

 the floor of the third ventricle, but which is seen at the base of the brain in many 

 animals, and is connected with the lateral nucleus of the body, is traceable to the 

 tegmentum and ultimately to the mesial part of the crusta (v. Gudden). In most 

 vertebrates there is but one (median) corpus albicans in place of two. 



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