THE DIENCEPHALON. II25 



rounded or sometimes elongated epithelial cells, which often contain brownish pig- 

 ment. With the exception of a few nerve-filaments in the anterior part probably 

 sympathetic in origin and destined for the blood-vessels, and a dense net-work of 

 neurogha fibres in the under part, 



the pineal body contains no ele- 1' *<*' 972. 



ments of a nervous character, nerve- Lenticular area 



111* i icctinil srcei 



cells being absent. Quite com- 

 monly the adult organ encloses a 

 variable number of concretions, 

 often called brain-sand {acervulus} , 

 which consist of laminated particles 

 composed of calcium carbonate 

 and phosphate mingled with or- 

 ganic material. They may be of 

 microscopic dimensions, or reach 

 the size of a millet seed, and by 

 aggregation assume a mammillated 

 form. 



Blood-vessel 



Diverticulum 



dividing into 



tubules 



Sagittal section of pineal organ of lizard (Lacerta agilis) 

 embryo. X 175. 



The significance of the pineal 

 body long remained an unsolved 

 riddle and served as the theme for 

 unrestrained speculation. The em- 

 bryological and comparative studies of 

 Graaf, Spencer and others have shown that in many of the lower animals, especially in the reptiles 

 (lizards), the pineal body reaches a high degree of development and is a flattened cup-shaped 

 organ connected with the brain by a stalk containing nerve-fibres. The structural resemblances 

 to the invertebrate visual organ suggested a possible similarity of purpose in the higher types, 

 an assumption that was strengthened by the fact that in certain lizards the pineal body not only 

 is borne by a stalk but reaches an interparietal subcutaneous position on the head by passing 

 through or lying within a special foramen in the skull. The organ was, therefore, designated 

 the pineal eye, although probably in no existing animal a functionating structure. While such a 

 superficial position in the adult is very exceptional, the embryonic relations in many reptiles 

 (Fig. 972) are very suggestive of the probable significance of the pineal body, at least in such 

 form as a rudimentary sense organ, although not necessarily an eye. These conclusions are 

 likewise suggestive in forming our conceptions concerning the pineal body in man, which is 



now by many regarded as representing a very imperfectly 

 developed and greatly modified sensory structure. 



Although strictly belonging to the telencephalon, men- 

 tion may here be made of a second evagination, know as the 

 paraphysis, which arises from the roof-plate of the fore-brain. 

 The pouch appears in advance of the pineal outgrowth and is 

 a temporary structure, seemingly being in nature comparable 

 to an outwardly directed choroid plexus. The paraphysis 

 has been described in the lower vertebrates, including reptiles 

 and birds, in some mammals and, indeed, according to 

 the observations of Francotte and of Ewing Taylor, it is not 

 improbable that a corresponding evagination is recognizable 

 in the early human embryo. 



FIG. 973. 



Small portion of pineal body, 

 showing constituent cells more highly 

 magnified. X 600. 



The posterior commissure (commissura poste- 

 rior cerebri) is a narrow but distinct cord-like band of 

 white matter which overlies the superior entrance into 

 the Sylvian aqueduct (Fig. 976) and is partially masked by the habenular commissure 

 and pineal peduncle above. Behind and laterally it is continuous with the superior 

 colliculi. The commissure provides the paths by which fibres from various sources 

 undergo median decussation, but the details and connections of its component fibres 

 are only imperfectly understood. Among its probable constituents are: (i) fibres 

 originating in the nucleus of the posterior commissure and also from the nucleus of 

 the posterior longitudinal fasciculus (nucleus fasciculi longitudinalis posterior), which 

 occupies the gray matter of the floor of the third ventricle near the mammillary 

 bodies (page 1117); (2) fibres from the posterior part of the thalamus of the 



