1422 A MANUAL OF ANATOMY 



red in colour, and somewhat conical in shape. Superiorly it is inti- 

 mately related to the pia mater, as that membrane passes through 

 the transverse cerebral fissure to form the velum interpositum, and 

 the splenium of the corpus callosum lies above it with the inter- 

 vention of the pia mater. Inferiorly it is in contact with the depres- 

 sion between the upper quadrigeminal bodies. Its apex, which is 

 directed downwards and backwards, is free. Its base is directed 

 upwards and forwards, and contains the pineal recess, which is con- 

 tinuous anteriorly with the cavity of the third ventricle. The 

 portion of the base which lies below this recess is connected with 

 the posterior commissure, which separates it from the upper opening* 

 of the aqueduct of Sylvius. The portion above the recess contains 

 the commissura habenularum and the superior commissure. Con- 

 nected with either side of the base of the pineal body there is a 

 bundle of white fibres. These two bundles constitute the peduncles 

 or stalks of the pineal body. Each stalk passes outwards, and soon 

 terminates in the trigonum habenulae. 



Structure ol the Pineal Body. — The pineal body is free from nervous con- 

 stituents. It consists of a number of follicles, lined with epithelial cells, 

 and containing a variable amount of calcareous matter, called acervulus 

 cerebri or brain-sand, which is composed of calcium phosphate, calcium 

 carbonate, magnesium phosphate, and ammonium phosphate. 



Development. — The pineal body is developed as a diverticulum of the 

 posterior part of the dorsal aspect of the thalamencephalon or diencephalon. 

 This diverticulum for the most part becomes solid, but a portion of it persists 

 as the pineal recess of the third ventricle. 



The pineal body is regarded as representing the pineal eye of Uzards. 



Striae Medullares or Pineales. — Each stria meduUaris is a narrow 

 strip of white longitudinal fibres, lying along the upper part of the 

 mesial surface of the corresponding optic thalamus. It constitutes 

 the habenula. Anteriorly, most of its fibres are derived from the 

 olfactory lobe, more particularly the olfactory bulb and locus 

 perforatus anticus. Some, however, may be derived from the 

 anterior pillar of the fornix, and through the fornix from the cells 

 of the hippocampus major. Posteriorly, the fibres are disposed in 

 two ways, (i) The lateral fibres enter the ganglion habenulae, and 

 terminate in connection with its cells. (2) The mesial fibres curve 

 inwards towards the base of the pineal body, in which they cross 

 to the opposite side, lying above the pineal recess. As they cross 

 the median line they decussate with the mesial fibres of the opposite 

 stria medullaris, and they terminate in the ganglion habenulae of 

 the side to which they have crossed. Their decussation is known 

 as the commissura habenularum. 



Trigonum Habenulae. — This is a small triangular area which is 

 bounded posteriorly by the upper quadrigeminal body, internally 

 by the posterior part of the stria medullaris, and externally by the 

 adjacent part of the optic thalamus. It contains an important 

 group of multipolar nerve-cells, known as the ganglion habenulae. 

 This ganglion belongs to the mesial area of the optic thalamus. It 

 receives some of the fibres of the stria medullaris, which come from 



