RELATIONS OF THE ADULT PINEAL ORGAN 417 



both sets of fibres belong to the sympathetic system. This fold has been 

 described as the posterior ligament (Calvet), whereas the reflections at 

 the side are styled the lateral ligaments. In some cases the body and apex 

 of the pineal organ are completely surrounded by a plexus of vessels 

 lying in the subpial tissue and containing calcareous concretions. In 

 old subjects this tissue is often very dense and thick, so that consider- 

 able difficulty may be experienced in freeing the body from its 

 surroundings. 



At the base of the organ are the superior and inferior peduncles and 

 an intermediate or lateral peduncle (Calvet) which connects the pineal 

 body with the medial surface of the thalamus. The superior peduncle 

 contains medullated nerve-fibres belonging to the superior or habenular 

 commissure, and the inferior peduncle conveys similar fibres of the 

 posterior commissure ; between the two commissures is the pineal recess. 



The superior peduncle is continued forward on each side as the 

 habenula (Fig. 281). This forms the inner boundary of the trigonum 

 habenulae, and anteriorly is continuous with the taenia thalami, which 

 marks the lateral limit of the roof of the third ventricle and the line along 

 which the ependyma on the lateral wall of the ventricle leaves the medial 

 surface of the thalamus. The habenular ganglion is situated in relation 

 with the posterior and median part of the optic thalamus, beneath the 

 trigonum habenulae. It receives afferent fibres from the stria medullaris 

 thalami, which if traced backwards divide into two bundles, of which one 

 joins the ganglion of the same side while the other crosses in the habenular 

 commissure to the ganglion of the opposite side. The stria medullaris 

 is connected in front with the anterior pillar of the fornix, these fibres 

 being derived from the cells in the hippocampal cortex, whereas a ventral 

 bundle of fibres comes from a collection of cells in the anterior perforated 

 substance. It is believed, therefore, that in the human subject the 

 habenular commissure is chiefly composed of decussating fibres belong- 

 ing to the olfactory system and that each habenular ganglion receives 

 relays of fibres from the olfactory organ of both the right and left side. 

 In lower vertebrates, however, such as the cyclostomes, in which definite 

 pineal sense-organs are present, the habenular ganglia receive afferent 

 fibres which arise in the ganglion cells of the retinae of the pineal eyes, 

 and in the human subject some of the fibres of the habenular commissure 

 appear to terminate in the basal part of the pineal body (see p. 408). 



The posterior commissure : in spite of the position of the posterior 

 commissure, as seen in median longitudinal sections of the brain, being 

 so familiar and such a valuable landmark, it has been found difficult to 

 trace its connections with certainty, and there is considerable difference 

 of opinion with regard to the origin of its fibres. Most authors are, 

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