THE THIRD VENTRICLE AND INTER-BRAIN. 135 



cone-shaped body, a quarter of an inch high and one- sixth of an 

 inch in diameter, joined to the roof of the third ventricle by a 

 flattened stalk or peduncle. It is also called the epiphysis. The 

 pineal body is situated in the floor of the transverse fissure of 

 the cerebrum, directly below the splenium of the corpus callosum 

 and rests between the superior colliculi of the quadrigeminal 

 bodies on the posterior surface of the mid-brain. It is closely 

 invested by pia mater. The pineal stalk splits into a dorsal and 

 a ventral lamina, which are separated by the pineal recess. The 

 ventral lamina fuses with the posterior commissure; but the 

 dorsal stretches forward over the commissure in continuity with 

 the roof epithelium. The border of the dorsal lamina is thick- 

 ened along the line of attachment to the thalamus and forms the 

 stria medullaris thalami (pineal stria). The thickening is due 

 to the presence of a bundle of fibers derived from the columna 

 of the fornix and the medial stria of the olfactory tract. Between 

 the medullary striae, at the posterior end, there is a transverse 

 band, called the habenula. The habenula contains the commis- 

 sura habenularum, through which the fibers of the striae partially 

 decussate to the nucleus habenulae in the thalamus. 



The interior of the pineal body is made up of closed follicles 

 surrounded by ingrowths of connective tissue. The follicles are 

 filled with epithelial cells mixed with calcareous matter, the 

 brain-sand (acervulus cerebri). Calcareous deposits are found 

 also on the pineal stalk and along the chorioid plexuses. The 

 function of the pineal body is unknown. It is supposed to repre- 

 sent a cyclopian eye. In the Hatteria, a New Zealand lizard, 

 it projects through the parietal foramen and presents an imper- 

 fect lens and retina and, in its long stalk, nerve fibers. 



The chorioid tela of the third ventricle (velum interpositum, 

 Figs. 37, 40 and 42) is the double triangular fold of pia mater 

 spread over the dorsum of the inter-brain. It lies underneath 

 the fornix and the chorioid epithelial lamina which stretches 

 from the body of the fornix lateralward to the stria terminalis. 

 Its apex is just behind the anterior commissure, and its base, 

 directed backward, is continuous, by the upper layer, with the 

 pia mater of the occipital lobes; and, by the inferior layer, it is 



