224 CHESTER H. HEUSER 



enular ganglion. The conspicuous ridge at the lower margin, 

 of this habenular concavity continues caudally into the pineal 

 recess. Between the pineal recess and the groove for the pos- 

 terior commissure there are two slight projections of the third 

 ventricle (infrapineal recesses). The very extensive fusion of 

 the thalami, forming the massa intermedia, has produced a hole 

 in the model of the third ventricle. It is oval in outline, measur- 

 ing about 4.6 mm. by 3 mm. 



MID-BRAIN 



It is well known that in young embryos the cavity of the mid- 

 brain is very large, forming the middle cerebral vesicle. In the 

 adult it is generally described as '^ reduced to a narrow slit— the 

 aqueduct of Sylvius." Cunningham states that the mid-brain 

 "is tunnelled by a narrow passage, the aqueduct of Sylvius, 

 which extends between the fourth and third ventricles, "and Bens- 

 ley ('10) writes: ''The mesencephalon is noteworthy in a mam- 

 mal as lacking a ventricle. Its cavity is a narrow canal, the 

 aqueduct of Sylvius . . . . " But Retzius ('00) described 

 a spindle-shaped expansion in the middle portion of the cavity 

 of the adult human mid-brain, and named it 'ventriculus mesen- 

 cephali.' It will be shown that a distinct cavity exists in the 

 mid-brain of the adult pig, comparable with that found by Ret- 

 zius in the human brain. 



In the 5.1-mm. pig (fig. 4) a slight constriction separates the 

 caudal part of the diencephalon from the cavity of the mid-brain. 

 Another slight constriction marks the position of the isthmus 

 between the mid-brain and the hind-brain. The angle formed 

 by a line passing through the axis of the diencephalon with an- 

 other through the isthmus and cavity of the medulla, is very acute 

 (30°) in this stage. In the adult it is very wide. 



The cavity of the mid-brain continues to expand and becomes 

 sharply marked off from that of the hind-brain at the isthmus, 

 as shown in the 12-mm. embryo (fig. 5). Between the mid-brain 

 and fore-brain, however, the cavity shows only a slight constric- 

 tion, which is limited to the lateral surface. As seen in the cast 



