CEREBRAL VENTRICLES IN THE PIG 227 



all concave, and in the central part of the mid-brain there are 

 five surfaces — two dorso-lateral, bounded by the median dorsal 

 ridge and the lateral ridges; two ventro-lateral, extending down- 

 ward to the sulci liniitantes; and a ventral surface between these 

 sulci. 



The form of the mid-brain ventricle in the embryo measuring 

 110 mm. has been described at length, since subsequently it 

 undergoes only slight modifications. This was determined by 

 modelling the ventricles in the 260-mm. pig (figs. 25 and 26) 

 and by making dissections of the adult. The ventricle of the 

 mid-brain continues to increase in size but it does not keep pace 

 with the growth of the adjoining cavities. The grooves remain 

 as described, and the ventral surfaces, between the sulci linii- 

 tantes, are always subdi^dded posteriorly by a forward extension 

 of the median ventral sulcus. 



HIND-BRAIN 



In the 5.1-mm. embryo the cavity of the hind-brain or third 

 cerebral vesicle is elongated and quite straight. Behind the ven- 

 tricle of the mid-brain (fig. 4) a well marked constriction indi- 

 cates the future isthmus. Caudad to this, the cavity, or fourth 

 ventricle, widens somewhat, and then slopes gradually through 

 the medulla to the spinal cord. The low ridge which follows the 

 line of greatest width corresponds at this stage with the line 

 of attachment of the thin roof-plate, and not with the sulcus 

 limitans. Along the ventral surface of the cast there is a sharp 

 median ridge which represents the sulcus medianus of the rhom- 

 boid fossa. 



Seven neuromeral grooves can be identified. The first pro- 

 duces a low but broad ridge on the cast anterior to the widest 

 part of the rhombencephalon. Dorsally it flattens out before 

 reaching the line of maximum width and ventrally it does not 

 extend quite to the median sulcus. The second neuromeral 

 groove is situated opposite the widest portion of the fourth ven- 

 tricle. The next four are about equalh' spaced. They are very 

 prominent in the ^-entral zone and some of them appear to reach 

 upward a short distance into the dorsal zone. The last one is 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 15, NO. 2 



