CEREBRAL VENTRICLES IN THE PIG 235 



This shifting in the mid-brain is brought about by the expansion 

 in the dorsal zone to form the cavities in the colHcuh. The 

 line of maximum width begins behind the constriction between 

 the mid-brain and fore-brain in the superior collicular cavity 

 and terminates at the caudal end of the mid-brain in the inferior 

 collicular recess. Anteriorly the hypothalamic sulcus is seen to 

 connect with the tegmental sulcus. This connection is most dis- 

 tinct in the 22. mm. embryo. 



His first called attention to the dorsal and ventral zones of 

 the brain and cord. A demarcation of the sensory from the 

 motor portion of the central nervous system is obviously of fun- 

 damental importance and Johnston ('09) has described the sulcus 

 limitans as the most important landmark in the brain. Diagrams 

 have appeared which show an uninterrupted line, extending from 

 the optic recess backward through the brain and spinal cord. 

 However, there is no real picture which shows a sulcus contin- 

 uous throughout the brain. It is distinct in each division of 

 the brain, but at the cephalic end of the isthmus it is interrupted. 



The hemisphere of course is very much larger in the human 

 embryos than in approximately corresponding stages of the pig. 

 The thalamus and habenular region are also further advanced 

 in the human stages. The tegmental folds never form conspicu- 

 ous projections into the ventricle of the mid-brain as they do 

 in early human embryos. The olfactory lobe becomes progres- 

 sively more conspicuous in the pig series — ^the reverse of the 

 condition in human embiyos, of later stages especially. In the 

 latter, the olfactory lobe is covered over by the rapidly expand- 

 ing pallium. The olfactory lobe in the pig always extends some 

 distance in front of the end of the hemisphere. The roof of the 

 fourth ventricle is not present in aii}^ of the His models, hence there 

 is no indication of a caudal protrusion of the fourth ventricle. 

 This protrusion is first well indicated in pig embryos about 22 

 mm. long. It expands very rapidly until the cerebellum grows 

 downward into this region. There being no foramen of Majendie 

 in the pig, the posterior medullary velum stretches nearly straight 

 across the caudal part of the fourth ventricle so that the caudal 

 protrusion is just recognizable in the adult. 



