CEREBRAL VENTRICLES IN THE PIG 217 



to a lamp board. The lamp board, with the sockets connected in paral- 

 lel, serves as a rheostat, and any desired temperature can be obtained 

 by turning on the proper lights. One 50 e.p. globe allows about the 

 desired amount of current to flow. A larger electrically heated smoother 

 for wax models and an electrically heated knife for cutting wax plates, 

 which were used in making the models, will be described at a future date. 



Those who study the nervous system from the standpoint of 

 comparative physiology regard the "functional system of neu- 

 rones" as the real unit of the nervous system. Thus Herrick 

 ('08), tin discussing the morphological subdivisions of the brain, 

 refers to the influence of metamerism as primitive, dominating 

 the subdivision of the nervous system of lower vertebrates. He 

 further states that transverse divisions, such as the diencephalon 

 and mesencephalon, are not ' natural regions' because the primary 

 metamerism has ceased to be an important factor. But the 

 anatomical subdivisions exist no less than the physiological, and 

 although their significance may not be profound, they form a 

 convenient basis for description. In the following account the 

 primary divisions of the brain will be considered in this order — 

 fore-brain, mid-brain, hind-brain. Reference will be made es- 

 pecially to the casts and models of the ventricles, but the wall 

 of the brain must also be studied as the changing structure which 

 modifies the cavities within. 



FORE-BRAIN 



A model of the cerebral ventricles of a 5.1-mm. embryo, the 

 youngest one studied, shows that the three divisions of the brain 

 are distinctly marked off from each other, and that the fore-brain 

 is aheady subdivided into telencephalon and diencephalon. John- 

 ston ('09) states that the boundary between the telencephalon and 

 the diencephalon is determined in mammals, as well as in lower 

 vertebrates, by the velum transversum above and the primitive 

 optic groove or postoptic recess below. This subdivision is dis- 

 tinct in the young pig studied as seen in figure 4. In the model 

 very slight lateral swellings from the telencephalon indicate the 

 cavities of the rudimentary * cerebral hemispheres. The dien- 

 cephalon shows a division into two parts or neuromeres, the first 



