Meek, Choroid Plexus. 



289 



blood returns through the choroid vein to theV. cerebri interna 

 which joins its fellow and forms the V. cerebri magna or vein of 

 Galen. 



It is generally believed that the choroid plexuses are largest in 

 the embryonic state, and that their volume diminishes as the brain 

 reaches its full development. Loeper ('04) states that in the 

 human foetus the plexuses are largest from the third to the sixth 

 month, after which they decrease in size. Possibly a more accu- 

 rate statement would be that after the sixth month they are out- 

 grown by the other parts, and the decrease in size is only propor- 

 tional. The His-Ziegler model for the three months human 



Fig. 2. Cross section through the brain of an embryo rat, showing the origin of the choroid plexus. 

 Magnification X "joo. 



foetus, shows the plexuses as large swollen glandular organs oc- 

 cupying practically all of the space in the lateral ventricles. Along 

 with this presentation, has grown the idea that the plexuses furnish 

 some kind of a fluid food necessary for the growth of the brain 

 during the foetal period. Accordingly, they have sometimes been 

 called the "cerebral placenta." 



Whatever the facts may be in man,^ it seems that the above 

 description does not apply to all forms. Fig. 2 shows the begin- 



^ A recent reconstruction of the encephalon of a human embryo (length 22.8 millimeters, age approxi- 

 mately 2 months) by Dr. Ewing Taylor, of the Universtiy of Pennsylvania, shows the plexuses of the 

 lateral ventricles as much smaller and more folded than they appear in the His-Ziegler model of three 

 months. A similar difference is shown by the model of the encephalon reconstructed from a slightly 

 larger embryo (length 30 mm., approximate age 55 days) by Dr. G. L. Streeter of the Wistar Institute 

 of Anatomy. In view of these results, the relations exhibited by the His-Ziegler model can hardly be 

 considered as typical. 



