20 LECTURES ON THE CENTRAL NERVOUS SYSTEM. 



It is, however, of no small interest to examine the manner 

 in which the development of the brain takes place in animals, 

 proceeding from the lowest to the highest vertebrates. The 

 limits set to these lectures will not permit us to enter the some- 

 times very important details of the course of fibres and the finer 

 structure. You will, perhaps, most easily get an idea of the 

 difference in arrangement, and see how now this and now that 

 portion of the brain is most prominently developed, if you will 

 examine the figures, 1 1-17. These figures represent, in a slightly 

 diagrammatic form, sagittal sections through the brains of the 

 various vertebrate classes. If .you will first make yourselves 



FIG. 11. 

 Diagram of a sagittal section through the brain of a vertebrate. 



Uinterh., Hind-brain. Prim. V. H., Primary fore-brain. 



Millelhlrn, Mid-brain. . Schhixn Pltittr, Embryonic terminal lamina. 



Nachh., After-brain. Si-rund. I'unlcrh., Secondary fore-brain. 



Zir. Him, Inter-brain. 



familiar with the general diagram of a vertebrate brain (Fig. 11) 

 you will easily understand the other figures. You here see that 

 the primary fore-brain gives origin to the secondary fore-brain 

 (hemispheres), by a bulging out of the lateral portions of the 

 embryonic terminal lamina. You see how it protrudes ventrad 

 into tlie infundibulum, and how its dorsal wall (driven inward 

 by blood-vessels) forms the choroid plexus. 



Farther back the roof is elongated into two sacs, of which 

 the anterior is called the cushion of the epiphysis, and the pos- 

 terior the epiphyseal tube. We recognize next the roof of the 

 mid-brain (the corpus opticum or corpora quadrigemina), and 

 adjoining this the involuted layer of the cerebellum. This 



