CH. XLIV.] 



THE PRIMITIVE BRAIN 



645 



a very small one. A highly intellectual man has a more elaborately 

 convoluted cerebrum than a savage. 



In spite of these differences, and many more might be mentioned, 

 there is throughout the vertebrate series from 

 fish up to man, the same general plan of con- 

 struction ; and the brain of the human embryo 

 is very much like the adult condition of the 

 brain of the fish. 



In the foetus the central nervous system 

 is formed by an infolding of a portion of the 

 surface epiblast. This becomes a tube of 

 nervous matter, which loses all connection 

 with the surface of the body, though later 

 in life this is in a sense re-established by the 

 nerves that grow from the brain and cord to 

 the surface. The anterior end of this tube 

 becomes greatly thickened, to form the 

 brain, its cavity becoming the cerebral ven- 

 tricles ; the rest of the tube becomes the 

 spinal cord. The primitive brain is at first 

 subdivided into three parts, the primary 

 cerebral vesicles; the first and third of these 

 again subdivide, so that there are ultimately 

 fivs divisions, which have received the 

 following names : 



1. Pros-encephalon, telencephalon or fore- 

 brain. This is developed into the cerebrum 

 with the corpora striata. It encloses the 

 lateral ventricles. 



2. Thalam-encephalon, diencephalon, or 

 twixt brain. This is developed into the parts 

 including the optic thalami, which enclose 

 the third ventricle. 



3. Mes-encephalon, or mid-brain, con- 

 sists of the parts which enclose the aque- 

 duct of Sylvius namely, the corpora 

 quadrigemina, which form its dorsal, and 

 the crura cerebri, which form its ventral 

 aspect. The corpora quadrigemina in many 

 lower animals instead of being four in number 

 are two, and are called the optic lobes. 



4. Met-encephalon, or hind-brain, which forms the cerebellum 

 and pons. 



5. Myel-encephalon, or after-brain, which forms the bulb or 

 medulla oblongata. 



M.o. 



FIG. 397. Diagrammatic hori- 

 zontal section of a vertebrate 

 brain. The figures serve both 

 for this and the next diagram. 

 Afb, mid-brain : what lies in 

 front of this is the fore-, and 

 what lies behind, the hind- 

 brain ; Lt, lamina terminalis ; 

 Olf, olfactory lobes ; Hmp, 

 hemispheres ; Th. E, thalam- 

 encephalon ; Pn, pineal 

 gland ; Py, pituitary body ; 

 P.M., foramen of Munro ; cs, 

 corpus striatum ; TA, optic 

 thalamus ; CC, crura cerebri : 

 the mass lying above the canal 

 represents the corpora quad- 

 rigemina ; Cb, cerebellum ; 

 M.o., medulla oblongata; 

 / IX, nine pairs of cranial 

 nerves ; 1, olfactory ventri- 

 cle ; 2, lateral ventricle ; 

 8, third ventricle ; 4, fourth 

 ventricle ; + , iter a tertio 

 ad quartum ventriculum, or 

 aqueduct of Sylvius. 



(Huxley.) 



