286 



HUMAN ANATOMY. 



in its margins the choroid plexus of the lateral and third ven- 

 tricles. It is perforated by two slit-like foramina, one com- 

 municating with each lateral ventricle. 



The tela choroidea inferior forms the roof of the lower 

 part of the fourth ventricle. It contains nerves and lymphatics. 



THE ENCEPHALON, OR THE BRAIN. 



The encephalon, or brain, consists of four distinct portions 

 the cerebrum, cerebellum, pons Varolii, and medulla oblongata. 

 The average weight of the brain in the male adult is forty-nine 

 and a half ounces, in the female forty-four ounces, of which the 

 cerebrum is about seven-eighths. The maximum male brain 

 weighs sixty-five ounces; minimum, thirty-four ounces. The 

 maximum female brain weighs fift} r -six ounces; minimum, 

 thirty-one ounces. The brains of idiots seldom weigh more than 

 twenty-three ounces. Excepting the whale and elephant, the 

 human brain is heavier than that of all the lower animals. 



The brain is developed from the anterior portion of the 

 primitive neural tube. This expands and later becomes con- 

 stricted into three primary brain-vesicles, which are called the 

 forebrain (prosencephalon), midbrain (mesencephalon), and 

 hind-brain (rhombencephalon). The forebrain becomes differ- 

 entiated later into the telencephalon and diencephalon ; and the 

 hind-brain likewise is differentiated into two parts: the meten- 

 cephalon and the myelencephalon. 



From these brain- vesicles are developed the following : 



Encephalon, 

 or Brain, 



Rhombencephalon, 

 or hind-brain, 



Myelenceph- 

 alon, 



Metenceph- 

 alon, 



C Medulla oblongata, 

 ^ Lower part of fourth 

 [_ ventricle. 



f Cerebellum, 

 I Pons Varolii, 

 1 Upper part of fourth 

 ventricle. 



Isthmus rhom- ^ 

 bencephali 

 (narrow part 

 con necting 

 with mesen- 

 cephalon), 



Superior cerebellar pe- 

 duncles, 

 Valve of Vieussens. 



( M e s e n c e p h- f Corpora quadrigemina, 

 Mesencephalon, or ^ Qr *^\ ^ ^^ 



midbrain, ] hi . :iii ; [ Aqueduct of Sylvius. 



