THE BRAIN. 171 



The interbrain gives origin to the optic thalami, pineal 

 gland, infundibulum, (part of the) pituitary body, the optic 

 nerve (primarily). The cavity within the interbrain remains 

 as the third ventricle. It is joined to the lateral ventricles 

 by the foramina of Monro, which are the remains of the 

 comparatively wide openings in the embryo, that connects 

 the fore and interbrain cavities. 



The midbrain furnishes the crura cerebri, corpora quad- 

 rigemina, secondary portion of the optic nerve. The narrow 

 canal which runs through it is the aqueduct of Sylvius. 



The hindbrain forms the cerebellum, valve of Vieussens, 

 pons, and the anterior part of the fourth ventricle. 



Diag. 14. Same stage as the preceding diagram. VERTICAL ANTEROPOSTE- 

 RIOR SECTION OF THE CEREBRAL VESICLES. (Modified from Huxley.} i, Olfactory 

 bulb. 2, Forebrain roof, or pallium. 3, Pineal gland. 4, Corpora quadrigemina. 5, 

 Cerebellum. 6, Pons, and behind it the medulla. 7, Crus cerebri. 8, Pituitary gland. 

 a, Opening of olfactory bulb into forebrain cavity, b, Infundibular pit. c, Corpus 

 striatum. d, Optic thalamus. e, Site of the foramen of Monro. f, Aqueduct of 

 Sylvius, g, Fourth ventricle, h, Portion of the roof that will be involuted in front of 

 the velum interpositum and choroid plexuses, x, Lamina terminalis. 



The afterbrain expands into the medulla. From it arises 

 the auditory nerve, and it completes the posterior part of 

 the fourth ventricle. 



To follow the changes a little more in detail that result 

 in producing the above results : The forebrain enlarges 

 rapidly forward, outward, upward, and backward until it 



