474 THE BRAIN 



lentiform part of the capsule, and then along the lateral 

 wall of the posterior horn of the lateral ventricle to their 

 distribution in the visual area of the occipital cortex, are called 

 the optic radiations (Fig. 191). 



Through the posterior part of the internal capsule in the 

 retro-lentiform area, but at a lower level than the optic 

 radiations, there passes a series of fibres, from the medial 

 geniculate body to the temporal lobe, which constitute the 

 acoustic radiations (Fig. 191). 



Capsula Externa. The external capsule is a relatively 

 thin lamina of white matter which intervenes between the 

 lentiform nucleus and the claustrum. It is continuous, 

 anteriorly and posteriorly, with the internal capsule, and, 

 above, with the corona radiata. It blends, below, with the 

 sheet of white fibres which separates the lentiform nucleus 

 from the roof of the inferior horn of the lateral ventricle. 



THE PARTS OF THE BRAIN WHICH LIE IN 

 THE POSTERIOR CRANIAL FOSSA. 



The parts of the brain which lie below the tentorium 

 cerebelli in the posterior cranial fossa are the lower part of the 

 mid-brain, the pons, the medulla oblongata, and the cerebellum. 

 The mid-brain has been considered already. The cerebellum, 

 the medulla oblongata, and the pons constitute collectively 

 the rhombencephalon or hind brain, and they are grouped 

 around the fourth ventricle of the brain. The fourth ventricle 

 is a cavity which communicates below with the central canal of 

 the medulla spinalis, and above with the aquaeductus cerebri. 



Medulla Oblongata. The medulla oblongata is the con- 

 tinuation of the spinal medulla into the brain. It is nearly 

 30 mm. long (rather more than one inch), and may be 

 reckoned as beginning at the level of the foramen magnum. 

 Thence it proceeds upwards, in an almost vertical direction 

 (Fig. 1 88), and it ends at the lower border of the pons. At 

 first, its girth is similar to that of the spinal medulla, but it 

 rapidly expands as it approaches the pons, and consequently 

 it presents a more or less conical appearance. Its anterior 

 surface lies in the groove on the basilar portion of the occi- 

 pital bone, and its posterior surface is sunk into the vallecula 

 of the cerebellum. 



