SPINAL CORD. 385 



alimentary passage, which has been replaced by surrounding 

 nervous material, and which ceased to be functional when 

 the permanent gut became a tube open at each end. 



In the brain the grey matter forms the ganglionic masses 

 of the optic thalami, the optic lobes, and the medulla ob- 

 longata, and is covered by a layer of white matter. In the 

 cerebellum, and in the cerebral hemispheres, the superficial 

 layer of white matter is very thin ; in both the grey matter 

 forms a cortex beneath the superficial white layer, in the 

 cerebral hemispheres internal ganglionic masses as well ; in 

 both, moreover, white matter occurs again within the grey. 



In Cyclostomata, Ganoids, and Teleosteans, the fore-brain 

 has no nervous roof, but is covered by an epithelial pallium 

 homologous with what is called the choroid plexus of the 

 third ventricle in higher Vertebrates. This choroid plexus 

 is a thin epithelium, with blood-vessels in it. But in 

 Elasmobranchs, Dipnoi, and Amphibians the basal parts of 

 the fore-brain have grown upwards to form a nervous roof, 

 and this persists in higher Vertebrates. The roof of the fourth 

 ventricle is always thin and epithelial in adults. 



Enswathing the brain and following its irregularities is a 

 delicate membrane — the pia mater — rich in blood-vessels 

 which supply the nervous system. Outside this in higher 

 Vertebrates there is another membrane — the arachnoid — 

 which does not follow the minor irregularities of the brain 

 so carefully as does the pia mater. Thirdly, a firm mem- 

 brane — the dura mater- — lines the brain-case, and is continued 

 down the spinal canal. In lower Vertebrates the dura 

 mater is double, in higher Vertebrates it is so in the region 

 of the spinal cord, where the outer part lines the bony 

 tunnel, while the inner ensheathes the cord itself In 

 Fishes the brain-case is much larger than the brain, 

 and a large lymph space lies between the dura and the pia 

 mater. 



Sense-Organs. — The central nervous system has doubtless 

 arisen in the course of history from the insinking of external 

 nerve-cells ; it does arise in development as an involution of 

 ectoderm or epiblast. The same layer gives origin to the 

 sense-organs, the nose, the ear, and the sensory structures of 

 the skin. The Vertebrate eye is somewhat divergent, for it 

 is formed in great part as an outgrowth from the brain, but 



2 B 



