344 THE ANIMAL KINGDOM. 



the different Vertebrate classes, and the mid-brain thus forms the anterior 

 extremity of the body. This bend is known as the cranial flexure. 

 The hind-brain is constricted into two lobes the cerebellum in front, 

 and the medulla oblongata behind. The cerebellum of the adult is the 

 roof, the pons Varolii, the floor of the first lobe. The medulla oblongata 

 is the floor of the second lobe. Its original roof widens and thins away 

 at the same time, and is eventually represented only by an epithelial layer. 

 The two ridges which carry the roots of the nerves are thus widely separated 

 from the middle line. The primitive cavities of the brain-vesicles persist 

 as the ventricles. The ventricles of the olfactory lobes are sometimes 

 obliterated ; those of the cerebral hemispheres only in some Pisces. The 

 spinal cord extends primitively throughout the whole length of the spinal 

 canal or backbone, and so persists in Pisces ; but in other Vertebrata it 

 shortens, its terminal portion being represented by a filum terminale. 



The original cellular walls of the spinal cord develope into an outer 

 layer of medullated nerve-fibres ( = white matter), and an inner mass of 

 ganglion cells and fine fibrils ( = grey matter) surrounding a canalis centralis, 

 a remnant only of the original cavity of the cord continuous anteriorly 

 with the ventricles of the brain, which is lined by a ciliated epithelium. 

 The grey matter in the thalami optici, mid-brain, and medulla oblongata, 

 forms ganglionic masses ; in the cerebellum an outer layer ; in the cerebral 

 hemispheres both ganglionic masses (corpus striatum) and an outer layer 

 or cortex. The superficial layer of white matter in the cerebellum and 

 hemispheres is exceedingly thin ; on the contrary white matter is largely 

 developed internally to the grey layer. In Amphibia and higher Vertebrata 

 the hemispheres are united anteriorly by a transverse band of fibres the an- 

 terior commissure. A similar band behind the root of the pineal gland forms 

 the posterior commissure of Elasmobranckii, Sauropsida, and Mammalia. 

 In the last-named class there is a grey or middle commissure uniting the 

 two optic thalami, and two bands, one of transverse fibres the corpus 

 callosum uniting the hemispheres across the roof of their ventricles, the 

 other of longitudinal fibres the fornix 1 . The brain and spinal cord are 

 invested by three membranes a pia mater, immediately applied to their 

 surfaces ; an arachnoid, covering the pia mater ; and a dura mater, which 

 lines the inner surface of the cranium, but in the spinal cord is split 

 into two layers, one lining the spinal canal, the other, or theca spinalis, 

 forming a sheath round the cord. 



The cranial or cerebral nerves are (i) olfactory, in connection with 



1 The corpus callosum appears to be represented in both Reptilia and Amphibia. The anterior 

 commissure of the brain in these classes is divisible into an upper and lower portion. The latter 

 represents the true anterior commissure; the former represents the corpus callosum, 'and contains 

 the fibres of the dorso-medial moiety of the hemispheres.' See Osborn, Z. A. ix. 1886. The corpus 

 callosum is also possibly indicated in Fish. 



