CHAP. IX.] NERVOUS SYSTEM AND ORGANS OF SENSE. 



259 



tlio neural canal till they come to the foramina appropriated to them 

 respectively. Thus a bundle of nerves passes backwards in the 

 hinder part of that canal, on each side of the filum terminale, and 

 the whole bundle of such fibres goes by the name of the cauda 

 equina. 



The spinal cord is composed principally of white fibres, while the 

 grey matter within it is so aggregated as to present the appearance, 

 in transverse sections, of two crescentic masses with their con- 

 vexities adjacent and placed one in each lateral half of the cord 

 (Fig. 124, C). 



Each grey crescent ends in what is called an anterior and 

 posterior horn, which approach respectively to the anterior and 

 posterior lateral furrows. The posterior horn is long, with a narrow 

 end. The anterior horn is shorter and thicker, with a rounded end. 



Fi.1,'. 125. — Lateral view of the Brain. 



F. Frontal Iolip. 

 T. Temporal lobe. 

 ol. Olfactory lobe. 

 ra. MeduUa oblougati. 

 ch. Cereljellwn. 

 V. Pons Varolii. 



S. Syhian fissure (the Sylvian fissui'e is the 

 sulcus which passes baclvwards and some- 



wliat upwards from the spot to wliich the 



letter S has been made to point, 

 s. Superior external g>'rus. 

 m. Mid'lle external gyrus. 

 i. Inferior external gyrus. 

 0. Supra-iirbital sulcus. 

 c. Crucial sulcus. 

 li. Hippocampal gyrus. To the right of h is 



.seen the cut end of the left optic nerve. 



The two crescents are united together by a band of grey tissue 

 running across transversely at the bottom of the posterior median 

 fissure, and called the grey or posterior commissure. Another band 

 of white tissue also runs transversely across at the bottom of the 

 anterior fissure, and is called the white or anterior commissure. 



A minute central canal runs backwards along the whole length 

 of the spinal cord and into the filum terminale. It traverses the 

 posterior or grey commissure, and is lined with a layer of cylindrical 

 cells of ciliated epithelium. It is called the canalis centralis. 



§ 7. The ERAix, or encephalon-, is that enlarged part of the 

 nervous centres which is contained within the cranium and is enve- 

 loped by the three membranes already described. It is a mass of soft, 

 but more or less solid, matter which fills up the whole cranial cavity, 

 fitting into all those depressions which we have found to exist in 

 the floor and other parts of that cavity. It consists of two large but 



very unequal parts, termed respectively the cerehrmn and cerehellmn, 



s 2 



