526 THE NEKVOUS SYSTEM. 



matter which separates the broad extremity of the anterior column from the sur- | 

 face of the spinal medulla. This latter portion of the anterior funiculus is 

 traversed by the emerging fila of the anterior nerve-roots. 



In cross-sections of the spinal medulla the partition of pia mater, which dips in 

 at the sulcus intermedius posterior and divides the posterior funiculus into the 

 medial fasciculus gracilis and the lateral fasciculus cuneatus, is very strongly marked 

 in the cervical regions, but as it is traced downwards into the thoracic region it 

 becomes shorter and fainter, and finally disappears altogether at the level of the 

 eighth thoracic nerve. Below this point there is no visible demarcation of the 

 posterior funiculus into two parts. 



The white matter is not present in equal quantity throughout the entire length 

 of the spinal medulla. It increases steadily from below upwards, and this increase is 

 most noticeable in the lateral and posterior funiculi. In the lower part of the conus 

 medullaris the amount of gray matter is actually greater than that of the white 

 matter : but very soon this state of affairs is changed, and in the lumbar region the 

 proportion of gray to white matter is approximately as 1 : 2*1 ; in the thoracic region 

 as 1:5; and in the cervical region as 1 : 5'1. When it is remembered how the gray 

 matter expands in the lumbar and cervical regions, and how greatly it becomes reduced 

 in the thoracic region, the significance of these figures will become more apparent. 



Canalis Centralis. As previously stated, the central canal is found in the 

 gray commissure. It is a very minute tunnel, barely visible to the naked eye 

 when seen in transverse section, and it traverses the entire length of the spinal 

 medulla. Above, it passes into the medulla oblongata, and finally opens into the 

 fourth ventricle of the brain ; below, it is continued for a variable distance into 

 the filum terminale, and in this it ends blindly. Only in the lumbar region does 

 the centra] canal occupy the centre of the spinal medulla. Above this level, 

 in the thoracic and cervical regions, it lies much nearer the anterior than 

 the posterior aspect of the spinal medulla; whilst below the lumbar region, as 

 it is traced down into the conus medullaris, it inclines backwards and approaches 

 the posterior aspect of the spinal medulla. The calibre of the canal also varies 

 somewhat in different parts of the spinal medulla. It is narrowest in the thoracic 

 region ; and in the lower part of the conus medullaris it expands into a distinct 

 fusiform dilatation (very nearly 1 mm. in transverse diameter), which is termed 

 the ventriculus terminalis (Krause). 



The central canal is lined with a layer of ciliated columnar cells, the deep taper- 

 ing ends of which are prolonged into slender processes which penetrate into the 

 substance of the spinal medulla. These cells constitute the lining ependymal cells 

 of the canal. The cilia of the epithelial cells are very early lost, and it is not un- 

 common to find the canal blocked up by epithelial debris. 



The central canal is of interest because it represents in the adult the relatively 

 wide lumen of the early ectoderrnal neural tube from which the spinal medulla 

 is developed. 



Filum Terminale. The delicate thread to which this name is applied is con- 

 tinuous with the inferior tapered end of the conus medullaris. It is easily distin- 

 guished, by its silvery and glistening appearance, from the numerous long nerve-roots 

 (cauda equina) amidst which it lies. It is about six inches long, and down to the 

 level of the second sacral vertebra it is enclosed with the surrounding nerve-roots 

 within the dura mater. Below this point the dura mater is applied directly to 

 the surface of the filum terminale and is called filum dura matris spinalis. The filum 

 terminale proceeds downwards in the sacral canal, and finally receives attachment to 

 the periosteum on the posterior aspect of thecoccyx (Fig. 460, p. 518). It is customary 

 to speak of the filurn as consisting of two parts, viz., the filum terminale internum and 

 the filum terminale externum, or the part inside and the part outside the tube of 

 dura mater. 



The filum terminale externum is simply a fibrous thread, strengthened by the pro- 

 longation it receives from the dura mater. The filum terminale internum is composed 

 largely of pia mater; but in its superior half it encloses the terminal part of the central 

 canal, and around this a variable amount of the gray substance of the spinal medulla 

 is prolonged downwards into the filum. When transverse sections are made through 



