SEC. 2. THE STRUCTURE OF THE SPINAL CORD. 



562. Lying within the vertebral canal the spinal cord is 

 protected by its 'membranes,' the dura mater, the arachnoid 

 membrane and the pia mater. The consideration of the arrange- 

 ment of these membranes and of the structure of the dura mater 

 and arachnoid we will leave until we come to speak of the vascular 

 and lymphatic supplies of the central nervous system ; the histo- 

 logy of the pia mater may more fitly come with that of the spinal 

 cord itself. 



Along its whole length from its junction with the bulb to 

 its termination in the filum terminate the spinal cord, while 

 possessing certain general features, is continually changing as to 

 special features. It will be convenient to study first the general 

 structure of some particular part, for instance the middle of the 

 thoracic (dorsal) 1 region, and afterwards to point out the special 

 features which obtain in the several regions. 



A transverse vertical section of either a fresh or a hardened and 

 prepared spinal cord at the thoracic region possesses an outline 

 which is roughly speaking circular. In the middle of the anterior 

 or ventral surface is a vertical fissure, the ventral or anterior fissure 

 (Fig. 96, A. F.) running some way across the thickness of the cord 

 from the ventral towards the dorsal surface. Opposite to it on 

 the posterior or dorsal surface is a corresponding, deeper but 

 narrower, dorsal or posterior fissure (Fig. 96, P. F.) which, 

 however, as we shall see, differs materially in nature from the 



1 It is very desirable to use the terms ' dorsal ' and ' ventral ' for the parts of the 

 cerebro-spinal axis which lie respectively near the dorsal or back part, and the 

 ventral or belly part of the body, instead of the terms posterior and anterior; but 

 if this is done, the use of the word dorsal to denote the region of the cord between 

 the lumbar and cervical regions is apt to lead to confusion ; hence the introduction 

 of the word thoracic. If this use of dorsal and ventral be adhered to, before and 

 behind, above and below, may conveniently be used to denote nearer the head and 

 nearer the tail (or coccyx) respectively ; anterior and posterior may also be used in 

 the same sense except in the case of anterior and posterior fissure and horn, which 

 terms seem too much honoured by time to be thrown aside. 



