THE STRUCTURE OF THE SPINAL CORD. 529 



that this method may be applied to the differentiation of tracts of fibres in 

 the brain and spinal cord. 



THE STRUCTURE OF THE SPINAL CORD. 



475. 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 arrangement 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 sys- 

 tem ; the histology 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 at its termination 

 in the filum terminals the spinal cord, while possessing certain general fea- 

 tures, 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 afterward to point out the special 

 features which obtain in the several regions. 



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

 pared 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 ver- 

 tical fissure, the ventral or anterior fissure (Fig. 119, A. F.\ running some 

 way across the thickness of the cord from the ventral toward the dorsal sur- 

 face. Opposite to it on the posterior or dorsal surface is a corresponding, 

 deeper but narrower, dorsal or posterior fissure (Fig. 119, P. F.) which, how- 

 ever, as we shall see, differs materially in nature from the anterior fissure, 

 and ought to be called a septum rather than a fissure. Between the two fis- 

 sures the substance of the cord is reduced to a narrow isthmus uniting the 

 two lateral halves, which in a normal cord are like each other in every 

 respect. In the middle of the isthmus lies the section of a small canal, the 

 central canal (Fig. 119, c. c.), which is all that remains of the relatively wide 

 neural canal of the embryo. 



Each lateral half consists of an outer zone of white matter surrounding, 

 except at the isthmus, an inner more or less crescentic, or comma-shaped 

 mass of gray matter. The convexity of each crescent is turned toward the 

 median line of the cord, the two crescents being placed back and back and 

 joined together by the isthmus just spoken of. The somewhat broader ante- 

 rior extremity of the crescent, or head of the comma, is called the anterior 

 cornu or horn; and the narrower posterior extremity of the crescent, or tail 

 of the comma, is called the posterior cornu or horn. The part by which each 

 horn is joined on to the middle part of the crescent is called the cervix, ante- 

 rior and posterior respectively. The isthmus joining the backs of the two 

 crescents, like the crescents themselves, consists, for the most part, of gray 

 matter, the band running posterior or dorsal to the central canal being called 

 the posterior gray commissure (Fig. 119, p. g. c.),and the band running ante- 

 rior or ventral to the canal being called the anterior gray commissure (Fig. 

 119, a. g. c.). The posterior fissure touches the posterior gray commissure, 



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, arid 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 cer- 

 vical 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) respect- 

 ively; 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 honored by time to 

 be thrown aside. 



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