382 DISSECTION OF THE SPINAL CORD. 



SEGMENTS OF THE CORD. Each half of the cord between the median 

 fissures is divided into two parts by the lateral sulcus (fig. 125, d) : the 

 piece in front of that slit and the posterior roots of the nerves is called 

 the antero-lateral column () ; and the piece behind, the posterior col- 

 umn (6). 



The antero-lateral column (fig. 125, ) includes rather more than two- 

 thirds of the half of the cord, extending backwards to the posterior roots 

 of the nerves, and gives attachment to the anterior roots. 



The posterior column (fig. 125, b) is situate between the lateral fissure 

 (rf), with the posterior roots of the nerves, and the central median fissure. 

 Near the median fissure is a slight groove (e), which marks off a slender 

 piece, the posterior median column (c) : this portion is best seen in the 

 cervical part of the cord. 



A central piece, or the commissure of the cord, connects the halves of 

 the medulla, and limits the depth of the median fissures. 



Different division of the cord. Each half of the cord is sometimes di- 

 vided into three columns anterior, lateral, and posterior, whose limits 

 are the following : The anterior reaches from the anterior roots of the 

 nerves to the median fissure in front. The lateral column is limited be- 

 fore and behind by the roots of the nerves. The posterior with its small 

 posterior median segment, is placed between the posterior roots and the 

 median fissure behind. 



COMPOSITION OF THE CORD (fig. 125). A horizontal section of the 

 medulla shows more distinctly its division into halves, with the commis- 

 sural or connecting piece between them. The same cut demonstrates the 

 existence of a mass of gray matter in the interior, which is arranged in 

 the form of two crescents (one in each half) united by a cross piece, and 

 surrounded by white substance. 



The commissure (fig. 125, h) consists of two parts, viz., a transverse 

 band of gray matter, with a white stratum in front, 



The gray transverse band {gray commissure) connects the opposite 

 crescents, and consists of nerve cells, and of transverse nerve fibres de- 

 rived from the halves of the cord and the roots of the nerves. 



In its centre is the shrunken canal of the spinal cord (fig. 125, #), 

 which is best seen in the fo3tus. It reaches the whole length of the me- 

 dulla, and a cross section of the cord shows it as a round spot. Above, it 

 opens on the fioor of the fourth ventricle, and below it is continued into 

 the filum terminate. It is lined by a columnar ciliated epithelium, and is 

 obstructed by a granular material near the upper opening (Clarke). 



The white piece of the commissure is formed partly by fibres of the an- 

 terior column ; and partly by fibrils of the anterior roots of the nerves, 

 which here decussate as they cross from the one half to the other. It is 

 best marked opposite the enlargements on the cord, and is least developed 

 in the dorsal region. 



The half of the medulla. In the half of the cord as in the commissure, 

 gray and white portions exist; the former is elongated from before back, 

 being crescentic in shape as before said, and is quite surrounded by the 

 latter. 



The gray crescent (//) is semilunar in form, with the horns or cornua of 

 the crescent directed towards the roots of the nerves, and the convexity to 

 the middle line (fig. 125). The crescentic masses in the opposite halves 

 of the cord are united by the gray commissure. 



The posterior cornu is long and slender (fig. 126, x ) and reaches near 



