576 NERVOUS SYSTEM. 



together by a transverse band of gray matter, the gray commissure. Each 

 crescentic mass has an anterior and posterior horn. The posterior horn is long 

 and narrow, and approaches the surface of the posterior lateral fissure, near 

 which it presents a slight enlargement. The anterior horn is short and thick, 

 and does not quite reach the surface, but extends towards the point of attach- 

 ment of the anterior roots of the nerves. Its margin presents a dentate or stel- 

 late appearance. Owing to the projections towards the surface of the anterior 

 and posterior horns of the gray matter, each half of the cord is divided, more 

 or less completely, into three columns, anterior, middle, and posterior; the 

 anterior and middle being joined to form the antero-lateral column, as the 

 anterior horn does not quite reach the surface. 



The gray commissure, which connects the two crescentic masses of gray 

 matter, is separated from the bottom of the anterior median fissure by the ante- 

 rior white commissure ; and from the bottom of the posterior fissure by the 

 posterior white commissure. The gray commissure consists of a transverse 



band of gray matter, and of white fibres, derived 



Fig. 323. Transverse Sections from the opposite half of the cord and the posterior 



roots of the nerves. The anterior commissure is 

 formed of fibres, partly from the anterior column, 

 and partly from the fibrils of the anterior roots of 

 the spinal nerve, which decussate as they pass across 

 tf cervical re 3 Jt fr m one to the other side. 



The mode of arrangement of the gray matter, and 

 its amount in proportion to the white, vary in dif- 

 ferent parts of the cord. Thus, the posterior horns 

 are long and narrow, in the cervical region ; short 

 and narrower in the dorsal ; short, but wider, in the 



Off oite itiddit. pfDortai rt 3 - lumbar region. In the cervical region, the crescentic 



portions are small, the white matter more abundant 

 than in any other region of the cord. In the dorsal 

 region, the gray matter is least developed, the white 

 matter being also small in quantity. In the lumbar 

 n , region, the gray matter is more abundant than in 



any other region of the cord. Towards the lower 



end of the cord, the white matter gradually ceases. The crescentic portions of 

 the gray matter soon blend into a single mass, which forms the only constituent 

 of the extreme point of the cord. The minute anatomy of the cord is described 

 in the Introduction. 



THE BRAIN AND ITS MEMBRANES. 



Dissection. To examine the bruin with its membranes, the sknllcap must be removed. In 

 order to effect this, saw through the external table, the section commencing, in front, about an 

 inch above the margin of the orbit, and extending, behind, to a level with the occipital protube- 

 rance. Then break the internal table with the chisel and hammer, to avoid injuring the investing 

 membranes or brain ; loosen, and forcibly detach the skull, when the dura mater will be exposed 

 The adhesion between the bone and the dura mater is very intimate, and much more so in the 

 young subject than in the adult. 



The Membranes of the Brain are, the dura mater, arachnoid membrane, and 

 pin muter. 



DURA MATER. 



The Dura Mater is a thick and dense inelastic fibrous membrane, which lines 

 the interior of the skull. Its outer surface is rough and fibrillated, and adheres 

 closely to the inner surface of the bones, forming their internal periosteum, this 

 adhesion being more intimate opposite the sutures and at the base of the skull; 

 at the margin of the foramen magnum, it becomes continuous with the dura 

 mater lining the spinal canal. Its inner surface is smooth and epitheliated, 





