835 



is best marked in the cervical region and becomes more abundant in the medulla 

 oblongata. / 



The mode of arrangement of the gray substance and its amount in proportion 

 to the white vary in different parts of the cord. Thus, the posterior horns are long 

 and narrow in the cervical region; short and narrower in the thoracic; short but 

 wider in the lumbar region. In the cervical region the crescentic portions are 

 small, and the white substance more abundant than in any other region of the 

 cord. In the thoracic region the gray substance is least developed, the white 

 substance being also small in quantity. In the lumbar region the gray substance 

 is more abundant than in any other region of the cord. Toward the lower end 

 of the cord the white substance gradually ceases. The crescentic portions of the 

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

 of the extreme point of the cord. 



SPINAL ROOT GANGLIO 



SPINAL NERVE 



VENTRAL FISSURE 



FIG. 608. Showing origin of two pairs of spinal nerves (schematic). 



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

 consists of myelinic and amyelinic nerve fibres and neuroglia. The fibres pass 

 transversely, spreading out at various angles, into the lateral gray masses. In 

 the gray commissure, and extending the whole length of the cord, is a minute 

 channel, the central canal (myelocele; canalis centralis], which is barely visible to 

 the naked eye, but is proportionately larger in some of the lower vertebrates. 

 Cephalad, in the medulla oblongata, it opens out into the fourth ventricle; caudad 

 it is continued for a short distance into the filum, in which it ends blindly. The 

 canal is very minute, less than 0.1 mm. (one-twenty-fifth inch), except in the 

 terminal part of the conus, where it expands into a fusiform dilatation, the sinus 

 terminalis (rhombocele; ventriculus terminalis [Krause]). The central canal is 

 lined by a layer of columnar cells which are seen to be ciliated in the embryo 

 and are in all respects identical with the ependymal cells lining the ventricles 

 of the brain. Surrounding the ependymal lining of the central canal and gradually 

 merging into the spongy substance which constitutes the remainder of the gray 

 commissure is a finely granular and reticulated substance, the gelatinosa centralis, 

 almost entirely composed of neuroglia, with a few fine fibrils apparently proceeding 

 from the ependymal cells, and having a translucent, gelatinous appearance. 



The gray substance of the cord is composed of (1) the gelatinosa or gliosa, 

 which envelops the head of the dorsal horn and which encircles the central canal 

 of the cord; (2) the spongiosa, which forms the crescentic horns (except the heads 

 of the posterior horns and the envelope of the central canal). Further, it may be 

 stated that the gray substance consists of nerve-fibres of variable but smaller 



