284 NORMAL HISTOLOGY. 



the surface of the nervous masses ; the pia does, dipping down into 

 the fissures and penetrating, as part of the velum interpositum, into 

 the interior of the brain. The inner layer of the pia is closely 

 united to the surface of the cord and the brain, while the vascular 

 tunic in places is less accurately adapted : thus the entire pia enters 

 the anterior median fissure of the spinal cord, while the inner layer 

 alone takes part in the formation of the posterior median septum, or 

 "fissure." 



Branched pigment-cells are not uncommon in the outer layer 

 of the pia ; these are especially well developed on the anterior sur- 

 face of the medulla, although frequently found in other positions 

 along the cord and at the base of the brain. A few non-medullated 

 nerve-fibres have been traced within the pia. 



THE SPINAL CORD. 



The spinal cord, or medulla spinalis, hangs, enveloped by its 

 membranes, within the vertebral canal, and extends from the upper 

 border of the atlas, where it becomes continuous with the medulla 

 above, to the lower border of the first lumbar vertebra below ; from 

 this level the conical end oTth~e corcf, thecomis_iriedullaris, is 

 continued into the attenuated filum terminaleTthe nervous matter 

 disappearing about the middle of this structure. While the several di- 

 visions of the cord are distinguished by individual peculiarities, certain 

 general features of arrangement are common throughout its length. 



The cord is formed of symmetrical halves partially separated 

 in the mid-line in front by a cleft, the anterior median fissure, and 

 behind by an ingrowth of pial connective tissue which constitutes 

 the posterior median fissure, but is really only a fibrous sep- 

 tum. Each half of the cord contains a crescentic mass of gray 

 matter ; the convexities of the crescents face, and are connected by 

 a horizontal bridge, the gray commissure, the gray matter of the 

 cord thus collectively forming an H-like mass. The horns of the 

 crescents are not equal, the anterior cornua being broad and robust, 

 while the posterior cornua are more slender and pointed and ex- 

 tend almost to the outer surface. 



The exterior of the cord is closely invested by the inner pial 

 layer, from which numerous fibrous septa extend into the substance 

 of the cord, dividing the white matter into certain pyramidal areas. 

 The anterior or motor roots of the spinal nerves are formed by 

 bundles of fibres which escape from the gray matter ; these bundles 

 pass from the anterior cornu to the surface of the cord associated in 

 groups, their exit being indicated by slight furrows. The position 

 at which the posterior or sensory root appears on the surface, on 

 the contrary, is marked by a distinct indentation. 



