524 THE NEEVOUS SYSTEM. 



segments ; followed in a downward direction it blends with the anterior column in 

 the lumbar swelling and contributes to the thickening of that column. 



The gray matter is for the most part mapped off from the surrounding white matter 

 with a considerable degree of sharpness ; but in the cervical region, on the lateral 

 aspect of the crescentic mass and in the angle between the anterior and posterior 

 columns, fine bands of gray matter penetrate the white matter, and, joining with each 

 other, form a network, the meshes of which enclose small islands of white matter. 

 This constitutes what is called the formatio reticularis. Although best marked 

 in the cervical region, traces of the same reticular formation may be detected 

 in lower segments of the spinal medulla. 



Characters presented by the Gray Matter in Different Regions of the 

 Spinal Medulla. The gray matter is not present in equal quantity nor does it 

 exhibit the same form in all regions of the spinal medulla. Indeed, each segment 

 presents its own special characters in both of these respects. It is not necessary, 

 however, in the present instance, to enter into this matter with any degree of 

 minute detail. It will be sufficient if the broad distinctions which are evident in 

 the different regions are pointed out. 



It may be regarded as a general law that, wherever there is an increase in the 

 size of the nerves- attached to a particular part of the spinal medulla, a correspond- 

 ing increase in the amount of gray matter will be observed. It follows from this 

 that the regions where the gray matter bulks most largely are the lumbar and 

 the cervical swellings. The great nerve-roots which go to form the nerves of the 

 large limb-plexuses enter and pass out from those portions of the spinal medulla. 

 In the thoracic region there is a reduction in the quantity of gray matter in 

 correspondence with the smaller size of the thoracic nerves. 



In the thoracic region (Fig. 467, B) both columns of gray matter are narrow, 

 although the distinction between the anterior column and the- still more attenuated 

 posterior column is sufficiently manifest. In this region the lateral column of 

 gray matter also is characteristic, and the substantia gelatinosa in transverse 

 section is pointed and spear-shaped. 



In the upper three segments of the cervical region the anterior columns of gray 

 matter are not large and they resemble the corresponding columns in the thoracic 

 region. A lateral column also is present. But in these segments (and more especi- 

 ally in the first and second) there is a marked attenuation of the neck of the 

 posterior column, and the posterior commissure is very broad. 



In the cervical swelling the contrast between the two columns is most striking ; 

 the anterior column is of great size and presents a very broad surface towards the 

 anterior aspect of the spinal medulla, whilst the posterior column remains narrow. 

 This great increase in the bulk of the anterior column is due to a marked addition 

 of gray matter on the lateral side of the column, and seeing that this additional 

 matter is traversed by a greater number of fibres, it stands out, in well-prepared 

 specimens, more or less distinctly from the part of the column which lies to the 

 medial side, and which may be considered to represent the entire anterior column 

 in the thoracic and upper cervical segments. Within this lateral addition to the 

 anterior column are placed those collections of cells which constitute the nuclei of 

 origin of the motor nerves of the muscles of the upper limb. The characteristic 

 thickening of the anterior column of gray matter is evident, therefore, in those 

 segments of the spinal medulla to which the nerves which enter the brachial plexus 

 are attached, viz., the lower five cervical segments and the first thoracic segment. 



In the lumbar swelling the anterior columns again broaden out, and for the same 

 reason as in the case of the corresponding columns in the cervical swelling. The 

 nuclear masses which contain the cells from which the motor fibres which supply 

 the muscles of the lower limbs take origin are added to the lateral aspect of the 

 columns and give them a very characteristic appearance. In this region of the 

 spinal medulla, however, the posterior columns also are broad and are capped 

 by substantia gelatinosa which in transverse section presents a semilunar outline. 

 There is consequently no difficulty in distinguishing, from an inspection of the 

 gray matter alone, between transverse sections of the spinal medulla taken from 

 the cervical and lumbar swellings of the spinal medulla. 



