THE SPINAL MEDULLA. 



519 



These are classified into eight cervical, twelve thoracic, five lumbar, five sacral, and 

 one coccygeal; and according to the attachments of these groups of nerves the 

 spinal medulla is arbitrarily subdivided into cervical, thoracic, lumbar, and sacral 

 regions. In employing these terms, therefore, for different districts of the spinal 

 medulla, it must be understood that the regions are determined by the . nerve 

 attachments and not by any direct relationship between these parts of the spinal 

 medulla and the sections of the vertebral column which bear the same names. 



Each spinal nerve is attached to the spinal medulla by an anterior or ventral 

 and a posterior or dorsal root, and as these are traced to their central attachments 

 they are seen to break up into a number of separate nerve fascicles or bundles, which 

 spread out, in some cases very widely from each other, as they approach the side of the 

 spinal medulla (Fig. 461). Each pair of nerves is therefore attached to a portion of 

 spinal medulla of some length, and such a portion, with its pair of nerves, receives the 

 name of a " segment of the spinal medulla." It must be clearly understood, how- 

 ever, that, in so far as the surface of the spinal medulla is concerned, there is no means 

 of marking off one segment from another except by the nerve attachments. 



In the cervical and lumbar regions of the spinal medulla the nerve-roots are somewhat crowded 

 together, so that little or no interval is left between the adjoining root fila or fascicles of neigh- 

 bouring nerves. In the thoracic region, however, distinct intervals may be observed, and the root 

 fila are more loosely arranged. From 

 this, it will be evident that the seg- 

 ments in different parts of the spinal 



medulla are not of equal length. In the ,,, . 



cervical region the segments measure about JfflKt~ vertebra 



12 mm. in length, in the thoracic region 

 from 20 to 24 mm., and in the lumbar 



region about 10 mm. The number of fila ^^mzZ&t&'SSi&ffim'&ftL Dura mater 



which attach the different nerve -roots to 

 the spinal medulla is very different in dif- 

 ferent nerves, and is not necessarily the 

 same in the same nerve -root in different 

 individuals. 



Arachnoid 



Conus medullaris 

 Roots of first 

 lumbar nerve 



Cauda equina 



FIG. 462. SECTION THROUGH THE CONUS MEDULLARIS AND 

 THE CAUDA EQUINA AS THEY LIE IN THE VERTEBRAL CANAL. 



Owing to the great difference 

 which exists between the length of 

 the spinal medulla and the length 

 of the vertebral column, the farther 

 we pass down the greater the dis- 

 tance becomes between the attach- 

 ment of the various nerve-roots to 

 the spinal medulla and the intervertebral foramina through which the corresponding 

 nerves leave the vertebral canal. The lower nerve-roots, therefore, have to traverse 

 the vertebral canal for .a considerable distance before they reach their apertures of 

 emergence. It thus happens that the nerve-roots which spring from the lumbar and 

 sacral regions of the spinal medulla attain a very great length and descend vertically 

 in the lower part of the vertebral canal in a bunch or leash, in the midst of which 

 lie the conus medullaris and the filum terminale. This great bundle of nerve-roots 

 receives the appropriate name of the cauda equina. 



Enlargements of the Spinal Medulla. Throughout the greater part of the 

 thoracic region, the spinal medulla presents a uniform girth and a very nearly 

 circular outline when seen in transverse section. In the cervical and lumbar 

 regions, however, it shows marked swellings. The intumescentia cervicalis or 

 cervical enlargement is the more evident of the two. It begins very gradually at 

 the upper end of the spinal medulla, attains its greatest breadth (12 to 14 mm.) 

 opposite the fifth or sixth cervical vertebra, and finally subsides opposite the 

 lecond thoracic vertebra. To this portion of the spinal medulla are attached the 

 reat nerves which supply the upper limbs. The intumescentia lumbalis or lumbar 

 enlargement begins at the level of the tenth thoracic vertebra, and acquires its 



cimum transverse diameter (11 to 13 mm.) opposite the last thoracic vertebra. 

 Below, it rapidly tapers away into the conus medullaris. To the lumbar enlarge- 

 ment are attached the great nerves of the lower limbs. 



34 c 



