I026 



HUMAN ANATOMY. 



Form of the Cord.- 



FlG. 8S3. 



Medulla 



ir 



Cervical 



Thoracic 



Lumbar 



Sacral 

 Coccygeal 



Filuni 



V 



Spinal cord denuded *f mem- 

 branes and nerves, showing: pro- 

 portions of its leTigth contributed 

 by different regions and position 

 and relative size of enlargements, as 

 viewed from before; semidiagram- 

 niatic, based on measurements ; 

 one-third actual size. 



coccygeal nerves, homologous 



-After removal of its membranes and the root-fibres, the 

 spinal cord is seen to differ from a simple cylinder in 

 the following respects. It is somewhat flattened in the 

 antero-posterior direction, so that the sagittal diameter is 

 always less than the transverse diameter, and its outline 

 in cross-sections, therefore, is not circular but more or 

 less oval ; its width is not uniform on account of two 

 conspicuous swellings that are associated with the origin 

 and reception of the large nerves supplying the limbs. 

 The upper or cervical enlargement (intumescentia 

 cervicalis) begins just below the upper end of the cord 

 and ends opposite the second thoracic vertebra, having 

 its greatest expansion at the level of the fifth and sixth 

 cervical vertebrae, where the sagittal diameter is about 

 9 mm. and the transverse from 13-14 mm. The lower 

 or lumbar enlargement (intumescentia lumbalis) begins 

 opposite the tenth thoracic vertebra, slightly above the 

 origin of the first lumbar nerve, and fades away in the 

 conus meduUaris below. It appears very gradually 

 and reaches its maximum opposite the twelfth thoracic 

 vertebra, where the cord has a sagittal diameter of 8. 5 mm. 

 and a transverse diameter of from 11-13 mm. (Ravenel). 

 The lumbar enlargement is associated with the great 

 nerve-trunks supplying the lower limbs. The inter- 

 vening part of the thoracic region is the smallest and most 

 uniform portion of the cord and is almost circular in out- 

 line. Where least expanded, opposite the middle of the 

 thoracic spine, the cord measures 8 mm. in its sagittal 

 and 10 mm. in its transverse diameter. These enlarge- 

 ments appear coincidently with the formation of the limbs, 

 are relatively small during fcetal life, and acquire their 

 full dimensions only after the limbs have attained their 

 definite growth. In a general way, a similar relation 

 between the size of the enlargements and the degree of 

 development of the limbs is observed in the lower animals. 

 At the tip of the conus meduUaris the spinal cord 

 is prolonged into a delicate tapering strand, the filum 

 terminale, that consists chiefiy of fibrous tissue con- 

 tinued from the pia mater and invested by arachnoid. 

 It extends to the bottom of the pointed and closed end 

 of the dural sac, which it pierces at the level of the second 

 sacral vertebra and, ensheathed by a prolongation of dura 

 {vagina terrninalis), as the Jihim tei'yninale extenmm, 

 proceeds downward through the lower end of the sacral 

 canal for a distance of about 8 cm. (3^ in.), finally to 

 be attached to the periosteum covering the posterior 

 surface of the coccyx. The part within the dural sac, 

 i\\Q Jih(m terminale internum, is about 16 cm. (6^ in.) 

 in length and surrounded by the nerve-bundles of the 

 Cauda equina (Fig. 882), from which it is readily dis- 

 tinguished by its glistening silvery appearance. 



The upper half or less of the internal filum contains the 

 terminal part of the central canal of the spinal cord walled by 

 a thin and variable layer of nervous substance in which small 

 nerve-cells are usually present. The minute bundles of nerve- 

 fibres often found adhering to the filum, which sometimes may be 

 followed to and even through the dural sheath, are regarded by 

 Rauber as representing one or two additional (second and third) 

 with the caudal nerves of the lower animals. 



