422 THE NERVOUS SYSTEM. 
Owing to the great difference which exists between the length of the spinal. 
cord and the length of the vertebral column, the farther we pass down the greater 
the distance becomes between the attachment of the various nerve-roots to the 
cord and the invertebral foramina through which the corresponding nerves leave 
the spinal canal. The lower nerve-roots, therefore, have to traverse the spinal 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 cord attain a very great length and descend vertically in the lower part of 
the spinal 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 Cord.—Throughout the greater part of the dorsal region, 
the spinal cord 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 cervical enlargement 
(intumescentia cervicalis) is the more evident 
of the two. It begins very gradually at the 
upper end of the cord, attains its greatest 
aroove breadth (12 to 14 mm.) opposite the fifth or 
Anterior nerve-root, First lumbar 
Posterior nerve-root vertebra 
Dura mater 
Arachnoidea 
Conus 
Roots of first 
lumbar nerve 
hes ' Cauda equina 
Spinal ganglion 
Anterior primary 
division of nerve 
Posterior primary 
| division of nerve 
Fic. 316. — THE Roots OF ORIGIN OF THE 
SEVENTH DorsaL NERVE (semi-diagram- Fic. 817.—SECTION THROUGH THE CONUS MEDULLARIS AND 
matic). THE CAUDA EQUINA AS THEY LIE IN THE SPINAL CANAL, 
sixth cervical vertebra, and finally subsides opposite the second dorsal vertebra. 
To this portion of the cord are attached the great nerves which supply the upper 
limbs. The lumbar enlargement (intumescentia lumbalis) begins at the level of 
the tenth dorsal vertebra, and acquires its maximum transverse diameter (11 to 
13 mm.) opposite the last dorsal vertebra. Below, it rapidly tapers away into the 
conus medullaris. To the lumbar enlargement are attached the great nerves of 
the lower limbs. 
These enlargements of the cord are associated with the outgrowth of the limbs. In 
the earlier developmental stages of the spinal cord they are not present, and they only 
take form as the limbs become developed. In the lower mammalia their size corresponds 
with the degree of development of the limbs. Thus, in the long-armed orang and gibbon, 
the cervical swelling stands out with a remarkable degree of prominence. It is, however, 
interesting to note that although in the cetacea there are no visible hind-limbs, there is 
nevertheless a well-marked lumbar enlargement of the cord. 
Fissures and Furrows of the Cord.— When cross-sections of the spinal cord 
are made, it is seen to be a bilateral structure which is partially subdivided into a 
right and left half by two median clefts—one upon the anterior and the other upon 
the posterior aspect. These clefts are termed the antero-median and the postero- 
median fissures, and they extend along the entire length of the cord. At the same 
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