THE SPINAL CORD 



765 



the second thoracic. Its maximum transverse diameter is about an inch (ca. 25 

 mm.) and its dorso-ventral nearly half an inch (ca. 12 mm.). The lumbar enlarge- 

 ment (Intumescentia lumbalis) is situated in the fourth and fifth lumbar vertebra}. 



It is a little narrower than the cervi- 

 cal enlargement, and its dorso-ventral 

 diameter is also slightly smaller. Be- 

 hind this the cord tapers rapidly to 

 form the conus meduUaris. The tip 

 of the latter is continued by a delicate 

 glistening strand, the filum terminale, 

 which is composed largely of fibrous 

 tissue continued from the pia mater, 

 covered by arachnoid. 



Fig. 626. — Venteal View of Medulla Oblongata 

 AND First and Second Segments of Spinal 

 Cord of Horse; the Membranes are Cut and 

 Reflected. 



1, Lig. suspensorium arachnoideale ; 2, right cere- 

 brospinal artery; 3, -5, digitations of lig. denticulatum ; 

 4, free border of lig. denticulatum; 6, middle spinal 

 artery; 7, basilar artery; 8, pons; 9, arachnoidea; 

 10, dura mater; 11, 12, ventral root-bundles of first 

 and second segments of spinal cord; VI , N. abducens; 

 IX, X, glosso-pharyngeus and vagus; XI, accessory, 

 medullary part; XI*, accessory, spinal part; a, line 

 between medulla oblongata and spinal cord. (Dexler, 

 in Ellenberger-Baum, Anat. d. Haustiere.) 



Fig. 627. — Cauda Equina. 

 1, Dura and arachnoidea divided and reflected; 2, 

 spinal cord; 3, nerve-roots. (From Leisering's Atlas, re- 

 duced.) 



The surface of the spinal cord is 

 divided into two similar halves by a 

 dorsal median groove and a ventral 

 median fissure. On either side of the 

 former is the dorso-lateral groove (Sul- 

 cus dorsalis lateralis), at which the fibers 



of the dorsal nerve-roots enter the cord; 

 it is faint except at the enlargements, and is represented by two grooves in the first 

 cervical segment. The ventral root-fibers as they emerge from the cord do not 

 form a continuous series, but arise from a zone (Area radicularis ventralis) 3 to 5 



