764 NERVOUS SYSTEM OF THE HORSE 



thickening, the Hnea splendens, along which the ventral spinal artery runs. On 

 each side the pia mater gives off a strong longitudinal band, the ligamentum den- 

 ticulatum, which is connected externally with the dura mater. The inner or pial 

 border extends in a line between the dorsal and ventral roots of the nerves. The 

 outer or dural border is denticulated and to a large extent free. The denticulations 

 are attached to the dura between the nerve-roots. 



NERVOUS SYSTEM OF THE HORSE 

 The Spinal Cord 



The spinal cord (Medulla spinalis) is the part of the central nervous system 

 which is situated in the vertebral canal. It extends from the foramen magnum to 

 about the middle of the sacrum. Its average length is about 76 to 78 inches (ca. 

 190-195 cm.), and its weight about 83''2 to 9 ounces (ca. 240-255 grams). 



It is approximately cylindrical, but more or less flattened dorso-ventrally. 

 There is no natural line of demarcation between it and the medulla oblongata, but 

 for descriptive purposes the division is usually assumed to be at the plane of the 

 foramen magnum. Its posterior part tapers rapidly to a point, forming the conus 

 medullaris. This is prolonged for a short distance by the slender filum terminale. 



Forty-two pairs of spinal nerves are connected with the sides of the spinal cord. 

 They are classified as eight cervical, eighteen thoracic, six lumbar, five sacral, and 

 five coccygeal. According to the attachments of these series of nerves the spinal 

 cord is divided into cervical, thoracic, lumbar, and sacral parts. ^ 



In the embryo these divisions correspond primitively to the regions of the 

 vertebral column, but later, through unequal growth of the cord and spine, the 

 correspondence between the two is not at all exact in the anterior regions and is 

 lost in the last two. The lumbar part of the cord in the horse ends at the junction 

 of the fifth and sixth lumbar vertebrae, so that the roots of the last lumbar nerve 

 must run backward the length of the last lumbar vertebra to reach the interverte- 

 bral foramen through which it emerges. The conus medullaris reaches only to the 

 anterior part of the sacral canal, so that the roots of the sacral and coccygeal nerves 

 extend backward in the spinal canal for a considerable distance, forming a leash 

 of bundles, in the center of which lie the conus medullaris and the filum terminale. 

 This arrangement is expressively designated the cauda equina. 



Each pair of spinal nerves is attached by its root-fibers to a certain length of 

 the cord, and the latter is, therefore, regarded as consisting of as many segments 

 as there are pairs of nerves. It is to be noted, however, that there is no line of 

 demarcation between the segments other than the intervals between the root-fibers 

 of adjacent nerves. 



The segments are of different lengths; the longest are the third to the sixth cervical, which 

 measure 11, 10, 10, and S.5 cm. respectively. The spinal nerves are in general designated accord- 

 ing to the ^•ertebrae behind which they emerge from the vertebral canal. In the neck, however, 

 there are eight pairs of nerves and only seven vertebra?; here the first nerve emerges through the 

 intervertebral foramen of the atlas and the eighth between the last cervical and the first thoracic 

 vertebrae. 



In the greater part of the thoracic region the spinal cord is fairly uniform in 

 size, but there are two conspicuous enlargements which involve the segments with 

 which the nerves of the limbs are connected. The cervical enlargement (Intumes- 

 centia cervicalis) begins gradually in the fifth cervical vertebra and subsides in 



^ In a horse about 163^ hands high these parts measured 65 cm. (ca. 26 in.), 86 cm. (ca. 34.4 

 in.), 27 cm. (ca. lO.S in.), and 15 cm. (ca. 6 in.) respectively (Dexler). 



