648 NERVOUS SYSTEM OF THE HORSE 



lated 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. 



In the horse its average length is about 76 to 78 inches (ca. 190 to 195 cm.), 

 and its weight about 83^ to 9 ounces (ca. 240 to 255 grams). 



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

 There is no line of demarcation between it and the medulla oblongata, but for prac- 

 tical purposes the division is usually assumed to be at the posterior border of the 

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

 meduUaris. 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 a horse about 16,^ 2 hands high these parts measured G5 cm, (ca. 26 in.), 86 cm. (ca. 34.4 

 in.), 27 cm. (ca. 10.8 in.), and 15 cm. (ca. 6 in.) respectively (Dexler). 



In the eml:)ryo these divisions correspond primitively to the regions of the 

 vertebral column, but later, through uncc}ual 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 lumljar vertelorte, so that the roots of the last lumbar nerve 

 must run backward the length of the last luml;)ar 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 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 verteljric behind which they emerge from the vertebral canal. In the neck, howe\er, 

 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 

 vertebra. 



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

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

 with which the nerves of the limbs are connected. The cervical enlargement 

 (Intumescentia cervicalis) begins gradually in the fifth cervical vertebra and sul)- 

 sides in the second thoracic. Its maximum transverse^ diameter is al)out an inch 

 (ca. 25 mm.) and its dorso-ventral nearly half an inch (ca. 12 mm.). The lumbar 

 enlargement (Intumescentia lumbalis) is situated in the fourth and fifth lumbar 

 vertebrae. It is a little narrower than the cervical enlargement, and its dorso- 



