HEAD AND NECK OF THE HORSE 167 



Leaving each side of the spinal cord are forty-two spinal nerves 

 (uervi spinales), classified according to the vertebrae behind which they 

 leave the vertebral canal. The number of nerves agrees with the 

 number of the vertebne in the thoracic, lumbar and sacral regions ; but 

 in the cervical region there are eight nerves, the first leaving the 

 vertebral canal by the intervertebral foramen of the atlas. The number 

 of coccygeal nerves is generally five. 



Each spinal nerve is formed by the union of two roots, a dorsal and 

 a ventral. Each root in its turn is compounded of a number of rootlets 

 that leave the lateral aspect of the spinal cord. The rootlets of the 

 dorsal root leave along a definite line (the dorsal lateral groove) ; but 

 those of the ventral root emerge from a narrow longitudinal area (area 

 radicularis ventralis). The two roots pierce the dura mater separately, 

 though close together, and join to form the mixed spinal nerve within 

 the intervertebral foramen, except in the sacral and coccygeal part of 

 the cord, where union takes place within the vertebral canal. Immedi- 

 ately before union is effected, the dorsal root expands into an oval grey 

 swelling, the spinal ganglion (ganglion spinale). 



It is customary to regard as a " segment " of the spinal cord the 

 length with which a pair of spinal nerves is connected. The whole cord 

 is therefore divided into as many segments as there are pairs of spinal 

 nerves. 



Up to a certain stage of embryonic development, the spinal cord is 

 co-extensive with the vertebral canal, and the various nerves arise from 

 the cord opposite their foramina of exit from the canal. But, owing to 

 disparity in the rate of growth in length of the two structures, the cord 

 is displaced, and it becomes necessary for many of the nerves to travel 

 for a longer or shorter distance before gaining egress from the vertebral 

 canal. To such a degree is this carried in the terminal part of the 

 spinal cord that it is surrounded by a cluster of nerve roots, known as 

 the Cauda equina. 



Dissection. — Remove the membranes from the cord so that its exterior 

 may be studied. 



By examination of the exterior of the spinal cord the dissector 

 may readily satisfy himself of its bilateral symmetry. A deep ventral 

 median fissure (fissura mediana ventralis) runs the full length of the 

 ventral aspect and, in conjunction with a faint dorsal median groove 

 (sulcus medianus dorsalis), divides the surface into two symmetrical 

 halves. Each half is subdivided into three funiculi (ventral, lateral 

 and dorsal) by the emergence of the ventral rootlets of spinal nerves 

 and by the dorsal lateral groove (sulcus lateralis dorsalis). In the 

 cervical region and the first part of the thoracic region a dorsal inter- 



