DISSECTION OF THE DOG 203 



of vertebrae 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 

 three or four. 



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 which 

 leave the lateral aspect of the cord. The rootlets of the dorsal root leave 

 the cord along a definite line ; but those of the ventral root emerge from a 

 narrow longitudinal area. The two roots pierce the dura mater separately, 

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

 vertebral foramen, except in the sacral and coccygeal regions, where union 

 takes place within the vertebral canal. Immediately before the union is 

 effected the dorsal root expands into an oval grey swelling, the spinal ganglion 

 (ganglion spinale). 



Because of the disparity in the length of the spinal cord and the vertebral 

 canal which contains it, and because the spinal cord only reaches as far as the 

 seventh lumbar vertebra, it follows that all the spinal nerves do not leave 

 the cord on a level with their foramina of exit. In other words, it is necessary 

 that most of the nerves must travel, for a longer or shorter distance, within 

 the vertebral canal before gaining egress. Only the first six nerves pass 

 straight and immediately from the spinal cord to the intervertebral foramina. 

 At the seventh cervical nerve commences an obliquity which gradually increases 

 to such a degree that the terminal part of the spinal cord is surrounded by a 

 cluster of nerve roots known as the cauda equina. 



Dissection. — The spinal cord should now be removed from the vertebral 

 canal. Cut the spinal nerves at the intervertebral foramina, when the 

 cord and its membranes can be raised by pulling upon the dura mater. 

 The membranes should be re-examined at this stage by pinning the 

 specimen out in a cork- or paraffin-lined tray. If the specimen be 

 well injected, the ventral spinal artery will be noted. When this is 

 done, the membranes must be removed, so that the exterior of the cord 

 itself may be studied. 



By examination of the exterior of the spinal cord the dissector may readily 

 satisfy himself as to its bilateral symmetry. A deep median fissure (fissura 

 mediana ventralis) runs the full length of the ventral aspect and, in conjunc- 

 tion 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 dorsal and ventral lateral grooves 

 (sulcus lateralis dorsalis et sulcus lateralis ventralis) along which the dorsal 

 and ventral rootlets of the spinal nerves emerge. Of the two, the dorsal 

 lateral groove is the more definite, and persists on the conus medullaris after 

 the ventral lateral groove has disappeared. In the cervical region, a dorsal 



