THORAX AND ABDOMEN OF THE HORSE 59 



tubular dura mater — must now be noted, as must also certain blood 

 vessels associated with the vertebral canal. 



The interior of the canal must be exposed by sawing through the 

 vertebi-al arches on each side as close as possible to the transverse pro- 

 cesses. In this way the greater part of the arches, with the spinous 

 processes and ligaments, may be removed from the thoracic and lumbar 

 regions of the vertcliral column in one continuous length. 



Nervi spin ales. — In the thoracic and lumbar regions the spinal 

 nerves agree in number with the vertebrae. There are, therefore, 

 eighteen thoracic and six lumbar nerves on each side of the body. Each 

 spinal nerve is connected with the spinal cord by two roots — dorsal and 

 ventral — and each root is composed of a number of converging bundles 

 of nerve fibres that pierce the dura mater of the spinal cord. A spinal 

 ganglion is developed on the dorsal root as it occupies an intervertebral 

 foramen. Immediately lateral to the ganglion the dorsal and ventral 

 roots unite : thus a mixed spinal nerve is formed. From the mixed 

 nerve a delicate recurrent nerve passes to the vertebrae, vertebral 

 liofaments, blood vessels within the vertebral canal and the dura mater 

 of the spinal cord. 



It will be noted that most of the spinal nerves gain immediate exit 

 from the vertebral canal by an intervertebral foramen. Towards the 

 end of the spinal cord, however, it is necessary for the nerves to travel 

 for some distance before they reach their foramina of exit ; and this 

 distance is greatest in the case of those nerves that arise from the ex- 

 treme end of the cord. This circumstance is further commented upon 

 and explained in connection with the dissection of the spinal cord itself. 



Each spinal nerve divides into two branches: (1) A dorsal branch 

 (ramus dorsalis) supplying the muscles and skin dorsal to the bodies of 

 the vertebrae ; and (2) a much larger ventral branch (ramus ventralis). 

 From the ventral branch arises a small communicating branch (ramus 

 communicans) that is connected with a ganglion of the sympathetic 

 cord. Rami communicantes consist of white fibres (white ramus com- 

 municans) passing from the spinal cord to the ganglia, and grey fibres 

 (grey ramus communicans) running in the opposite direction. Some- 

 times the grey and white rami are merged into one: sometimes they 

 are independent. It is largely by way of the rami comnmnicantes that 

 nerve impulses are conveyed from the central nervous system to the 

 abdominal and thoracic viscera, and vice versa. Typically a slender 

 filament from the ramus communicans joins the recurrent nerve 

 mentioned above as leaving the mixed spinal nerve. 



In the thoracic region the ventral branches of spinal nerves con- 

 stitute the intercostal nerves; while in the lumbar region they form 

 the roots of the ilio-hypogastric, ilio-inguinal, genito-femoral, lateral 



