346 ANATOMY OF THE RABBIT 



extend ventrolaterally dose to the cord and unite just 

 lateral to it, where the root expands into a spinal ganglion. 

 The more slender anterior or ventral root (radix anterior) lies 

 directly ventral to the dorsal root and, like it, consists of a 

 row of separate filaments. These are composed of efferent 

 fibres emerging from the ventrolateral surface of the cord. 

 They converge dorsolaterad to meet and combine with the 

 dorsal root close to the spinal ganglion, thus forming a 

 single nerve, which breaks up a little further laterally into 

 three primary branches, the posterior or dorsal, the anterior 

 or ventral, and the communicating ramus. Each branch 

 contains fibres from both roots. The roots lie within the 

 dura mater and this extends into each intervertebral foramen 

 there to become continuous with the connective tissue 

 sheath of the nerve. 



The posterior (dorsal) ramus is an inconspicuous branch 

 (except in the first two cervical nerves) passing to the 

 dorsal musculature and skin. The anterior (ventral) ramus 

 is the chief portion of the spinal nerve, the successive 

 anterior rami appearing as the components of the cervical 

 and lumbosacral plexuses or as individual spinal nerves. 

 The ramus communicans is a slender filament passing 

 ventrad to join the sympathetic trunk. . 



Each spinal nerve has a grey ramus communicans and 

 certain ones have also a white ramus communicans. In 

 the rabbit, the latter are all the thoracic and the first five 

 lumbar nerves and similar parasympathetic white rami 

 occur in the second, third, and fourth sacral nerves (see 

 pp. 73-75). 



2. A small portion of the spinal cord may be excised and examined 

 (preferably under water) for the following (see Fig. 18, p. 31). 



(a) The cord is divided into lateral halves by two median 

 depressions, the ventral or anterior median fissure (fissura 

 mediana anterior) and the dorsal or posterior median 

 sulcus (sulcus medianus posterior). 



(b) Each half of the cord is further marked off into three 

 columns by shallow grooves, the ventrolateral and dorso- 



