SPLXAL M'lRVES 111 



saeiuiii, the nerve-roots }jassin^ within the verteljral eanul beyond the cord form a 

 great sheaf of fibres, the cauda equina (fig. 437). The distance of the points of 

 emergence (superficial origins) of certain of the nerves from the corresponding 

 intervcrteljral foramina is given in the following table. This table gives the 

 measurements made by Testut in a suljject of eighteen years. Tlie length of the 

 spinal cord was in this case forty-one centimetres. 



Right side Left side 

 mm. nitii. 



Third pair of cervical nerves 18 17 



Fifth ,, „ ,, 25 25 



First ,, tlioracic , 3;> 32 



Fifth ,. „ ,, 47 47 



Tenth ,, „ ,, 68 68 



Twelfth ., „ , Ill 110 



Fii-st ,, lumbar 114 114 



Second ,, , 138 134 



Third ,, 151 151 



Fourtli ,, „ ., 163 164 



Fifth ,, „ , 181 180 



First ., sacral 188 188 



Fiftli „ ,, ,, 280 280 



Each spinal nerve, as it enters the intervertebral foramen, is enclosed in a strong 

 tubular slieath formed by the dura mater. This sheath is at first divided l)y a par- 

 tition into two compartments, one for the anterior and the other for the posterior 

 root. The sejjtum soon disappears and allows the posterior and anterior roots to 

 intermingle their fibres. Immediately beyond the ganglion on the posterior root 

 the sheath thins away into the epineurium and perineurium of the nerve. The 

 arachnoid and pia mater also send tubular processes around the nerves; these 

 become continuous with the perineurium, and a connection is thus estaljlished 

 between the l3'mph spaces around the nerves and the subdural and subarachnoid 

 spaces. 



Topography. — Tlie relation of the superficial origins of the spinal nerves to the 

 spinous processes of the vertebne has been investigated by Nuhn, and more recently 

 by Reid. In the folhjwing table, which is given by the latter anatomist, A signi- 

 fies the highest point of origin; B the lowest point of origin; it gives the extreme 

 limits of origin as observed in six subjects. For example, the origin of the sixth 

 thoracic nerve mnr/ extend as high as the lower border of the spine of the second 

 thoracic vertebra, or as low as the upper border of the spine of the fifth, but it does 

 not necessarily extend either as high or as low as the points indicated. 



Second cersMcal (.\) A little above the posterior arch of atlas. 



(B) Midway between posterior arch of atlas and spine of axis. 

 Third (A) A little below posterior arch of atlas. 



(B) Junction of upper two-thirds and lower third of si)iiie of axis. 

 Fourtli ., (A) Just below upper biirdi-r of spine of axis. 



(B) 3Iiddle of spine of third ccivical vertebra. 

 Fifth ., (A) Just below lower border of si)iiie of axis. 



(B) Just below lower border of spine of fourth cervical vertebra. 

 Sixth .. (A) Lower border of spine of third cervical vertebra. 



(B) Lower border of sj)iue of fifth cers'ical vertebra. 

 Seventh ., (A) Just below upper border of spine of fourth cervical vertebra. 



(B) Just above lower border of spine of sixth cervical vertebra. 

 Kiirhth (A) Upper border of spine of fifth eer\ncal vertebra. 



(B) Upper border of spine of seventh cervical vertebra. 

 First thoracic (A) Midway between spines of fifth cervical and sixth cervical vertebrai. 



(B) Junction of upper two-thirds and lower third of interval between seventh 

 cervical and first thoracic vertebne. 

 Second .. (A) Tjower border of spine of sixth cervical vertebra. 



(B) Just above lr>wer border of spine of first thoracic vertebra. 

 Third ,, (A) Just above middle of spine of seventh ceivical vertebra. 



(B) Lower border of spine of second thoracic vertebra. 



