380 SPECIAL ANATOMY. 



many places with the pia mater. Behind it is free ; before it is 

 connected by processes with lig. longitudinal posterius. Laterally, 

 sheaths pass off with the nerves through the foramina interverte- 

 bralia, for the posterior and anterior roots of which there are 

 openings (two) in it. The superior extremity is firmly attached to 

 the circumference of theforam. magnum, and passes into the dura 

 mater of the brain ; the artt. vertebrales perforate it at this point. 

 The inferior extremity forms a wide sac around the cauda equina, 

 divides into several sheaths (from five to six) for the sacral nerves, 

 and is firmly connected with lig. sacro-coccyg. postic. Vessels : 

 arteries are rami spinales, the artt. cervicales, dorsales, lumbales 

 and sacrales. The veins pass into the plex. spinales inter ni. 

 Nerves have not yet been found, the dura mater is however sen- 

 sitive. 



2. Arachnoidea. Between the dura and pia mater an epithe- 

 lium layer is placed, as in the brain, which covers the uniting tis- 

 sue of the two surfaces, opposed to each other, and is regarded as 

 a proper tunic ; that is, the arachnoid membrane. It envelopes the 

 nerves of the cauda only, as far as theforamm. intervertebr., and 

 is then reflected upon the dura mater. Between dura and pia 

 mater a space is also found, filled with cerebro-spinal fluid, which 

 facilitates the movement of the spinal cord. 



3. Pia mater, is stronger than that of the brain, dirty white, 

 sometimes, in the cervical region, dotted with blackish spots. It 

 forms the neurilemma of the medulla spinalis, to which it is firmly 

 applied, into the fissures of which it enters, and envelopes the 

 nervous fasciculi. It is strengthened by fasciculi of fibres, which 

 are only lost in the cranial cavity ; it is formed besides of uniting 

 tissue and vascular rete. On either side the thin edge of the fibrous 

 lig. denticulatum is attached, from the free, thicker edge of which 

 (from twenty to twenty-one) denticulations pass outwards, and 

 connect themselves with the dura mater in the intervals between 

 the anterior and posterior roots of the nerves. The first denticu- 

 lation is placed in the foram. magn. between art. vertebral, and 

 n. hypogloss. / the last at the first lumbar vertebra. From this 

 point, the inferior extremity of conus medullaris, the pia mater 

 terminates in a delicate thread, filum terminate, which above, only, 

 contains a little gray medullary matter, and is attached below in 

 the sacral canal to the termination of the dura mater. 



The Peripheral portion of the Nervous System. 

 629. A. The cerebral nerves, nervi cerebrales. 

 There are twelve pairs, of which the first, second, and eighth 



