134 ANATOMY OF UPPER EXTREMITY, ETC. 



parting from the spine of the axis form a star with si: 

 points ; the inferior points being formed by the semi-spi 

 nales colli, the lateral by the obliqui inferiores, and th< 

 superior by the recti capitis majores. 



SPINAL CORD AND MEMBRANES. 



The muscles are to be dissected away from the sides of the verte- 

 bral spines. With the chisel and saw their arches are to be divided 

 upon each side close to the articular processes ; the hones can only 

 be removed piecemeal Mnd with difficulty. This done, the membranes 

 of the spinal cord will be exposed. 



The membranes of the spinal cord are covered exter- 

 nally by veins and by a loose areolar tissue containing 

 fat and, especially at the lower part, a little fluid ; they are 

 a continuation of those of the brain, and, like them, con- 

 sist of dura mater, arachnoid, and pia mater. 



The DURA MATER envelops the cord loosely, and sends 

 tubular prolongations along the spinal nerves issuing at 

 the intervertebral foramina; at its lower part these pro- 

 longations become longer and lie for some distance within 

 the spinal canal. The dura mater terminates in an imper- 

 vious fibrous process, which blends with the periosteum 

 covering the back of the coccyx. 



The dura mater is to be opened lengthwise with the scissors ; this 

 will expose the arachnoid. 



The ARACHNOID is a serous membrane enveloping the 

 spinal cord, and reflected upon the internal surface of the 

 dura mater. That portion attached to the dura mater is 

 closely adherent to it, while that in relation to the cord is 

 loose ; the interval between the cord and membrane con- 

 stitutes the sub-arachnoid space, and is filled by a fluid, 

 called the cerebro-spinal (p. 80). The arachnoid envelops 

 each spinal nerve and the collection of nerves which ter- 

 minates the cord. 



The loose arachnoid is to be removed and the pia mater will then 

 be exposed. 



The PIA MATER is a thin and stout membrane closely 

 investing the spinal cord; it forms a sheath for the spinal 

 nerves and inferiorly is prolonged downward in a slender 

 process called the filum terminals, which blends with the 

 terminal prolongation of the dura mater. 



On each side of the spinal cord, extending its whole 



