SPINAL CORD. 431 



The Peduncles of the pineal gland must also be regarded as commis- 

 sures, assisted in their function by the grey substance of the gland. 



The Pons Varolii is the commissure to the two hemispheres of the 

 cerebellum. It consists of transverse fibres, which are split into two 

 layers by the passage of the fasciculi of the corpora pyramidalia and 

 olivaria. These two layers, the superior and inferior, are collected 

 together on each side, in the formation of the crura cerebelli. 



SPINAL CORD. 



The dissection of the spinal cord requires that the spinal column 

 should be opened throughout its entire length by sawing through the 

 laminae of the vertebrae, close to the roots of the transverse processes, 

 and raising the arches with a chisel ; the muscles of the back having 

 been removed as a preliminary step. 



The Spinal column contains the spinal, cord, or medulla spinalis ; 

 the roots of the spinal nerves ; and the membranes of the cord, viz. the 

 dura mater, arachnoid., pia mater, and membrana dentata. 



The Dura mater spinalis (tJieca vertebralis) is a cylindrical sheath of 

 fibrous membrane, identical in structure with the dura mater of the 

 skull, and continuous with that membrane. At the margin of the 

 occipital foramen it is closely adherent to the bone ; by its anterior 

 surface it is attached to the posterior common ligament, and below by 

 means of its pointed extremity to the coccyx. In the rest of its 

 extent it is comparatively free, being connected, by a very loose 

 areolar tissue only, to the walls of the spinal canal. In this areolar 

 tissue there exists a quantity of reddish, oily, adipose substance, 

 somewhat analogous to the marrow of long bones. On either side 

 and below, the dura mater forms a sheath for each of the spinal 

 nerves, to which it is closely adherent. Upon its inner surface it is 

 smooth, being lined by the arachnoid ; and on its sides may be seen 

 double openings for the two roots of each of the spinal nerves. 



The Arachnoid is a continuation of the serous membrane of the 

 brain. It encloses the cord very loosely, being connected to it only by 

 long slender filaments of fibro-cellular tissue, and by a longitudinal 

 lamella which is attached to the posterior aspect of the cord. The 

 fibro-cellular tissue is most abundant in the cervical region, and dimi- 

 nishes in quantity from above downwards ; and the longitudinal 

 lamella is complete only in the dorsal region. The arachnoid passes 

 off on either side with the spinal nerves, to which it forms a sheath ; 

 and is then reflected upon the dura mater, to constitute its serous 

 surface. A connection exists in several situations between the arach- 

 noid of the cord and that of the dura mater. The space between the 

 arachnoid and the spinal cord is identical with that already described 

 as existing between the same parts in the brain, the sub-arachnoidean 

 space. It is occupied by a serous fluid, sufficient in quantity to 

 expand the arachnoid, and fill completely the cavity of the theca 



