SPINAL CORD. 389 



the fibres from the posterior lobes are assembled. The fibres which curve 

 backwards into the posterior lobes from the splenium of the corpus callo- 

 suin have been termed forceps, those which pass directly outwards into 

 the middle lobes from the same point, tapetum, and those which curve 

 forwards and inwards from the genii to the anterior lobes, forceps anterior. 



The Fornix is an antero-posterior commissure, and serves to connect a 

 number of parts. Below, it is associated with the thalami optici ; on each 

 side, by means of the corpora fimbriata, with the middle lobes of the 

 brain ; and, above, with the corpus callosum, and consequently with the 

 hemispheres. 



The Septum luddum is a perpendicular commissure between the fornix 

 and corpus callosum. 



The Anterior commissure traverses the corpus striatum, and connects 

 the anterior and middle lobes of opposite hemispheres. 



The Middle commissure is a layer of grey substance, uniting the thalami 

 optici. 



The Posterior commissure is a white rounded cord, connecting the 

 thalami optici. 



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 cere- 

 bellum. It consists of transverse fibres, which are split into two layers by 

 the passage of the fasciculi of the corpora pyramidalia arid corpora olivaria. 

 These two layers, the superior and inferior, are collected together on each 

 side, in the formation of the crura cerebelli. 



SPINAL C ORD. 



The dissection of the spinal cord requires that the spinal column should 

 be opened through 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 core?, 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 (theca 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 fora- 

 men it is closely adherent to the bone ; by its anterior surface it is attached 

 o 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 



