180 OUTLINES OF EQUINE ANATOMY. 



•eitlier side we may expose tlie spinal cord, in situ, covered 

 "by its membranes. The dura mater of the cord differs 

 from that of the brain in that it does not form the peri- 

 osteum lining the bony cavity in which it is placed. It is 

 continuous anteriorly with the corresponding membrane 

 of the brain, in passing to the margin of foramen mag- 

 num of the occiput. Posteriorly it forms the fihim ter- 

 w^inale in becoming attached to the posterior extremity 

 of the bony canal. On either side it is closely attached to 

 the intervertebral foramina, and is reflected across these 

 openings to close them, so that here it is thick, and pre- 

 sents two rows of foramina, through which the collections of 

 nei-ve-fibres pass. The pia mater is directly continuous 

 with that of the brain and closely invests the cord. It in 

 structure closely resembles that of the brain, and laterally 

 is connected to the dura mater by the ligamentum denti- 

 «culatum. This is a band of white fibrous tissue extending 

 along the lateral part of the cord, attached to the external 

 surface of the pia mater, between the superior and inferior 

 roots of the nerves. It sends processes outwards opposite 

 the bodies of the vertebrae, which become attached to the 

 inner surface of the dura mater. The arachnoid mem- 

 l)rane is continuous with the cranial arachnoid, lines the 

 dura mater, covers the pia mater loosely so as to j)roduce 

 .a suharaclmoid space, and invests the processes of liga- 

 iQientum denticulatum, and those portions of the nerves 

 ■which pass through the arachnoid space. 



The Spinal cord consists of a number of nerve-centres 

 united together by white fibrous nerve cords, which render 

 it a continuous and extremely elongated mass extending 

 irom the brain anteriorly, through the spinal column as far 

 rfxs ahout opposite the second sacral hone. It varies in size in 

 'different parts, and this variation is due to the number of 

 aierves it must give off. Thus those parts which supply 

 merves to the fore and hind extremities are very large. It 

 is moderate in size in the central part of the cervical region, 

 Ibut in the posterior cervical and anterior dorsal vertebrae 

 it grows larger, and from this decreases in size until the 

 lumbar region, where it becomes large in giving off nerves 

 -to the hind limb. As a rule the nerves pass straight out- 

 wards to the intervertebral foramina, but in the sacral region 

 Ihey run obliquely backwards, and the superior fibres unite 

 with the inferior and again separate, forming the superior 



