AND OF THE SYMPATHETIC NERVE. 163 



and the activity of the parts by which it is surrounded. The 

 head and neck being in constant motion, the spinal marrow 

 within the cervical region is of the medium size ; however, 

 as it approaches the fore extremity, that is, about the last 

 three cervical and the first four dorsal vertebrse, it gradually 

 and considerably enlarges. Then within the dorsal region it 

 becomes the smallest of all ; but, gaining the lumbar, from 

 which the huge hind limbs are supplied, it grows larger than 

 ever, and there it terminates in the horse. Animals with a 

 short spinal marrow have a w^ell-marked cordae equinee, or 

 hanging bunch of nerves, from the extremity of the spinal 

 marrow, supposed to resemble a horse's tail. In those with 

 a longer spinal marrow the similitude is less marked ; and 

 in those with this part longer still, the most distant likeness 

 is hard to detect. 



The spinal marrow receives the same coverings as the 

 brain ; or the first having three membranes, these are also 

 continuous over the last organ. There is no difference 

 in their position. In each the dura mater is external ; the 

 pia mater internal; and the arachnoid, or a serous membrane, 

 holds the middle place ; and in each their structure is the 

 same. There is, however, this difterence : the dura mater 

 is not, in the spinal, fixed to the canal, or to the bone ; but 

 upon its outer surface is found a quantity of fat. Then the 

 arachnoid does not lie close upon the spinal marrow ; but is 

 raised far from it by a limpid fluid, as clear as distilled 

 water, which is situated within the cells of the pia mater. 

 Besides this, however, the spinal marrow is peculiar for 

 having several fine ligaments, which run obliquely from the 

 bone ; and a straight ligament, which is continuous through 

 its entire course. They are situated — the straight ligament, 

 W'hich is termed the membrana dentata — within the sheath, 

 and between the true motor and sentient nerves ; and the 

 oblique (which have received no name) are external, and 

 belong particularly to the dura mater. Their use will be 

 referred to hereafter. 



The ganglia. — Wherever there is found the red and white 

 nervous matter mingled, there is a ganglion. The brain is 

 only a large ganglion ; with this difterence however, that, 

 unlike the rest, it is able to take cognizance of the sensation 

 conveyed by the nerves with which it is in connexion. 



m2 



