MAMMALIA. 147 



motions of the spine. The arterial circles placed at the more posterior parts of the 

 bodies of the vertebrae, and described by Willis, offer some explanation. The arterial 

 circles receive blood, like the spinal arteries, from the vertebral, intercostal, lumbar, 

 and sacral arteries. The branches leading to the spinal arteries are conducted along 

 the bundles of nerves, and, by the communications of similar branches with the arterial 

 circles, allow a uniform supply of blood continually throughout the cord, whilst the 

 plexuses of veins are fitted for accommodating any returning blood and preventing its 

 delay in the cord. 



The nerves arising from the spinal cord consist of an anterior and posterior row 

 of bundles on each side. The fibrils of the posterior bundles are by far the coarsest, 

 and have ganglia attached to them on leaving the spinal sheath. The ganglia are not 

 always placed as in man ; but in the baboon, the dorsal, lumbar, and sacral are con- 

 tained within the spinal canal ; and in some animals, as the ass, calf, and goat, are 

 not so compact as in man, the baboon, jaguar, dog, fox, pig, and others, but of a 

 coarser texture, and have less of the red matter intervening between the nervous 

 fibrils ; at the anterior point each ganglion is joined by the anterior bundle to form a 

 nerve, in which the fibrils of both become intermixed. In the ass, the different 

 bundles of fibrils composing a nerve, having passed through separate openings in the 

 sheath of the dura mater, form little ganglia, which communicate together just at the 

 beginning of the nerve ; it is nearly the same in the calf and goat. The principal 

 difference of their structure in numerous animals consists of a greater or less separation 

 of the fibrils into more or fewer ganglia, and a varying quantity of red matter. The 

 posterior fasciculi in the porpoise are rather smaller than the anterior, probably on 

 account of the small extent of skin and great development and power of the muscles ; 

 they form close and fleshy ganglia externally to the sheath of dura mater ; those of 

 the caudal are not so distinct as the cervical and dorsal. The spinal cord is embraced 

 closely by the sheath of dura mater; but, as there is not more than a very slight 

 motion between the vertebrae, much room for bending is not required. The vascular 

 net-work occupies a large portion of the canal. 



In the jaguar the nine, and in the sow the seven, first bundles of dorsal nerves 



u 2 



