SUMMARY. 239 



it varies either in breadth or length, according to the required motion of the spine, 

 and the number and size of the nerves ; it may be short and broad, or long and 

 narrow, with enlargements in places from which larger nerves are to proceed ; it may 

 form a longer or shorter cauda equina. In birds, it appears knotted, and has its 

 dorsal part closely surrounded by bone, and has a lumbar ventricle ; it reaches to the 

 tail in birds, and generally in amphibia and fishes. 



The number of spinal nerves varies in each class. There is an anterior and 

 posterior origin of nerves from each portion of it in the four superior classes, a ganglion 

 is generally formed by the posterior, this however may not only vary in structure, in 

 being either thready or fleshy, but in the disposition of the nerves attached to it, as 

 in the cod. Ganglia are both fleshy and thready in mammalia, fleshy in birds and 

 amphibia, "but much more indistinct in fishes ; in the skate, a very small ganglion is 

 attached to each posterior fibril, and in the cod, most of the anterior and posterior 

 bundles, after leaving the spinal cord, are not only arranged differently, but the 

 ganglionic structure is very equivocal. In the ventral surface of the cord of the 

 lobster there are ganglia, but not in the ring of the crab, nor in the nerves proceeding 

 from it. In almost the whole of mammalia, the number of cervical nerves is the 

 same as in man ; in birds, it is very various ; also in amphibia ; in some there is very 

 little distinction as well as in fishes, the nerves of the toes covered by a hoof are 

 smaller in proportion to those covered only by cuticle. The phrenic nerve exists in 

 mammalia only. In animals having upper extremities, the lower cervical and first 

 dorsal nerves generally form an axillary plexus from which these parts are supplied ; it 

 is composed of more or fewer spinal nerves, and generally sends off similar trunks, 

 which are larger or smaller, or differently arranged, according to the size, and the 

 required direction of the muscles. In mammalia, birds, and some of amphibia, as the 

 turtle, the axillary plexus is produced by the lower cervical and first dorsal nerves ; 

 in the frog there is one large nerve instead of the plexus ; in the snake it is absent ; 

 in fishes there is some resemblance of it, and particularly in the skate. It divides into 

 similar nerves in mammalia and birds ; in the turtle they resemble most the median, 

 spiral, and circumflex ; in fishes they hardly bear any comparison. In mammalia, the 



