OF SOUTH CAROLINA. 21 
This must also be the case in the arrangement of the sub-kingdom Radiata, as far as relates to the 
groups under the classes which cannot follow each other, but must be arranged side by side. 
It seems, then, that we must not look for a scale of beings that will fairly represent the gradation 
of species, from the lowest organism up to man; and although it is true, to a certain extent, as 
regards the great groups, which do exhibit relative degrees of complication or perfection in their 
structure, yet among the subdivisions of these groups many wide breaks oceur. 
Vertebrata.—The osseous character of the skeletons of these animals is highly favorable to their 
preservation, when enclosed in sedimentary deposits, and hence the vast number of the remains of 
vertebrata found in the rocks of every country. 
The great characteristic of this sub-kingdom is the spinal column, which is continued, with won- 
derful constancy, from the lowest to the highest animal included in it. With the exception of some 
of the lowest fishes, in which it consists of a cartilaginous tube, the spinal column is divided into a 
number of articulating osseous pieces, called vertebra. And as these present modifications 
characteristic of the several classes of vertebrata, they are entitled to special consideration. 
Fig. 7. This figure represents the essential parts, or elements, 
as they are called, of a vertebra. a is the centrum or 
body, the ends of which constitute the articulating sur- 
faces of the vertebra; 6 b are two bony plates that pro- 
ceed from the centrum, and form the sides of an arch, 
whose office is the protection of the spinal cord, and is 
hence called neurapophysis. 'These plates are united at 
the top by the spinous process, d, which compietes the 
arch. Corresponding with the preceding are two other 
plates that extend from the under side of the centrum, and 
are connected, in like manner, by a spinous process—form- 
ing a canal for the protection of the large blood vessels 
that pass along the under side of the centrum. It is there- 
fore called hemapophysis. 'This corresponds with the 
chevron bone of some writers, and is found in fishes and 
cetacea as well as in the caudal vertebre of reptiles.— 
Proceeding from the sides of the centrum are two plates, 
ee, called transverse processes, with which the ribs are connected. Besides these there are other 
processes forming articular surfaces, by which contiguous vertebra are united, but they are not 
constant. In fishes other bones are developed for special purposes, and are called interspinous 
bones. 
The relation between the spinous and transverse processes and the flexures and motions of the 
spinal coiumn is curious and interest ng. When these processes are much developed there can be 
but little motion in the direction in which the development takes place—for instance, in the osseous 
fishes the processes extend in the vertical direction, and are but slightly extended laterally. The 
flexures of the spine are consequently made with ease laterally, but it has scarcely any motion in a 
vertical direction. In the Cetacea, on the contrary, the transverse processes are large and the 
6 
