60 
less—animals. The three systems of assimilation or nutrition, circulation, and 
respiration, are intimately connected with each other-—so much so, that they 
ought, perhaps, to be considered as parts of one compound system,—-the vital sys¬ 
tem, or that by means of which the body of the animal is originally formed and 
maintained during the period of its life in the exercise of those functions which 
belong to its species. We need not mention that the first part of this compound 
system, in its organization, consists of the whole alimentary passage, together with 
the accessory parts which promote digestion, and those by which the assimilated 
food is conveyed to the blood; that the second consists of the circulating vessels, 
whether their contents be blood in the arterial or venous state, or any other cir¬ 
culating liquid; and that the third part consists of that apparatus by which the 
waste (in most cases apparently the surplus carbon) of the system is conveyed by- 
means of air or water to the general mass of inorganic matter. In the greater 
number of animals, whether vertebrated or not, the alimentary, or assimilating* 
part of the system, is internal; the circulating part is, also, more or less distri¬ 
buted throughout the whole body ; and the respiratory part is variously placed— 
being internal in the warm-blooded vertebrata, and in many invertebrated animals, 
but more or less external in others. The nervous system is very differently situ¬ 
ated ; though it is always internal in what may be considered as its most essential 
parts, and more or less ramified through the body in the others. 
In all vertebrated animals, the nervous system is really the central part; for 
the brain and its spinal elongation, from which the nerves proceed to all parts of 
the body, are always lodged within the vertebral part of the skeleton. In them, 
too, the three parts of what we have described as the vital system, are internal as 
regards the whole mass of the body, but external as regards the spinal column. 
They proceed from the opening of the mouth, and are lodged in cavities of the 
chest and abdomen, suspended upon one side of the vertebral column, and corres¬ 
pondingly on the same side of it, in all the classes of the grand division; but 
though they are supported on the spine, they are never contained in the same cavi¬ 
ty with its essential contents. Of the system of reproduction we do not speak, 
because this is connected with the succession of generations in the animal, and not 
with any one animal considered as an individual. 
In animals of this grand division, therefore, the several parts of the more im¬ 
portant systems are kept separate from each other, and each enjoys a different 
degree of protection from external injury, and even from injury by the working of 
the mechanical system. The central parts of the nervous system are wholly 
enclosed within the bones of the vertebral column, so that no external injury 
can happen to them, except from the fracture or dislocation of this column; and 
the processes or projections of the different vertebrae are so formed, that disloca¬ 
tion of the column is next to impossible, by any ordinary strain to which the ani¬ 
mal can be subjected. The breathing apparatus, and the heart, or centre of 
