community of office is observed: each organ becomes disembarrassed of the duties of the others, and performs its own particular function alone. At length, when the animal organisation has reached its highest point of development in Man, we find Locomotion executed exclusively by its appropriate membersthe Lower Extremities: Prehension executed exclusively by its appropriate members-the Upper Extremities: and Mastication by its appropriate instruments-Jaws and Teeth.

And here it may be observed, that, when the Lower Extremities, by their perfect construction as implements of locomotion, have emancipated the Upper Extremities from sharing in progression, so that the Hand and Arm are independent, and available for all acts of prehension, the Jaws and Teeth cease to be employed for seizing and holding. The mouth is exempt from performing any other duty but that of Mastication. Consequently, the bones of the face, jaws, and teeth admit of being reduced in dimensions: and the cavity of the mouth adapted, in size and form, to be an important part of the organ of Voice and Speech, in relation to man's highest endowmenthis Mind.

Such being the association of organs necessary for the construction of an Animal, viewed in the abstract, the question next arises-How is the mechanism to be animated; or, what must be the combined properties of a Nervous System, designed to meet the wants of the organism? By following that inquiry, we may be led to ascertain what is the most elementary form of a System of Nerves.

In the simplest view that can be taken of a nervous system, we must suppose the existence, in some part of it, of a central organ, corresponding to the Brain in the highest animals; an organ from which motor power may be transmitted to the muscles, and to which one or more senses may be conveyed from the surface. It is also reasonable to assume, however difficult it may be to prove the fact anatomically, that in the elementary brain the particular structure which initiates motor influence will be different from that which receives impressions from the senses.

With the doubly-constituted organ thus introduced, it must be supposed that nerves will be provided as media of communication-one set for giving Motion, and the other for giving

Sense. Two questions therefore present themselves, bearing directly on the immediate subject of the inquiry-What will be the particular nerves of motion; and What those of sense?

The answer in regard to the nerves of Motion is simple. Nerves of that kind will require to be allotted to the various organs enumerated as composing the frame of the animal. First, they will be sent to the Locomotive organs, represented in man by the Lower Extremities; next they will be sent to the Prehensile organs, represented by the Upper Extremities; and lastly, to the Manducatory organ, represented by the Jaws and Teeth.

As to the nerves of Sense. It is reasonable to suppose that, of all the senses known to belong to animals generally, that of Touch, or common Sensation, extending over the whole body, will be the most essential for a creature placed lowest in the ranks of the animal kingdom. And the next sense in importance will be that of Taste, the guardian of the opening by which food is passed into the stomach.

It will now be perceived, that, by following this course of observation, and imagining an Animal, of the simplest form, having the fewest attributes consistent with its ranking above Vegetables, we arrive at the conception of a system of nerves, which, for the properties supposed to belong to it, agrees precisely with the class of Spinal nerves and Fifth, in the higher animals and Man. By the former extensive series-the nerves of the spine-power of motion is given to the upper and to the lower extremities, the representatives of the organs of Prehension and of Locomotion; and by the small root of the Fifth, distributed exclusively to the muscles of the jaws, motor power is bestowed on the part which represents the organ of Mastication. Again, by the Spinal nerves, common Sensation or the sense of Touch is supplied to all the surfaces of the body (except the head); and by the large root of the Fifth, the same property, Touch, is given to the whole head, together with the special sense of Taste.

The conclusion, therefore, to which the author came was, that the series of Spinal nerves and Fifth constituted a class which belonged to animals in all grades of the animal kingdom; that it ministered to functions and endowments equally necessary to those high and low in the scale; that in animals of earliest and

simplest construction, it existed in the rudimentary form of a nervous system; but that, by a gradual process of development, it attained the perfect condition exhibited in the Vertebrata, and in Man. Wherefore, in arranging the series together in his classification, he applied to them the name "Original" system of nerves.

Respiratory System of Nerves.

II. The nerves next claiming attention are those which, we have seen, are distinguished by arising, in limited number, from a small circumscribed portion of the base of the brain, by single roots, and which diverge, in an apparently irregular and scattered manner, across the other nerves, to be distributed to the face, throat, neck, and chest-that is, to the region where the organ of Respiration, with its concomitant parts, is situated. The fact that these nerves seek the organ of breathing for their destination, naturally induced the author to study closely all relating to that part of the body, in the hope of obtaining facts which might assist in his inquiry. In that manner he was eventually led to examine with peculiar interest a series of extensive changes which gradually takes place in the scheme and uses of the apparatus, during its development from the lowest to the highest animals.

The organ of respiration, in the language of physiologists, is commonly understood to refer only to that structure in animals which is designed to expose the blood to the influence of the air, so that the vital fluid may be purified and rendered fit to circulate over the body. But essential as that office is to all living beings, and a similar process of aërating and renovating the fluids is carried on in vegetables as well as animals, it is not to be regarded as the most elevated application of the organ. Without diminution of its efficiency in that capacity, the mechanism is adapted, in Man, to an object altogether foreign to its original use: the structure is so ordered and arranged that the air, which has been employed in oxygenating the blood, is utilised, in the act of being expelled from the body, to produce audible sounds—the elements of Human Voice and Speech.

The addition to the organ of respiration of that high office,

connected with Man's noblest endowment, the Mind, is effected, in the course of its gradual development through the animal kingdom, by a series of changes successively made in its construction. As it belongs directly to our subject to examine these, I propose to give a general view of them, by tracing the mechanism, with its various modifications, through the different grades of animals, from the lowest to the highest.

First, it is important to notice that the process of respiration is conducted in animals on two widely distinct plans. One of these is termed the "Diffuse;" the other, the "Concentrated" system.

By the Diffuse system, is meant that mode of breathing by which the air, or the water charged with air, is brought in proximity to the blood by playing on surfaces upon which the vital fluid circulates more or less abundantly, but which surfaces are open and unenclosed. Here, then, is the distinctive peculiarity of the method-that the air made use of is not confined within a cavity. When it has performed its office, it is dispersed and lost.

In the Concentrated plan, the chief characteristic is the introduction into the mechanism of a membranous sac, capable of holding air; and of a tube, which communicates with the external atmosphere. The blood to be aërated circulates on the surface of the air-sac: and the air can be constantly changed, by the alternate expansion and compression of the walls surrounding the sac. From that general description it will be perceived that the sac containing air is the representative of lungs; the tube, of windpipe; and the enveloping walls, of thorax. It will also be seen that in an apparatus consisting of such parts, elements are supplied for the formation of an organ of Voice. It only requires that the air confined in the bag shall be expelled along the pipe, with a force sufficiently great to cause the special vibrations of sound, that it may be applied to that office.

Next, of the animals to which each of these distinct modes of respiration belongs. In arranging the animal kingdom, the most comprehensive division is into the two great sub-kingdoms-the Vertebrata, and the Invertebrata. Now, it is an interesting fact, in connexion with our subject, that in the whole extent of the lower of these divisions, there is not a single example of an animal which breathes according to the Concen

trated system: the only method is the Diffuse. That is equivalent to saying that in no animal of the Invertebrate sub-kingdom, is the mechanism formed on a plan consistent with its producing sounds adequate for Voice.

It is not till we mount up to that high stage in the animal kingdom, where animals begin to be formed on the Vertebral type, that breathing by the Concentrated method is met with. We then, for the first time, perceive introduced into the animal framework, an apparatus analogous to thorax, lungs, and windpipe. In the inferior classes of the sub-kingdom,. we see, and that obscurely, only the elementary rudiments of the construction. But by degrees, and in proportion as animals approximate in their general structure to Man, the mechanism becomes better calculated for performing its double office,—first, of purifying the blood; secondly, of ministering to Voice.

I may now point out, in a general manner, the more prominent changes wrought upon the organ, during its course of development through the five different classes of Vertebrata.

1. In Fishes, which form the lowest class of Vertebrata, the organ of respiration exhibits a transition state between the Diffuse and the Concentrated systems. The gills are constructed according to the former plan; yet the process of taking in the water by the Mouth, that it may be swept over the gills, is according to the latter. Indeed, the employment of the mouth in connexion with the operation of breathing, which is general in all the Vertebrata, is altogether unknown in the Invertebrata. In the latter, the oral orifice has relation exclusively to the digestive functions. The first time, in the animal kingdom, that the mouth is found to serve the double purpose of being a cavity for receiving food for the stomach, and an opening for the passage of the air in the vital operation of breathing, is in Fishes.

But it is not the gills which, in this class, claim principal interest in reference to the development of the organ of respiration. In certain fishes, there are lodged, within that part which corresponds most nearly to their chest, a membranous sac, and a tube; and these are the true first representatives, in animals, of Lungs and a Trachea. Disguised by performing an office not directly connected with breathing, they are not easily recognised as having any relation to that organ. The sac is filled