minute filaments occasionally proceed to other parts of the body.

c. Homogangliata, animals whose ganglia are symmetrically arranged.-Owen.

This division comprehends (together with less known animals) leeches, earth-worms, scolopendras, insects, scorpions, spiders, lobsters, crabs, &c. This division is characterized by having the nervous masses (ganglia) distributed over the body at regular intervals, corresponding to its well-defined segments.

d. Heterogangliata, animals in which the arrangement of the ganglia is not symmetrical.-Owen. In this division are found barnacles, oysters, muscles, snails, cuttle-fish, &c.

In this large group of animals, that symmetrical arrangement of parts so conspicuous in the ray of the star-fish, the segment of the insect, &c., is no longer observable; and the nervous system is as irregular in its distribution as the organs which it supplies are disproportionate to each other in their size.

e. Myelencephalia, animals possessing a brain in a bony skull.-Owen.

This group requires no additional description at present; it comprehends fishes, frogs, reptiles, birds, mammalia, and at the head of these, Man.

3. It will be observed in the foregoing table that a nervous system has been traced in all animals (that is, in all beings that can feel and move), except in those comprehended in the division a. But it is extremely probable that this system also exists in the cryptoneura, although its presence has not yet been detected in them,* since they exhibit sensation

* Carpenter's Inaugural Dissertation, p. 76.

and voluntary powers. The nervous system consists,

1. Of nerves and irregularly shaped masses of nervous matter, called ganglia.

2. Of a prolonged cord of nervous matter both vesicular and fibrous, sheathed in its proper membranes ;-or as some think, of a series of connected ganglia ;-the whole protected by the vertebræ of the spine, which are hollowed to receive it.

3. Of a superadded brain, or, as some have considered it, a collection of ganglia connected with the organs of sense. This brain is always defended by a bony case, or skull. It is found only in the animals of division e.

4. The nervous matter which forms this complicated system is varied both in appearance, and most probably in function. It may be classed under two divisions, the vesicular and the fibrous. "The vesicular nervous matter is gray or cineritious in color, and granular in texture: it contains nucleated nerve vesicles,* and is largely supplied with blood. It is more immediately associated with the mind, and is the seat in which originates the force manifested in the nervous system. The fibrous nervous matter, on

*The essential elements of the gray nervous matter are vesicles, or cells containing nuclei and nucleoli. They have been called nerve or ganglion globules. The wall of each vesicle consists of an exceedingly delicate membrane, containing a soft but tenacious, finely granular mass. The nucleus of the cell is generally eccentric, much smaller than the containing vesicle, and adherent to some part of its interior. Its structure is apparently the same as that of the outer vesicle. The nucleolus is a minute, remarkably clear, and brilliant body, also vesicular, enclosed within the nucleus. It forms a most characteristic and often conspicuous part of the nerve-vesicle." Todd and Bowman's Physiology, vol. i. p. 212.

the other hand, is, in most situations, white, and composed of tubular fibres, though in some parts it is gray, and consists of solid fibres. It is less vascular than the other, and is simply the propagator of impressions made upon it. When these two kinds of nervous matter are united together in a mass of variable shape or size, the body so formed is called a nervous centre, and the threads of fibrous matter which pass to or from it are called nerves. The latter are internuncial in their office: they establish a communication between the nervous centres and the various parts of the body, and vice versâ. smaller nervous centres are called ganglions: the larger ones, the brain and spinal cord."*

The

5. When examined by the naked eye and the finger, a nerve is a soft, white, thread-like substance. In its course it resembles a leafless branch. It spreads out into small nervelets or filaments, and thus diminishes or increases in size according as it is traced from or towards the central cord or ring in which it originates. But when carefully viewed by the microscope,t each nerve is found to be a mere bundle of extremely small tubular filaments, containing a sort of half fluid pith. These are separately enclosed and connected together by a covering of a very delicate texture, and the whole is cased in a thin membranous sheath. These fibrils sometimes unite with, sometimes cross over each other, sometimes form new groups with detachments from other bundles, sometimes are twisted over each other: but in no instance does the minutest fibril so penetrate another that there can be a mixture of their component particles. Therefore, although the number of

* Todd and Bowman's Physiology, vol. i. p. 205.
+ Carpenter's Principles of Physiology, p. 42.

nervous filaments is immense, there can be no confusion in the discharge of their functions. "The nerves appear to be formed after the same manner as the muscles, i. e., by the fusion of a number of primary cells arranged in rows into a secondary cell. The primary nervous cell, however, has not yet been seen with perfect precision by reason of the difficulty of distinguishing nervous cells, while yet in their primary state, from the indifferent cells, out of which entire organs are evolved. When first a nerve can be distinguished as such, it presents itself as a pale cord with a longitudinal fibrillation, and in this cord a multitude of nuclei are apparent.' "According to Valentin's description, the following is the process of development of the nerve vesicles. In the very young embryos of mammalia, as the sheep or calf, the cerebral mass in the course of formation contains in the midst of a transparent blastema, transparent cells of great delicacy with a reddish-yellow nucleus. Around these primitive cells, which we find likewise formed after the same type in the spinal cord, a finely granular mass becomes deposited, which probably is at first surrounded by an enveloping cell membrane. At this early period of formation the primitive cell still preserves its first delicacy to such a degree that the action of water causes it to burst immediately."*

6. The functions of the nerves are various: experience has shown that the intervention of nerves is absolutely necessary; 1, to the continuance of animal life; 2, to the reception of sensation; and 3, to the production of movements in all the higher orders of animals; and from analogy it has been conceived that even if it have hitherto eluded observation,

* Todd and Bowman's Phys., vol. i. pp. 227, 228.

nervous matter does exist, even in the lower tribes; for organic life is possessed no less by the rooted zoophyte, which seems scarcely to have any consciousness of the exterior world, than by man. But the actions of man being all destined to be modified by his rational faculties, a more complex arrangement is requisite in his case, in order to bring the whole system into harmony. The functions of animal life, therefore, are carried on by a machinery which, though capable of acting alone, is yet so connected with the organs of the higher faculties, as to be placed in great measure under subjection to them.

7. At the head of this machinery stands that set of ganglia, and their connecting nerves, which is known to anatomists as the sympathetic, ganglionic, or tri-splanchnic system, or sometimes, in older writers, as the great intercostal nerve. This is found extending from the base of the skull in a double chain of ganglia on each side of the vertebral column, interiorly as regards the body, and passing within the ribs towards the lower part of the trunk. Throughout its course, numerous nerve fibres are thrown out to supply the viscera both of the thorax and abdomen, and "branches attach themselves to the exterior of arteries, forming very intricate plexuses, which entwine around them, hederæ ad mo

* So called from oλayxva, viscera : "We may with de Blainville consider it as divisible into two parts, one placed in front of the spine (prævertebral) composed of plexus and ganglia (semilunar and cardiac), whose branches are distributed to the primary organs of digestion and circulation: the other consisting of two knotted cords, extended along the whole length of the spine, communicating with the prævertebral plexus on the one hand, and with the cerebro-spinal nerves on the other."--Quain and Wilson's Anatomy of the Nerves, p. 4.