Nervous System

In all these lower vertebrates except the Anura (frogs and toads), the cord fills the whole length of the spinal canal, but in the higher mammals (Primates, Chiroptera and Insectivora) it grows less rapidly, and so the posterior part of the canal contains the cauda equina within its sheath of Jura mater. In mammals below the anthropoid apes there are no direct pyramidal tracts in the cord, since the decussation of the pyramids in the medulla is complete. Moreover, the crossed tracts vary very much in their proportional size to the rest of the cord in different animals. In man, for example, they form 11.87% of the total cross area of the cord, in the cat 7.76%, in the rabbit 5.3%, in the guinea-pig 3%, and in the mouse 1.14%. In the frog no pyramidal tract is found. It is obvious, therefore, that in the lower vertebrates the motor fibres of the cord are not so completely gathered into definite tracts as they are in man.

A good deal of interest has lately been taken in a nerve bundle which in the lower vertebrates runs through the centre of the cen tral canal of the cord, and takes its origin in the optic reflex cells in close relation to the posterior commissure of the brain. More posteriorly (caudad) it probably acquires a connection with the motor cells of the cord and is looked upon as a means by which the muscles can be made to respond to the stimulus of light. It is known as Reissner's fibre, and its morphology and physiology have been studied most carefully in cyclostomes and fishes. It is said to be present in the mouse, but hitherto no trace of it has been found in man. It was discovered in 186o, but for forty years was looked upon as an artifact.

See P. E. Sargent, "Optic Reflex Apparatus of Vertebrates," Bull. Mus. Comp. Zool. Harvard, vol. xlv. No. 3 (July, 1904) ; also for general details R. Wiedersheim, Comparative Anatomy of Vertebrates (London, 1907) ; Lenhossek, Bau des Nervensystems (1895).

(F.

G. P.) The spinal cord is but a part of the nervous organ which throughout the length of the body forms the meeting-place of the nerve-paths arriving from and issuing to all regions with which nerve fibres communicate. To gain a true point of view for under standing the working of the spinal cord one must refer to the general function of the nervous system in the bodily economy.

Relation to General Nervous System.

An animal of mi croscopic size may continue throughout its life to be constituted entirely by one single cell. Animals of larger bulk, although each begins its existence as a single cell, attain their development by the multiplication of the original single cell, so that from it there comes to be formed a coherent mass of cells very many millions in number. In these multicellular animals each of the constituent

cells is a minute self-centred organism, individually born, leading its own life and destined for individual death. The corporate power of the complex animal is the sum of the powers of those manifold individual existences, its cells. Of the agencies which integrate the complex animal, one of the most potent is nervous action. A certain number of the unit cells composing the animal are specially differentiated from the rest to bind the whole to gether by nervous action. These specially differentiated cells are called "neurones." They constitute living threads along which waves of physicochemical disturbance are transmitted to act as releasing forces for the energy in distant cells.

It is characteristic of this nervous system, the system of neurones, that, although ramifying far and wide through the body, it is a continuum from end to end. The peripheral nerves are formed of bundles of neurones lying side by side, but these, al though packed close together, are strictly isolated one from another as conductors and remain isolated throughout the whole length of the nerve. The points of functional nexus of the neurones one with another are confined to one region only of the whole system. All their conductive connections one with another take place solely in the central nervous mass which constitute the so-called central nervous system, a nervous organ extending axially along the length of the body midway between the body's lateral halves. Thither the neurones converge in vast numbers, those of each body segment converging to that fraction of the central organ which belongs to their body segment. The central nervous organ thus receiving these neurones is, where it lies in the head, called the brain, the rest of it is called in vertebrates the "spinal cord," in worms and arthropods the "nerve-cord." The central organ not only receives neurones which converge to it from outside, but many of its own neurones thrust out their con ductive arms from it as nerve fibres carrying nervous influence outwards to regulate the activity of glands and muscles. In the vertebrates the ingoing neurones for each segment and similarly the outgoing neurone fibres are collected into a segmental nerve. To the spinal cord these are each attached by two roots, one dorsal, consisting of the afferent fibres, the other ventral, con sisting of the efferent fibres.