The embryo is composed of such segments placed with their centers in the same axial line. Each segment contains in association which is morphologic, physiologic, and anatomical, a segment of nerve matter and a somatic (body) segment. The neural segments are arranged end to end so as to form the rudimentary beginning of the complete central nerve axis of the adult human body; the somatic segments blend together with somewhat indefinite lines of cleavage which are to become much more indefinite and obscure by changes in relative form due to differences in the growth rate of different parts or to involuntionary changes following functional inutility at various periods. Gray says, “The intrinsically segmental nature of the spinal cord is expressed by the association of each definite segment with the somatic segment supplied by its nerve.”
Within each segment there may be observed at an early period cell migrations from the walls of the primitive neural tube and amoeboid projection of axonic and dendritic processes from these cells, which serve to bring the other tissues of the segment under the control of the nerve elements; there is an assumption of command, as it were, by the nervous system, so that the epithelial, connective, and muscular tissues of each segment are linked in sensomotor and vegetative co-ordination by the contact association of the nerves which ramify them—sensomotor because the nerves are presently to carry the only force capable of inciting activity of any kind in other tissues, vegetative because the functions of growth, nutrition, and repair, in each somatic cell, depend upon the continuity of communication between it and the lowest nerve cell in the nerve pathway which connects it with the higher motor and sensor centers.
Development of the Nerve System
Already may be noted a hint and a prophecy of that future segmental organization by which it becomes possible for some spinal vertebra to become displaced and thus begin a morbid process which may diffuse itself throughout an entire body segment, involving neural and somatic elements together. Already the simple organization begins to become rapidly complex and difficult to trace.
Cell masses begin to migrate from the walls of the primitive neural tube to a position laterad to become the spinal ganglia; these send out long dendritic processes which marvellously thread their way to a predetermined peripheral connection which is to bring some cutaneous, or muscular, or joint tissue into sensor relation with the dorsal, or Sensor, portion of the cord and through it with the brain; at the same time they send their axonic processes inward to mingle with and communicate with the dendrites of other sensor cells remaining in the central axis to form the gray matter of the cord, and thus, migrating, keep up communication both with the central axis and the periphery. Other cell masses migrate ventrolaterad to form the sympathetic ganglia and they also send out afferent and efferent processes which make a connection on the one hand with the periphery and on the other with the source from which the cells developed, the situation to be occupied by the cord. From this view it is seen that the sympathetic system is merely an offshoot from the same source with all the rest of the peripheral nerve system, merely a mechanism for the proper distribution of nerve impulses from the central organs, and that it retains its connection in all its parts with those organs. Its ganglia, like those of the cord, are always and from the beginning under the domination of the upper or cephalic end of the neural tube.
This cephalic end rapidly expands. Its growth is faster than the rest of the neural tube and from its walls, by proliferation, develop the structures of the cerebrum, mid-brain, and hind-brain. It also gives off ganglionic masses from which grow sensor processes to form the afferent elements of the cranial nerves and contains, like the cord, motor nuclei, or nidi, from which motor axons grow toward the periphery to come into relation with definitely predetermined organs.
The Spinal Column and Cranium
Now appear the primitive cartilaginous and membranous elements from which a bony wall is to be built around the central nerve axis, primitive vertebrae, the upper known as cranial and numbering four, and the lower, or spinal, numbering usually thirty-three. These bone structures develop around the brain and spinal cord. Later the cephalic vertebrae fuse into a solid vault, the cranium, completely enclosed except for various foramina for the passage of spinal cord, nerves, and blood-vessels. The succeeding twenty-four vertebrae remain separate and movable upon each other and leave between them the openings for the emergence of the spinal nerves. The last nine segments fuse eventually into two immovable or false vertebrae called Sacrum and Coccyx. These latter also contain foramina from which nerves issue.