General Construction and Development of the Nervous System

General Construction and Development of the Nervous SystemAs animals ascend in the scale of organisation from the simplest types they assume a bilateral symmetry which, however, is seldom rigidly adhered to. Speaking generally, the external protective and supporting (somatic) structures are similar on the right and left sides of the body; the internal (visceral organs of nutrition, etc., rarely show complete symmetry. In addition, as as strikingly seen in worms, the animal is built up of a series of transverse discs or segments, many of which are similar to one another; each contains the organs essential to life, and, in some animals, is capable of independent excistence. In the higher forms bilateral symmetry and transverse segmentation are still evident except in the visceral system; in the head the transverse segmentation is very obscure.

These factors influence the form which the nervous system ultimately. To meet the requirements of the transverse segmentation of the body the nervous system is also segmented, i.e. each body segment contains its own group of neurons. For facility of inter-communication and for protection, the cell bodies of the neurons are grouped together into ganglia and are withdrawn from the surface. Each segment, being bilaterally symmetric contains a right and a left ganglionic mass. In many circumstances it is advantageous for the segment to respond as a whole to external stimuli; this is effected in the first instance by cross communications (commissures) between the ganglionic masses, and at a later stage of evolution by the fusion of the two masses into one.

Similarly, when the body segments join together and lose their individuality, these ganglionic masses also fuse, and a long median column of nerve cells and fibres results. From this column nerve fibres come off in bundles corresponding to the original segments. Primitively each segment is capable of only simple reflex actions. As the segments combine the reflexes become more intricate and each increase in intricacy renders more necessary the establishment of a common co-ordinating nerve centre. The head end of the nervous column takes on this complicated function and is differentiated as the brain. Another factor in increasing the complication of the brain is the great concurrent increase in development of the special sense organs, the impulses from which have all to be co-ordinated and transferred to the proper responsive mechanisms.