CENTBAL OKGAN AND PERIPHERAL NERVES (PHYSIOLOGICAL). 33 



(phylogenetic), and of other parts which, only by use during the person's 

 life, derive their relationships (ontogenetic). 



Congenital mechanisms are found in all parts of the nervous system, 

 but observation of movements of embryos and infants shows that, at least 

 in respect to the nervous apparatus connected with the vegetative functions, 

 as in the sympathetic and in the large territory of the spinal cord and the 

 bulb, such mechanisms predominate. Probably to these may be added a 

 large part of the midbrain and cerebellum. Comparative anatomy teaches 

 that, up to the higher mammals, the apparatus lying anterior to these parts 

 are capable of still greater variations; and observation of the cerebral cortex 

 in its particular development specially shows that here, in cases of indi- 

 viduals, new paths may, by practice, become fixed. 



In so far as motor phenomena are considered as the result of irritations, 

 the necessary apparatus is called the movement-complex. This word was 

 coined by Exner, to whom we are indebted for an excellent review of many 

 of the related facts. One should not imagine, however, such collections of 

 ganglion-cells as entirely simple. The majority of movements require some 

 time for their execution, during which numerous other muscles may come 

 into play besides those first concerned. There must, therefore, be paths 

 leading from one collection of cells to another, the latter group being 

 affected by the irritation only when the action of the former is ended. 



Such processes are known (Exner) as successive movement-complexes. 

 Exner determined from physiological observations that, when one searches 

 carefully the nervous system of the invertebrates, one may easily find 

 anatomical series, which, once meeting with an irritation, may discharge 

 successive movements in perfect order. Especially the nervous system of 

 annulata^ — for example, the earth-worm, which we understand well since 

 the excellent researches of Eetzius — shows how, from afferent sensory fibers, 

 first a single motor cell-group is set in action, and then, through the proc- 

 esses of large association-cells, the impulse may be transmitted to the next 

 ganglion (Fig. 12). Besides, every ganglion contains other motor cells 

 whose neuraxons do not connect with the nerves of the corresponding meta- 

 mere, but which end in muscles that are anterior or posterior to this. So an 

 impression, which is received by an animal in any part of its body-surface, 

 may bring into action first the muscles of that part, and then also those of 

 metameres lying anterior or posterior to it. When such a successive move- 

 ment has once begun, another element enters in to regulate it. With the 

 changing position of the muscles and limbs, change also the sensory im- 

 pressions received by them. Consider the above-mentioned earth-worm. 

 The muscles of the first metamere contract frOm irritation of the apparatus 

 of touch; perhaps, too, those of the second. But by reason of these con- 

 tractions other portions of the integument come into contact with the 



