890 REPORT—1899. 
Regeneration, Specific Nerve Energy.—One other problem presented by this 
involuntary system we may say a few words about. You know that when a nerve 
in the hand or arm is cut the nerve will in proper conditions grow again; and 
the lost feeling and the lost power over the muscles will return, The recovery is 
brought about by the part of the nerve which is attached to the spinal cord growing 
along its old track and spreading out as before in the muscle, skin, and other tissue. 
At any rate that is the method for which there is most evidence. You may know 
also that when the nerve-fibresin the spinal cord are similarly injured, they do not 
recover function. Regeneration in the latter case implies that the nerve-fibres 
have to form fresh endings in connection with nerve-cells, If this were more 
difficult than the formation of nerve-endings in muscle and other non-nervous 
tissues, the difference which exists as regards recovery of function between the 
nerve-fibres of the limb, and nerve-fibres of the spinal cord, would be readily ex- 
plainable. But recent experiments show that the nerve-fibres which run from the 
spinal cord to the peripheral ganglia—z.e. pre-ganglionic fibres—re-form with ease their 
connection with nerve-cells, so that we may probably seek in mechanical condi- 
tions for the reason of the absence of regeneration of the fibres in the spinal cord. 
Possibly some way may be found of improving the mechanical conditions, and so 
obtaining regeneration. That question, however, we need not enter into. 
The regeneration of the pre-ganglionic nerves presents some very remarkable 
features. The nerve-fibres which end in a sympathetic ganglion are rarely, if ever, 
all of one kind—that is to say, they do not all produce the same effects. Thus, of 
those which run to the ganglion in the upper part of the neck, some cause the eye- 
lids to move apart, some cause the pupil to dilate, some cause the face to become 
pale, some cause the glands of the mouth or skin to secrete, and others have other 
effects. These different kinds of nerve-fibres run, in large part at any rate, to 
different nerve-cells in the ganglion. There are in the ganglion several thousands 
of nerve-cells closely packed together. And it would seem hopeless for each kind 
of nerve-fibre as it grows again into the ganglion during regeneration to find its 
proper kind of nerve-cell. Nevertheless, nearly all of them succeed in doing this. 
The nerve-fibres which normally cause separation of the eyelids, or dilatation of 
the pupil, or pallor of the face, or secretion from the glands, produce the same 
effects after several inches of their peripheral ends have formed anew. 
The fact offers at first sight a striking proof of a specific difference between the 
different classes of nerve-fibres and different classes of nerve-cells. Through the 
matted mass formed by the delicate interlacing arms of the nerve-cells, the 
ingrowing fibres pursue their tortuous course, passing between and about hundreds 
of near relations until they find their immediate stock, whom they clasp with a 
spray of greeting tendrils and so come to rest. 
Absolute laws seem unfitted for a workaday world, For closer observation 
shows that the fibres have not always this marked preference for their own stock. 
The nerve-fibres of the cervical sympathetic, the nerve I have spoken of above, do 
not often go astray, at any rate so far as is known. But they do sometimes; thus 
it may happen that some nerve-fibres which ought to find their home with nerve- 
cells governing the blood-vessels, take up with nerve-cells governing the dilator 
structures of the pupil. 
And if we turn to other nerves, greater aberrations are found. We have seen 
that the nerves running from the central nervous system to involuntary structures 
may be divided into two sets: the sympathetic nerves on the one hand, and the 
cranial and sacral nerves on the other. An important cranial nerve is the vagus; 
it causes, when in action, cessation of the heart-beat, contraction of the cesophagus, 
contraction or inhibition of the stomach, and various other effects. It does not 
send nerve-fibres to any of those structures of the head which we have seen the 
sympathetic ganglion at the top of the neck—the superior cervical ganglion—so 
liberally supplies. And yet the vagus nerve, if it has a proper opportunity of 
growing into the superior cervical ganglion, will do so, and there establish connec- 
tions with the nerve-cells. Thus the nerve which properly exercises control over 
certain viscera in the thorax and abdomen is capable of exercising control over 
structures in the head, such as the iris, the blood-vessels, and the glands. The 
