NERVOUS SYSTEMS, PERIPHERAL AND CENTRAL 473 



muscle ; a considerable number of smooth muscle fibres are required to be in 

 action at the same time. But, even if they exist, it is obvious that such networks 

 are not the same thing as the nerve centres of the lower invertebrates. 



An exception must, perhaps, be made in the case of Auerbach's plexus in the 

 alimentary canal. This does actually possess reflex functions as we have seen 

 (page 367), and ganglion cells are readily to be detected in it. This system, 

 however, seems to possess the properties of a synaptic system, such as excitation 

 and inhibition in definite directions, rather than the indifference of the typical 

 network. 



It is of interest in this connection that Meek (1911) has found that the plexus connections 

 are regenerated in about four months after t^ansection of the intestine. The muscular and 

 epithelial regeneration is completed considerably earlier than the capacity of transmitting a 

 wave of inhibition, thus giving additional evidence that the " my enteric reflex" is of nervous 

 origin. 



Hofmann (1907), as the result of extensive and detailed investigation of certain 

 peripheral networks, especially those in the heart muscle and those innervating the 

 chromatophores of the Cephalopods, comes to the following conclusions, which he 

 finds to agree with the observations made by other workers, with regard to the 

 smooth muscle of vertebrates in general. The nerve bundles supplying such 

 muscular structures form a primary plexus by branching and by division of the 

 coarse nerve fibres contained in them. This plexus is, as regards the direction of 

 its fibres, independent of that of the muscular strands, and often passes transversely 

 across them. These fibres do not anastomose with each other. In fact, Hofmann 

 had previously shown (1904) that each nerve fibre has its own area of distribution 

 and that there is no spreading of excitation, such as we meet with in the Medusae. 

 From this plexus, again, fibres are given off which form another, " terminal," plexus, 

 whose single fibres run close beside the muscle cells, not forming definite " endings " 

 in or upon them, but finally looping backwards and joining other branches of the 

 same fibre, or perhaps other fibres of the same nerve, although it was not possible 

 to decide whether the latter was really the case and whether a continuous network 

 of the branches of the same nerve bundle may exist. The possibility of fine short 

 fibrils passing to the muscle substance from the loops is not excluded by these 

 results and it does not seem unlikely that there is some further connection between 

 the nerve fibre and the muscle cell than the mere contiguity of the loops. There 

 is nothing of the nature of ganglion cells present in these plexuses. What have 

 been taken for them can be seen in good preparations to be separate from the nerve 

 fibres and appear to be nuclei of connective tissue sheaths to the nerves. 



The importance of these facts with respect to the heart will be seen later. 



Peripheral networks of the kind described above clearly serve only for conduc- 

 tion and not for origination of nervous impulses, nor as reflex centres. 



Owing to the fact that nerve fibres conduct in both directions, it will be seen 

 that, if one branch of a nerve fibre, which has divided, is put into excitation by 

 some means, the impulses will spread over all the other branches of the same fibre 

 and to any other fibres with which these may be connected in a network. We 

 have, in such a case, an " axone reflex" as defined by Langley (see page 425). 

 Excitation must spread to any effector cells supplied by any of the fibres. I refer 

 to this fact here on account of certain curious phenomena connected with the 

 vaso-dilator mechanism of the skin. When the peripheral ends of dorsal roots are 

 stimulated, the region which has its sensory innervation in the particular root 

 stimulated shows that its arterioles have dilated. I was able to show (1901, 2) 

 that the nerve fibres concerned have the same kind of anatomical connections, and 

 show the same degeneration relations, as the ordinary afferent fibres. It is well 

 known that certain substances, such as mustard, placed on the skin, produce 

 inflammation there. Spiess (1906) observed that such inflammation does not occur 

 if the area of skin is anaesthetised, as by cocaine, and thought that the inflammatory 

 process was brought about by a reflex from the spinal cord to vaso-dilator nerves. 

 Ninian Bruce (1910), however, found that the inflammation was still produced 

 after mere section of the dorsal roots, so that it could not be a spinal reflex. It 

 was not present, on the other hand, if sufficient time had been allowed for the 



