CHAPTER XII. 



SUMMARY. 



IN the previous chapters I have taken separately and examined 

 in detail the various systems of involuntary muscles, which occur 

 in vertebrates, and also the various nervous groups or systems 

 which are connected with them. In this final chapter I will 

 attempt to shortly put together all these systems and give my 

 present views on their inter-relationships and possible evolution. 

 The unstriped muscles of vertebrate animals belong to certain 

 definite groups characterized by their innervation and by response 

 to certain substances formed naturally in the body. I recognize 

 the following groups of muscles : 



1. A vascular group. 



2. A group of muscles underlying the skin or epidermis, 

 which I have called the dermal or ectodermal musculature. 



3. A group of muscles underlying the surface of the gut or 

 endoderm, which I have called the endodermal musculature. 



4. A group of muscles around the segmental duct. 



5. A group of muscles forming part of the gut walls which 

 especially constitute the system of sphincter muscles. 



6. A group of muscles connected with the adjustment of 

 vision. 



The motor nerves of all these muscles arise from nerve cells 

 which are situated, not in the central nervous system, as in the 

 case of all striated muscles, but in the periphery. Further taking 

 the first five groups these can be arranged definitely into two 

 groups : i , those which contract in the presence of a very 

 minute amount of adrenaline, and 2, those which contract in the 

 presence of a still smaller amount of acetyl-choline. The groups 

 I, 2, 4, 5, are all united into one group, which may be called the 

 adrenaline group, while 3, the endodermal musculature, forms 

 alone the acetyl-choline group. The striking action of these two 

 substances, adrenalin and acetyl-choline, is correlated with the 



150 



