XnANSACTIONS OF SECTION D. DEPT. ANATOMY AND PHYSIOLOGY. 641 



What were the steps by wliicli tliis remarkable process took place ? How has 

 it come about that there are nerves passing from the central nervous system to all 

 parts of the skin, and also to the muscles ? How have the arrangements for reflex 

 actions arisen by which stimuli received on the surface of the body are carried to 

 the central part of the nervous system, and are thence transmitted to the appropriate 

 muscles, and cause them to contract ? All these questions require to be answered 

 before we can be said to possess a satisfactory knowledge of the origin of the 

 nervous system. As yet, however, the knowledge of these points derived from 

 embryology is imperfect, although there is every hope that further investigation 

 will render it less so ? Fortunately, however, a study of comparative anatomy, 

 especially that of the Coelenterata, hils up some of the gaps left from our study of 

 embryology. 



From embryology we learn that the ganglion-cells of the central part of the 

 nervous system are originally derived from the simple undifferentiated epithelial 

 cells of the surface of the body. We further learn that the nerves are out-growths 

 of the central nervous system. It was supposed till quite recently that the nerves 

 in Vertebrates were derived from parts of the middle germinal layer or mesoblast, 

 and that they only became secondarily connected with the central nervous system. 

 This is now known not to be the case, but the nerves are formed as processes 

 growing out from the central part of the nervous system. 



Another important fact shown by embryology is that the central nervous system, 

 and percipient portion of the organs of special sense, are often formed from the 

 same part of the primitive epidermis. Thus, in ourselves and in other vertebrate 

 animals the sensitive part of the eye, known as the retina, is formed from two 

 lateral lobes of the front part of the primitive brain. The crystalline lens and 

 cornea of the eye are, however, subsequently formed from the skin. 



The same is true for the peculiar compound eyes of crabs or Crustacea. The 

 most important part of the central nervous system of these animals is the 

 supraoesophageal ganglia, often known as the brain, and these are formed in the 

 embryo from two thickened patches of the skin at the front end of the body. These 

 thickened patches become gradually detached from the surface, remaining covered 

 over by a layer of skin. They then constitute the supracesophageal ganglia ; but 

 they form not only the ganglia, but also the rhabdons or retinal elements of the 

 eye — the parts in fact which correspond to the rods and cones in our own retina. 

 The layer of epidermis or skin which lies immediately above the supraoesophageal 

 ganglia becomes gradually converted into the refractive media of the crustacean eye. 

 A cuticle which lies on its surface forms the peculiar facets on the surface of the 

 eye, which are known as the corneal lenses, while the cells of the epidermis give 

 rise to lens-like bodies known as the crystalline cones. 



It would be easy to quote further instances of the same kind, but I trust that 

 the two which I have given will be sufficient to show the kind of relation which 

 often exists between the organs of special sense, especially those of vision, and 

 the central nervous system. It might have been anticipated djmori that organs 

 of special sense would only appear in animals provided with a well-developed 

 central nervous system. This, however, is not the case. Special cells, with long 

 delicate hairs, which are imdoubtedly highly sensitive structures, are present in 

 animals in which as yet nothing has been found which could be called a central 

 nervous system ; and there is ever}' reason to think that the organs of special sense 

 originated ;;wj jjrtSi'ii with the central nervous system. It is probable that in the 

 simplest organisms the whole body is sensitive to light, but that with the ap- 

 pearance of pigment-cells in certain parts of the body, the sensitiveness to light 

 became localised to the areas where the pigment-cells were present. Since, how- 

 ever, it was necessary that stimuli received in such areas should be communicated 

 to other parts of the body, some of the epidermic cells in the neighbourhood of the 

 pigment-spots, which were at first only sensitive in the same manner as other cells 

 of the epidermis, became gradually differentiated into special nerve-cells. As to 

 the details of this differentiation, embryology does not as yet throw any great 

 light ; but from the study of comparative anatomy there are grounds for thinking 

 that it was somewhat as follows : —Cells placed on the surface sent protoplasmic 

 1880. T T 



