NERVOUS TISSUE. 37 



and how they gradually become less superficial, we shall see in our 

 systematic studies. 



Structure. — Let us consider first the ganglionic cells which receive 

 stimuli and shunt them, which regulate the whole life of the organism, 

 and are the physical conditions of "spontaneous" activity and in- 

 telligence. The simplest are prolonged at one pole into an outgrowth 

 which branches into an afferent and eiferent nerve-fibre. Most, how- 

 ever, give off outgrowths from two poles or on all sides. Internally 

 they exhibit a kernel or nucleus, and they consist in great part of a 

 network or coil of fine fibrils. Within ganglia the ganglionic cells 

 usually lie embedded in a fibrous cellular substance called neuroglia, 

 which most histologists regard as an ensheathing and supporting material. 



In all but a few of the simplest multicellular animals, the nerve-fibres 

 are surrounded by a sheath called the neurilemma, which is said to be 

 formed by adjacent connective tissue. Several nerve-fibres may combine 

 to form a nerve, but each still remains ensheathed in its neurilemma. 

 In vertebrate animals, each nerve-fibre usually consists of an internal 

 "axis cylinder," the important part, and an external unessentialmeduUary 

 sheath whose texture suggests fat. But even in the higher Vertebrates, 

 ' ' non-meduUated " or simply-contoured nerve-fibres are found in the 

 sympathetic and olfactory nerves, and this simpler type alone occurs in 

 hagfish, lamprey, and lancelet, as well as in all the Invertebrates with 

 distinct nerves. Furthermore, it should be noted that nerves are usually 

 surrounded by an enveloping nucleated layer called Schwann's sheath, 

 or else by neuroglia. 



Careful preparation of a nerve-fibre shows that it consists of numerous 

 fibrils like those seen within a ganglion-cell. These are usually regarded 

 as the essential elements in conducting impressions, but some maintain, 

 whether rightly or wrongly I am not able to judge, that the essential 

 part is the less compact, sometimes well-nigh fluid stuff between the 

 fibrils, or that the fibrils are but the walls of tubes within which the 

 essentially nervous stuff lies. As in other cases, the microscopic 

 morphologists discover intricacies in regard to the import of which 

 physiological conclusions are hardly possible. 



But you may reasonably ask what these nerve-fibres are. I do not 

 think that any one can at present give a decisive answer. According to 

 most authorities, they are extensive prolongations of the ganglion-cells, 

 and there is no doubt that the nerves of Vertebrates grow from the 

 central system outwards. But to others it seems plausible that the 

 neuroglia or other ensheathing elements contribute to the extension of 

 the nerve-fibres, or rather that special cells make both sheath and fibre. 

 It is possible that both theories are right. 



