40 THE ELEMENTS OF STRUCTURE. 



the connections between sensory cells and ganglionic cells, and between 

 the latter and muscles, are represented by well-developed nerves. 



So far as we know, nervous tissue always arises from the outer or 

 ectodermic layer of the embryo, as we would expect from the fact that 

 this is the layer which, in the course of history, has been most directly 

 subjected to external stimulus. 



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 efferent nerve fibre. Most, how- 

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

 they consist in great part of a network or coil of fine fibrils, amid which 

 lies the usual cell kernel or nucleus. Ganglionic cells, aggregated to 

 form ganglia, generally lie embedded in a fibrous cellular substance called 

 neuroglia, usually regarded as an ensheathing and supporting material. 



In all but a few of the simplest Metazoa, the nerve fibres are sur- 

 rounded by a sheath called the neurilemma, 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 unessential medullary sheath. But 

 even in the higher Vertebrates, " non-medullated " or simply contoured 

 nerve fibres are found in the sympathetic and olfactory nerves, and this 

 simpler type alone occurs in hag, lamprey, and lancelet, as well as in all 

 the Invertebrates with distinct nerves. Furthermore, nerves are usually 

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

 or else by neuroglia. 



A nerve fibre consists of numerous fibrils like those seen within a 

 ganglion cell. These are regarded by some as the essential elements in 

 conducting stimuli, while others maintain 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. 



According to some authorities, the nerve fibres are extensive pro- 

 longations of the ganglion cells ; according to others the neuroglia or 

 other ensheathing elements contribute to the extension of the nerve 

 fibres, or rather special neuroblast cells make both sheath and fibre. 



IV. CELLS. 



In discussing tissues, it was necessary to refer to the 

 component cells. Let us now consider the chief charac- 

 teristics of these elements. 



A cell is a unit mass of living matter. Most of the 

 simplest animals and plants (Protozoa and Protophyta) are 

 single cells ; eggs and male elements are single cells ; in 

 multicellular organisms the cells are combined into tissues 

 and organs. 



