THE PHENOMENON OF CONDUCTION. 79 



The fact that lecithin and cholesterin usually occur together 

 has suggested that they have some physiological connection. It 

 has been supposed, for example, that they act as a check upon each 

 other. Lecithin under certain conditions favors hemolysis of red 

 corpuscles, or the action of lipase on fat, while cholesterin inhibits 

 both of these activities. No application of this antagonistic rela- 

 tionship is possible at present in the case of the myelin sheath. 



Cerebrosides or Cerebrogalaclosides. This name is given to a 

 group of bodies containing nitrogen, but no phosphorus. In the 

 myelin they are found in connection with and possibly in com- 

 bination with the lecithin. They belong to the group of glucosides, 

 that is, on hydrolytic decomposition they give rise to a carbo- 

 hydrate group, in this case galactose. Fatty acids and a nitrogenous 

 base also result from this decomposition. The cerebroside material 

 obtained from the white matter has been named specifically 

 cerebrin or phrenosin, but little is known of its exact structure. 



Union of Nerve Fibers into Nerves or Nerve Trunks. The 

 assembling of nerve fibers into larger or smaller nerve trunks re- 

 sembles histologically the combination of muscle fibers to form a 

 muscle. Physiologically, however, there is no similarity. The 

 various fibers in a muscle act together in a co-ordinated way as a 

 physiological unit. On the other hand, the hundreds or thou- 

 sands of nerve fibers found in a nerve may form groups which are 

 entirely independent in their physiological activity. In the 

 vagus nerve, for instance, we have nerve fibers running side by 

 side, some of which supply the heart, some the muscles of the 

 larynx, some the muscles of the stomach or intestines, some the 

 glands of the stomach or pancreas, and so on. Nerves are, 

 therefore, anatomical units simply, containing groups of fibers 

 which have very different activities and which may function 

 entirely independently of one another. As a nerve-trunk is con- 

 stituted it consists chiefly of the connective tissue binding the 

 fibers together. It is estimated (Ellison) that in the median nerve 

 the connective tissue forms 63 per cent, of the whole trunk, while 

 myelin sheaths make up 28 per cent., and the axis cylinders only 

 9 per cent. 



Afferent and Efferent Nerve Fibers. The older physiologists 

 believed that one and the same nerve or nerve fiber might conduct 

 sensory impulses toward the central nervous system or motor im- 

 pulses from the central nervous system to the periphery. Bell and 

 Magendie succeeded in establishing the great truth that a nerve 

 fiber cannot be both motor and sensory. Since their time it has 

 been recognized that we must divide the nerve fibers connected 

 with the central nervous system into two great groups : the efferent 

 fibers, which carry impulses outwardly from the nervous system 



