382 BIOLOGY. [BOOK vi. 



The superior mammifer is a sort of summary of the entire king- 

 dom. In him are combined all the tissues, all the apparatus 

 scattered through the entire series : he has a special nervous 

 system, but he possesses nevertheless a portion of the ganglionary 

 system of the invertebrates, and in him as in them this gangli- 

 onary system is constituted especially by gelatiniform fibres. 



In effect the nervous fibre with double contour is not common 

 among the invertebrates. It is absolutely lacking in the inferior 

 invertebrates : it appears, but it is still rare, in the arthropods. 

 Even the nervous system of the inferior vertebrates, the cyclo- 

 stomes, is constituted especially by granulous fibres, enveloped 

 with a delicate sheath and destitute of oily covering. In this 

 case, as in many others, is verified the current saying : Natura 

 non facit saltus. 



There is 110 sudden bound, no hiatus, between the grey nervous 

 fibres and the nervous fibres with large diameter. The connecting 

 link is formed by the thin nervous tubes. These are nervous fibres 

 with double contour, of a diameter much smaller than that of 

 the other fibres. Some of them moreover seem to appertain to 

 motricity, others to sensibility : for certain of them are provided 

 with a cellular expansion, which is also of small size. The thin 

 nervous tubes are met with to some extent everywhere in the 

 nervous system : but they abound especially in the great sym- 

 pathetic. This authorises us to consider them as the first stadium 

 of perfectionrnent of the gelatiniform fibres. 



The complete fibres despoil themselves habitually of their 

 accessory parts, in their terminal portions, and reduced then to 

 their axile threads, they have a considerable affinity with the 

 gelatiniform fibres. 



According to recent researches of Dr. Luys, the nervous cells 

 and even their nuclei appear, when much magnified under the 

 microscope, to turn themselves into intricate fibrils (Fig. 67). 

 But the experiments of Dr. Luys having been generally made 

 on specimens hardened by chromic acid, it is possible that this 

 fibrillary severance is merely the result of the chemical agent, 



