52 COMPARATIVE ANATOMY chap. 



fibres : (1) Sensory nerve fibres, which transmit sensory impressions 

 from the peripheral sensory cells to the central organ; (2) Motor 

 nerve fibres, which transmit stimuli from the central organ to the 

 muscles. The majority of sensory cells also do not remain in their 

 undifferentiated condition. Division of labour steps in here also. 

 Some cells seem specially suited for the reception of light and colour 

 sensations, others for those of sound, others again for sensations of 

 smell and taste. Tactile cells still remain in the most undifferentiated 

 condition. Many sensory cells which are qualified to receive one and 

 the same class of stimuli become, by the addition of accessory tissues, 

 combined into complicated sensory apparati — the specific sensory 

 organs : the organs of sight, hearing, smell, taste, and touch. 



The ganglion cells possess one or more processes (unipolar, bi-polar, 

 multi-polar ganglion cells), one or more of which pass over into the 

 nerve fibres, while others only serve perhaps for the nourishment of 

 the cells. The process of a unipolar ganglion cell sooner or later 

 divides into at least two branches, one motor and the other sensory. 

 The ganglion cells in peripheral ganglion centres are often enclosed 

 in envelopes of connective tissue. 



Nerve fibres may have many branches, and are often finely 

 striated in a longitudinal direction. They are either naked — in some 

 of the lowest Metazoa — or enveloped in a sheath, the neurilemma, 

 which is supplied by the surrounding connective tissue. When many 

 nerve fibres form one nerve, the single fibres of this nerve are, among 

 all the higher animals, kept apart from one another by this sheath ; 

 the whole then in transverse section produces the effect of a spongy 

 tissue, in whose larger or smaller meshes the transverse sections of 

 the nerve fibres lie. The neurilemma is not generally continued on 

 to the ganglion cells. 



A further distinction between two sorts of nerves has been made, 

 especially among the Vertebrata : (1) nerves without medulla, which 

 remain simple ; and (2) nerves containing meduUa, in whose fibres 

 two parts are found — an outer oleagenous tubular medullary sheath, and 

 a fibre surrounded by this — the axis-cylinder. On entering a ganglion 

 cell, the latter alone penetrates its process — it alone represents the path 

 of transmission. Both these sorts of nerve fibre are enclosed in neuri- 

 lemma sheaths. 



In many of the lower animals the nervous system remains for the 

 most part in its original place of formation, i.e. in the body epithelium. 

 In the higher animals the nervous system remains in connection with 

 the body epithelium through the sensory apparatus. This helps us to 

 understand why, in the embryonic development of the highest animals, 

 the nervous system is always produced by the outer epithelial layer. 



Literature. 



Th. Schwann. MiTciosJcopische Untersucliungen iiber die Uebereinstimimmg in der 

 Structur imd dem Wachsthum der Thiere und PJlanzen. Berlin, 1839. 



