244 SUMMARY. 



pigment in the membranes of the brain of sheep. Almost the whole nervous system of 

 the crocodile is enveloped in the same black membrane. This variation shows that as 

 the internal parts of the nerve are white, it is an adventitious production connected 

 with peculiarities of the animal. 



There is a great variation in the texture of the component parts of very numerous 

 animals, and as the nerves must be connected with them, so there is generally a 

 proportionate degree of firmness or softness. Many nerves have been constituted with 

 a view to the mechanical operations in which they must be involved. When all the 

 organs have a softer texture, the nerves generally partake of a like consistency, and 

 when a greater capability of resistance is required, it is produced by a stronger 

 neurilema or peculiar attachments of the cellular membrane. 



If nerves had been altogether passive, they must have been very long for their 

 adaptation to moving parts, and very inconveniently placed amongst them. As their 

 medullary portions will not bear either extension or pressure, they require a particular 

 construction. The medullary fibres have generally a tortuous appearance, whilst 

 surrounded by the neurilema in an animal recently dead, and become straight by 

 extension, and tortuous again by relaxation ; there is an elasticity of the neurilema, 

 but not in the same degree of the medullary fibres, the elasticity allows a nerve to be 

 extended with the surrounding structures and shortened with their relaxation, and thus 

 to become accommodated to the changes of position caused by the motions of the body. 

 In the jaguar there is a great elasticity of some nerves, particularly of those of the 

 pharynx and the sterno-hyoid and sterno-thyroid muscles ; it is very little in those of 

 the tongue or in the par vagum and accessory of the same animal. There is a 

 tortuous appearance of the numerous small nerves in the very extensible parts of the 

 snake ; it is produced by connecting processes of the surrounding cellular membrane, 

 which in returning to its contracted state after extension draws them into folds, and by 

 this contrivance they are easily elongated when the body is in motion, or distended 

 with food. The tortuous state of the medullary fibril, depending on the neurilema, and 

 that of the whole nerve on bands surrounding cellular membrane, answer similar 

 intentions. 



