32 COMPARATIVE ANATOMY. 



membrane (the sarcolemina), and contains several nuclei, with 

 remnants of protoplasm along its axis ; or the contractile substance 

 forms a solid cylinder, on the surface of which, and immediately 

 below the sarcoleniina, are the nuclei with the remains of the proto- 

 plasm. 



Further, there are two varieties of this form of muscle-tissue 

 in which the contractile substance is respectively more homogeneous 

 or more heterogeneous. If more homogeneous the fibres resemble 

 the so-called smooth fibrous cells, from which indeed they differ only 

 by the fact that they do not correspond to a single cell, but to a 

 multiple of cells, as is clear from the number of nuclei appertaining 

 to the fibre. In the other condition, owing to the differentiation 

 of the contractile substance, they resemble the second form of 

 simple muscular fibres, and, like them, are transversely striped. 

 These also correspond to a number of cells although they are 

 derived from a single cell, and owe their elongation to its growth. 

 The reaction to stimulus is, in transversely-striped fibres, rapid. 



Nervous Tissue. 

 § 27. 



Nervous tissue appears (as has been already explained) at the 

 same time as muscular tissue in the Animal Kingdom, and is 

 distinguished by its functions, even in its lower conditions, from 

 other tissues. It receives and passes on stimuli, converts them into 

 sensations, and produces voluntary excitations. Two conditions are 

 to be distinguished in the morphological characters of the elementary 

 parts, nerve-fibres and nerve-cells. The former are mostly present 

 in the peripheral portion of the nervous system, and are the con- 

 ducting organs, while the latter form the central elements. 



1) Nerve-fibres have not always the same relations, and their 

 different conditions are to be regarded as stages of differentiation. 



a) In their simplest form they are elongated homogeneous 

 band-like bundles composed of fibres which are so slightly separated 

 from one another that they appear to be merely striated. For the 

 majority of Invertebrata the relation of nerve-trunks of this kind 

 and their branches to the histological form-elements is not thoroughly 

 made out ; and even the question whether the numerous striations 

 of the nerve-trunks are to be regarded as the indication of their 

 being composed of separate fibres, is an open one. The presence of 

 nuclei in their structures is the sole fact which points to their 

 relation to cells. In other cases fibres united into bundles may be 

 distinguished as individual elements of structure. The fibre consists 

 of an apparently homogeneous substance which is limited super- 

 ficially by a fine membrane, beneath which are the nuclei. Remains 

 of protoplasm may be at times made out around the nucleus, which 

 shows that the rest of the fibre is a differentiated substance. The 



