68 NERVOUS TISSUE. [SECT. 31. 



the various products of decomposition mentioned above, and also 

 from the circumstance, that their functional activity is soon extin- 

 guished on the stoppage of the circulation of the blood in them. 

 Wounds of muscles never heal by means of transversely striated 

 muscular substance ; on the other hand, accidental formations of 

 this tissue are met with, although rarely. 



Transversely striped muscular tissue occurs in the following 

 parts : 



1. In the muscles of the trunk and of the extremities; in the 

 external muscles of the eye, and all the muscles of the ear. 



2. In the muscles of many viscera ; as the larynx, pharynx, the 

 tongue, and oesophagus (upper half), the end of the rectum 

 (sphincter externus, levator ani), the genitals (bulbo-ischio-caver- 

 nosus, urethralis communis transversus, transversus perincei, cre- 

 master, muscular fibres of the round ligaments of the uterus, in 

 part) . 



3. In certain parts of the vascular system, viz., in the heart, and 

 in the walls of the large veins where they open into it. 



Literature. — W. Bowman, article Muscle and Muscular Motion, in Cyclop. 

 of Anat. ; and On the Minute Structure of Voluntary Muscle, in Phil. Trans., 

 184.0, 1 841. J. Holst, De Structurd Musculorum in yencre et Annulatorum 

 3fusculis in specie. Dorp., 1846. M. Barry, Neue Unters. uher die schrauben- 

 fbrmige Bcschajfhiheit, der Elementarfasem der Muslteln, nehst Beohachtunyen 

 uber die muskulbse Natur der Flimmerhdrchen (Mull. Arch,, 1850, p. 529). 



4. Nervous Tissue. 



§31. The essential elements of the nervous tissue are of two 

 kinds, nerve-tubes and nerve-cells. The nerve-tubes, or nervous 

 primitive fibres, are either medullated or non-medullated. The 

 former consist of three parts : of a structureless delicate envelope, 

 sheath of the primitive tube ; of a soft but elastic fibre, situated in 

 the centre, the central or axial fibre (axis-cylinder, Purhinje ; pri- 

 mitive band, Remah); and of a viscid, diffluent white layer between 

 these two, the medullary sheath. In the non-medullated fibres, 

 which, in man, occur only in certain peripheral terminal expan- 

 sions (retina, organ of hearing, organ of smelling, cornea, Pacinian 

 bodies), the structureless envelope encloses nothing but a homo- 

 geneous or finely granular, clear substance, which appears to agree 

 with the axial fibre of the other tubes, or, at any rate, may be con- 

 sidered analogous to it, so that the medullary sheath is absent in 

 these fibres. The primitive nerve-fibres are of very various dimen- 

 sions, and, accordingly, they may be distinguished as fine, o - ooo5'" 



