398 MUSCULAR MOTION. 



Experiments, by M. Magendie, and by Dr. Kronenberg, 1 of Moscow, 

 have shown, that a portion of the fibres of the sensitive roots extends 

 to the point of union between them and the anterior roots, and is re- 

 flected to the anterior column of the spinal marrow; the return or 

 reflection of the fibres taking place near the junction of the two roots. 

 This arrangement of the fibres accounts for the fact, often noticed by 

 physiologists, that some degree of sensibility appears to be manifested, 

 in experiments on animals, when the motor roots of the nerves are 

 irritated. The sensibility of the portio dura, a motor nerve, has been 

 long known, and properly ascribed to its receiving filaments of the fifth 

 pair. Motions can be excited by irritating the posterior root, which 

 are owing to its connexion with the spinal cord. This irritation does 

 not act immediately upon the muscles through the trunk of the nerve, 

 which the posterior root contributes to form; but it excites a motor 

 impulse in the spinal cord, which is propagated through the anterior 

 roots to the periphery of the system. 



In the ordinary case of the action of a voluntary striped or striated 

 muscle, the nervous influence, emanating from some part of the cerebro- 

 spinal axis, under the guidance of volition, proceeds along the nerves 

 with the rapidity of lightning, and excites the muscle to contraction. 

 The muscle, which was before smooth, becomes rugous; the belly more 

 tumid; the ends approximate, and the whole organ is rendered thicker, 

 firmer, and shorter. The researches of Mr. Bowman 2 have shown, that 

 in the state of contraction the transverse striae, before described as ex- 

 isting in each fibre, approach each other; whilst its diameter is increased; 

 hence the solid parts are more closely approximated, and the fluid which 

 previously existed between them is pressed out so as to form bullae in 

 the sarcolemma, as represented in Fig. 166, from Mr. Bowman. The 



Fig. 166. 



Muscular Fibre of Dytiscus in contraction. (Bowman.) 



marginal representations, Fig. 167, of the muscular fibre of the skate, 

 at rest and in contraction, are also from Mr. Bowman. It is proper to 

 remark, however, that these representations are of muscular fibres 

 when in an unnatural condition, separated, that is, from the rest of the 

 economy, and it cannot be considered established, that contraction ex- 

 cited by the agency of the nerves is accomplished in precisely the same 

 manner. 



With regard to the degree of contraction or shortening, which a 

 muscle experiences, some difference of sentiment has prevailed. Ber- 



1 Muller's Archiv., Heft v. 1839. 



3 Art. Muscular Motion, in Cyclop, of Anat. and Physiol.,Part xxiv. p. 525, London, July, 

 1842; and Philosophical Transactions for 1840-1841. 



