MUSCULAR MOTION. 129 



means essential to the continuance of the secretion, or to complete 

 chymification, (Todd and Bowman.) 



The sphial accessory is so called in consequence of its extensive 

 connexion with the upper part of the spinal cord. Of its two 

 branches, the i7iter7ial is probably sensitive. The external is dis- 

 tributed to the sterno-cleido-mastoideus and trapezius muscles, some 

 of its filaments inosculating with the cervical plexus, and is motor in 

 its functions. The movements excited by irritating it, are of a direct, 

 and not of a reflex character. 



MUSCULAR MOTION. 



The muscular system forms part of the apparatus of animal life, 

 inasmuch as it is the instrument by which nervous energy operates 

 upon external objects. The contractility which it manifests on the 

 application of a stimulus, is an endowment derived from its own 

 structure, and not from the nervous system. The presence of this 

 contractility is connected with the healthy nutrition of the tissue, and 

 with its due supply of arterial blood : the separation of any muscular 

 part from its nervous connexions has none but an indirect influence 

 on its properties. 



The development of muscles, their physical and vital properties, 

 their mechanical adaptation as moving agents, having been already 

 described, (pages 36, 37,) but little remains to be added. 



There is one form o^ contractility which produces a constant ten- 

 dency to contraction in the muscular fibre, but which difl?ers from 

 simple elasticity from the fact that it exists after death and before de- 

 composition takes place. This is called tonicity, and it manifests 

 itself in the separation or retraction that takes place between the 

 divided ends of a living muscle, as in amputation. It also shows 

 itself in the permanent flexure of the joints when the extensors are 

 paralysed, as in lead palsy. In the healthy state the tonicity of the 

 several groups of muscles is counterpoised, but the balance is de- 

 .stroyed when the tonicity of one set is lost or impaired. 



The tonicity is greater in the non-striated than in the striated ; it 

 is this property that keeps the walls of the arteries contracted upon 

 their contents. It is increased by cold, and diminished by heat. 

 The rigor tnortis, as already described, is probably to be regarded 

 as a manifestation of this property. 



The energy of muscular contractions is seen in the various feats 

 of strength performed by jugglers, &c. It is very much increased 

 by continued exercise. Much, however, depends upon the mechani- 

 cally advantageous application of the power; persons of ordinary 

 strength may in this way perform feats that would seem incredible. 



An idea of the rapidity of muscular contractions may be had by 

 estimating the number of letters that can be pronounced in a given 

 time. Some persons can pronounce 1500 in a minute; each of 



