SECTION I. 

 Physiology of Movement. 



1. The Contractile Tissues. — It was stated that the function of 

 contractility represents one of the fundamental properties of protoplasm, 

 and in the simplest forms of life, consisting of undifferentiated proto- 

 plasm, as in the structureless amoeba, the contractility of protoplasm 

 renders locomotion possible. The first attempt at localization of this 

 function of contractility, in the general specialization of function in the 

 development of the animal kingdom, was noted in the development of 

 protoplasmic prolongations of cells, so-called cilia, which in numerous 

 infusoria constitute organs of locomotion ; in various shell-fish, organs 

 for aiding the prehension of food and the functions of respiration, and in 

 the higher animals for producing motion of particles brought in contact 

 with them. The first attempt, therefore, at the development of organs 

 in which the function of contractility is specialized is seen in the develop- 

 ment of vibratile cilia. In a step higher in the progress of specialization, 

 contractility reaches its highest degree in the muscular tissue, which may 

 he regarded as a mass of protoplasm inclosed within a cj'linclrical or 

 polygonal cell. In the higher animals each of these three different 

 representations of contractile substances are met with : undifferentiated 

 protoplasm, as found in the lymph-corpuscles, white blood-corpuscles, 

 connective-tissue corpuscles, mucus- and pus-cells ; ciliated cells, lining 

 various mucous cavities in the body ; and muscular tissues of the striped 

 and unstriped varieties. 



The characteristics of motion as occurring in undifferentiated proto- 

 plasm and ciliated cells have already been studied. The conditions of 

 muscular contractility now deserve attention. In muscular tissue the 

 contractile substance is inclosed in a tubular sheath, constituting a mus- 

 cular fibre. Muscular fibres may be either striped or voluntary fibres, or 

 unstriped or involuntary fibres. The striped or voluntary muscles, which 

 have a red appearance, constitute the great mass of contractile tissues 

 of the body. They are ordinarily connected with the bones, and are 

 therefore spoken of as skeletal muscles; their contractions, as a rule, are 

 under the control of the will. Each muscle-fibre is more or less cylin- 

 drical, varies in length from one one-hundredth to one six-hundredth of 

 an inch, and consists of the sarcolemma, an elastic sheath, probably of the 

 nature of connective tissue, with transverse partitions which stretch 



(701) 



