92 APPAEATTJS OF MOTION. 



209. The muscles may be classified according as they are 

 more or less under the control of the will. The contractions 

 of some of them are entirely dependent on the will, as in the 

 muscles of the limbs which are used for locomotion. Others 

 are quite independent of it, like the contractions of the heart 

 and stomach. The muscles of respiration ordinarily act in- 

 dependently of the will, but are partially subject to it ; thus, 

 when we attempt to hold the breath, we arrest, for the mo- 

 ment, the action of the diaphragm. 



[ 210. The movements of animals are therefore divided 

 into VOLUNTARY and INVOLUNTARY ; the immediate agent of 

 the former is the muscular tissue, which is most intimately 

 associated with the nervous system, and is brought thereby 

 under the control of the will. The motions characterised as 

 involuntary, are for the most part effected by means of mus- 

 cular tissue ; but the fibres of the involuntary muscles present 

 histological characters, which distinguish them from that of 

 the voluntary class. The muscular tissue passes by insensible 

 gradations into other forms of contractile fibrous tissue, so 

 that it is difficult to define the limits between them. 



[ 211. Besides muscular movements, animals execute mo- 

 tions which appear to be altogether independent either of the 

 muscular or the nervous systems. These are called CILIARY 

 MOTIONS ; they are most extensively performed, and may be 

 best studied in the lowest classes of the invertebrata, although 

 they take place in connection with some of the organic func- 

 tions in all. 



[ 212. When studied by the aid of the microscope, with a 

 quarter of an inch object-glass, true muscular fibres present 

 two distinct histological forms. 1st. The simple unstreaked 

 fibrillee of organic life. 2nd. The compound streaked fibrillae 

 of animal life. 



[ 213. The first class consists of pale-coloured smooth 

 cylindrical fibres, arranged parallel to each other, and forming 

 bundles connected by a delicate cellular tissue. This class 

 is met with in the form of layers, investing the hollow 

 organs, as the stomach, intestines, and bladder ; it is likewise 

 found surrounding the excretory ducts of the larger glands, 

 and enters into the structure of the veins. The ultimate 

 fibrillse are estimated at about 1-1 000th of a line in diameter. 



[ 214. The second class consists of fibrillse mostly of a red 



