MAMMALIA. 313 



so keeps up a state of contraction in the muscle up to 

 a certain point, when the contractions become gradually 

 more feeble, and finally cease altogether ; the muscle is 

 then exhausted, but, if allowed to rest awhile, it again 

 obtains its contractile power. This peculiar contractile 

 power is a physical one, and exists for a short time 

 after death, that is, until its perfect structure is altered 

 by the beginning of decomposition ; and what is a curious 

 fact, the muscles of a dead body stimulated to contraction 

 by pricking or galvanism, will tire and again obtain their 

 contractility by rest, just as in the living body. The 

 nature of the stimulus which causes the muscles to contract 

 during life is not known ; but, whatever it may be, it 

 is conveyed from the spinal cord and brain, through 

 the medium of a set of nerves, called the " motor nerves," 

 or nerves of motion, to the muscles, to every fibre of which 

 they are distributed. There is, however, another set of 

 nerves which have nothing to do with motion, although 

 exactly similar in appearance and generally associated and 

 bound up in the same sheath with them ; these are the 

 nerves of sensation. They take their origin from every 

 part possessed of sensibility or feeling more especially the 

 skin and convey every impression of feeling to the brain. 

 The muscles are not all under the control of the will, for 

 the muscles of the heart, bowels, &c. are quite out of our 

 control. Those which are subservient to our will are called 

 " voluntary " muscles, or the " muscles of animal life," while 

 those out of the control of the will, are the " involuntary " 

 muscles, or "muscles of organic life" (fig. 79). These 

 last have a structure different to the voluntary muscles, 

 being composed of flattened fibres containing granules and 

 overlapping each other. The tendons are the cord-like 

 extremities of the muscles which connect them to the 

 bones ; they are fibrous and slightly elastic, flexible, and 

 exceedingly strong ; while moist they have a splendent 

 appearance, somewhat like mother-of-pearl. 



The ligaments are the strong fibrous bands which connect 

 the bones together. Cartilages are substances of a white 

 colour, smooth in their texture, and not very flexible ; 



