HIGH SCHOOL ZOOLOGY. 29 



lature of the body. They reply slowly to a stimulus, and are 

 not under the iuflucwce of tlie will, while those which unite the 

 various parts of the skeleton are called voluntary, because they 

 are controllable by the will, and rei)ly ra})idly to a stimulus. 



Both kinds of muscles are formed of fibres, which contract on the 

 receipt of a stimulus through motor nerve-fibres which terminate in 

 them. The involuntary are formed of bands of simple muscle-cells— 

 tlie voluntary of bundles of striated fibres, but in both the muscle-tissue 

 has a framework of connective-tissue which suspends the vessels and 

 nerves distributed to the muscle-tissue. 



32. The voluntary muscles are also called " skeletal," for it is 

 obvious that a very important relationship must exist between 

 the skeleton and the muscles. Where muscles are of large size, 

 they must have a sufficient surface for their origin and in- 

 sertion, and where they cause two parts of the skeleton to 

 move ujx)n each other, the natui'e and extent of the movements 

 must determine the character of the joint. Thus those parts of 

 the body where the most complicated movements are carried 

 out will have the more differentiated muscles, and those where 

 the movements are simpler, will have the less s])ecialised 

 muscles. In the catfish the more specialised muscles ai-e those 

 which work the jaws, the pai'ts of the viscei-al skeleton, the 

 gill-cover, and the spines of the pectoral and dorsal fins, while 

 the less specialised are those which form the fleshy mass of the 

 trunk and tail. The latter exhibit the same metamery which 

 we have seen to characterize this region of the skeleton, for the 

 muscles are divided into myomeres, separated by membranous 

 partitions which are attached to the ribs and vertebrae, but 

 the planes of these partitions are not vertical ones, as we 

 may see from a cut through the tail, or from the curved form 

 of the myomeres or flakes into which the flesh of the fish sejj- 

 arates when boiled. Special muscular slips extend into the fins, 

 and serve to depress or erect the rays, but these fin-muscles do 

 not attain the size which the limb-muscles have in higher Yev- 



