20 



Anatomy of the Rabbit. 



which apparently form a continuous network. Like striated muscle, it 

 possesses characteristic transverse markings, but, like involuntary 

 muscle, it is under the control of the sympathetic nervous system. 



As gross structures the voluntary muscles are functional units, each 

 of which has a particular action according to the movement per- 

 mitted by the parts of the skeleton to which it is attached. They 

 present a longitudinal striation which is roughly referred to as the direc- 

 tion of the fibres, and which is of great value in identification. The 

 striation is due to the circumstance that the fibres are arranged 

 in parallel groups or muscle bundles, each of which is surrounded and 

 separated from the adjacent bundles by a connective tissue enclosure, 

 the perimysium. 



A muscle is typically spindle-shaped, consisting of a middle fleshy 

 portion, termed the belly of the muscle, and of tapering ends which 

 provide for attachment. The attachment is effected by a strong 

 band of fibrous connective tissue, the muscle tendon (Fig. 6). In 

 contraction it is usually seen that one end, thus designated as the 

 origin of the muscle, remains stationary, while the other, known as the 



insertion, assumes a position nearer 

 the origin, carrying with it either the 

 structure which is to be moved, or 

 another portion determined by the 

 point of leverage or the position of 

 the joint. The action of the muscle 

 is estimated in a direct line between 

 origin and insertion, although the 

 effect of the contraction, on account 

 of the position of the joint, may be 

 otherwise. From these more typical 

 conditions, however, ■ many modifica- 

 tions occur. 



Some muscles, such as those of the 

 abdominal wall, are disposed in the 

 form of flattened sheets, the ends of 

 which are attached by broad, thin 

 sheets of connective tissue, the apon- 

 euroses. In unipennate muscles the 

 fibres are attached obliquely to the side of the tendon, or in bipennate 

 muscles to both sides, like the vane of a feather. In the so-called biceps, 

 triceps and quadriceps muscles of the limbs, the origin is divided into two, 

 three or four portions. Furthermore, the recognition of origin and 

 insertion depends on usual but not invariable relations. The exact 

 effect of muscle contraction also depends as a rule on the synchronous 

 action of other muscles. A muscle like the diaphragm does not 

 possess an insertion after the fashion of ordinary muscles; and in some 

 cases, as in the intrinsic muscle of the tongue or the so-called orbicular 

 or sphincter muscles, both origin and insertion may be absent. 



Involuntary muscle is distinguished by its white or greyish coloration 

 and by its smooth or homogeneous appearance. It forms characteristic 



Fig. 15. From a section of the pyloric end 

 (antrum pyloricum) of the stomach of the 

 rabbit: m.m., muscularis mucosae; t.m.c. 

 circular layer of the muscular tunic; t.m.l., 

 longitudinal layer of the muscular tunic; 

 t.ms., mucous tunic; t.s., tela submucosa; 

 t.sr., serous tunic. 



