500 SPHINCTER AND OTHER MUSCLES. 



of the suture is that the bones can still grow at their edges, which thus renders possible the 

 distension of the cavity enclosed by the bones (Hcrm. v. Meyer). 



306. ARRANGEMENT AND USES OF MUSCLES. The muscles form 45 

 per cent, of the total mass of the body, those of the right side being heavier than 

 those on the left. Muscles may be arranged in the following groups, as far as their 

 mechanical actions are concerned : 



A. Muscles without a definite origin and insertion : 



1. The hollow muscles surrounding globular, oval, or irregular cavities, such 

 as the urinary bladder, gall-bladder, uterus, and heart ; or the walls of more or less 

 cylindrical canals (intestinal tract, muscular gland ducts, ureters, Fallopian tubes, 

 vasa deferentia, blood-vessels, lymphatics). In all these cases the muscular fibres 

 are arranged in several layers, e. g., in a longitudinal and a circular layer, and some- 

 times also in an oblique layer. All these layers act together and thus diminish the 

 cavity. It is inadmissible to ascribe different mechanical effects to the different 

 layers, e.g., that the circular fibres of the intestine narrow it, while the longitudinal 

 dilate it. Both sets of fibres rather seem to act simultaneously, and diminish the 

 cavity by making it narrower and shorter at the same time. The only case where 

 muscular fibres may act in partially dilating the cavity is when, owing to pressure 

 from without, or from partial contraction of some fibres, a fold, projecting into the 

 lumen, has been formed. When the fibres, necessarily stretching across the depres- 

 sion thereby produced, contract, they must tend to undo it, i.e., enlarge the cavity. 

 The various layers are all innervated from the same motor source, which supports 

 the view of their conjoint action. 



2. The sphincters surround an opening or a short canal, and by their action 

 they either constrict or close it, e.g., sphincter pupillae, palpebrarum, oris, pylori, 

 ani, cunni, urethras. 



B. Muscles with a definite origin and insertion : 



1. The origin is completely fixed when the muscle is in action. The course 

 of the muscular fibres, as they pass to where they are inserted, permits of the 

 insertion being approximated in a straight line towards their origin during con- 

 traction, e.g., the attolens, attrahens, and retrahentes of the outer ear, and the 

 rhomboidei. Some of these muscles are inserted into soft parts which necessarily 

 must follow the line of traction, e.g., the azygos uvulae, levator palati mollis, and 

 most of the muscles which arise from bone and are inserted into the skin, such as 

 the muscles of the face, styloglossus, stylopharyngeus, tfec. 



2. Both Origin and Insertion movable. In this case the movements of both 

 points are inversely as the resistance to be overcome. The resistance is often 

 voluntary, which may be increased either at the origin or insertion of the muscle. 

 Thus, the sternocleidomastoid may act either as a depressor of the head or as an 

 elevator of the chest ; the pectoralis minor may act as an abductor and depressor 

 of the shoulder, or as an elevator of the 3rd to 5th ribs (w r hen the shoulder-girdle 

 is fixed). 



3. Angular Course. Many muscles having a fixed origin are diverted from 

 their straight course ; either their fibres or their tendons may be bent out of the 

 straight course. Sometimes the curving is slight, as in the occipito-frontalis and 

 levator palpebral superioris, or the tendon may form an angle round some bony 

 process, whereby the muscular traction acts in quite a different direction, i.e., as if 

 the muscle acted directly from this process upon its point of insertion, e.g, the 

 obliquus oculi superior, tensor tympani, tensor veli palatini, obturator internus. 



4. Many of the muscles of the extremities act upon the long bones as upon 

 levers: (a) Some act upon a lever with one arm, in which case the insertion of 

 the muscle (power) and the weight lie upon one side of the fulcrum or point of 

 support, e.g., biceps, deltoid. The insertion (or power) often lies very close to 

 the fulcrum. In such a case, the rapidity of the movement at the end of the lever 



