504 



COLLEGE ZOOLOGY 



the axis and atlas, which permits turning 

 the head from side to side; and (4) ball 

 and socket joints with a rounded head lying 

 in a cuplike cavity, such as the shoulder 

 and hip joints. 



MUSCLES AND MOVEMENT 



Movements in animals are effected prin- 

 cipally as a result of the contractility of mus- 

 cle fibers, but contractility is a general 

 property of protoplasm and can be observed 

 in the absence of muscle fibers, as in the 

 flagella and cilia of protozoans. Certain pro- 

 tozoans also contain contractile fibrils, the 

 myonemes, that resemble myofibrils in the 

 muscle fiber of a vertebrate. As already 

 noted, the epitheliomuscular cells of coelen- 

 terates are the simplest of all cells that are 

 specialized for purposes of contraction. 



Smooth muscle 



Muscular tissue in higher animals is usu- 

 ally classified, according to its structure and 

 localization, into three types (Fig. 373): 

 (1) smooth, (2) skeletal,* and (3) cardiac. 

 Cardiac muscle occurs only in the hearts of 

 vertebrates. Smooth muscle is characteristic 

 of invertebrates and of the visceral organs 

 of vertebrates. It is composed of spindle- 

 shaped cells, from 0.015 to 0.5 mm. long, 

 and from 0.002 to 0.02 mm. in diameter, 

 each with a single nucleus near the center. 

 Smooth muscle cells may be either isolated 

 or aggregated into small groups; or they may 

 exist in parallel layers of considerable size. 

 Smooth muscles contract more slowly than 

 other types. Changes in the shape and size 

 of the visceral organs are due to the contrac- 

 tion of smooth muscles, such as peristalsis 

 in the intestine. Such muscles are present in 



Smooth 



jC^^^CSkc 



' "^•iimmiti^m^; :[;';;/ 



■!'" ■ . 1 -■;';) ' ' i:. i ' i y i ' i '.i^ i^ Cj' ' '■ 



Skeletal 



Cardiac (heart) 



Figure 373. Microscopic structure of the three types of muscle cells (fibers). Skeletal muscle 

 is also called striated; but inasmuch as cardiac (heart) muscle is also striated, this terminology 

 results in some confusion. 



the walls of blood vessels and the bladder, 

 around the openings of glands, in the skin 

 where they may raise the hair, in the 



* Skeletal muscle is also called striated, but inas- 

 much as cardiac muscle too is striated, the use of 

 the term striated for skeletal muscle may result in 

 some confusion. 



trachea and bronchi, and in the reproduc- 

 tive ducts. Since they are not under volun- 

 tary control, they are called involuntary mus- 

 cles. Smooth muscles, because they are the 

 first kind to have developed in animals, are 

 thought to be the most primitive. 



