82 HANDBOOK OF PHYSIOLOGY. 



when they break they often have abruptly rounded or square extremities. 

 Each cell of these consists of a fine sheath, probably elastic; of a centra] 

 bundle of fibrils representing the contractile substance; and of an ob- 

 long nucleus, which includes within a membrane a fine network anasto- 

 mosing at the poles of the nucleus with the contractile fibrils. The 

 ends of fibres are usually single, sometimes divided. Between the fibres 

 is an albuminous cementing material or endomysium in which are found 



Fig. 82. Plexus of bundles of unstriped muscle cells from the pulmonary pleura of the Guinea-pig. 

 X 1HO. (Klein and Noble Smith.) A, Branching fibres; B, their long central nuclei. 



connective-tissue corpuscles, and a few fibres. The perimysium is con- 

 tinuous with the endomysium in the fibrous connective tissue surround- 

 ing and separating the bundles of muscle cells. 



Striated Muscle. 



Distribution. Striated or striped muscle is found in the following 

 situations. It constitutes the whole of the muscular apparatus of the 

 skeleton, of the walls of the abdomen, etc., the whole of those muscles 

 which are under the control of the will and hence termed voluntary, as 

 well as certain other muscles, e.g., of the internal ear and pharynx not 

 directly under the control of the will, and the heart. 



Structure. For the sake of description, striated muscular tissue may 

 be divided into two classes, (a.) skeletal, which comprises the whole of 

 the striated muscles of the body except (b.) the heart : 



(a.) Skeletal Muscle. In the majority of cases a skeletal muscle 

 is inclosed in a sheath of areolar tissue called the epimysium, which in 

 some cases is a very thick and distinct investment, while in other cases 

 it is much thinner. The sheath sends in partitions which serve to sup- 

 port the fasciculi or bundles of fibres, of which the muscle is made up, 

 forming more or less distinct sheaths for them, called perimysium. The 



