MUSCULAR SYSTEM. 275 



secondary bundles, to join finally with the perimysia interna to 

 make up a continuous connective-tissue framework. The parts 

 of this framework which enter the primary bundles to surround 

 the individual muscle fibres usually contain very few elastic 

 fibres and no fat cells ; while the larger strands separating the 

 primary and secondary bundles are rich in both these elements. 

 Blood-vessels and nerves enter the muscle in the connective- 

 tissue septa and surround the muscle fibres 



The blood supply shows an exceptionally rich branching of 

 capillaries around the muscle fibres. The blood-vessels enter 

 the perimysium and run more or less parallel to the course of 

 the muscle fibres (Fig. 201). In the perimysium between the 

 primary bundles fine arterial branches proceed at right angles 

 from the larger trunks between the muscle fibres. From these, 

 there run again at right angles i e., parallel to the course of 

 the fibres the capillaries, which form a fine network surround- 

 ing the individual fibres. They run in large part parallel with 

 the fibres, and send off quite frequently fine anastomosing 

 branches, so that the meshes of the network are for the most 

 part rectangular or rhomboidal. Each fibre is surrounded on 

 all sides by capillaries, as may be seen in a cross-section of an 

 injected muscle (Fig. 202). The veins arising from the capil- 

 laries are characterized by the presence of valves, even in the 

 finest branches. In the red muscles of the rabbit there are 

 sinuses in many places between the arterial and venous ends 

 of the network (Ranvier). 



There is in all muscles a definite blood vascular unit, special 

 attention to which was called by Spalteholz. Arteries can 

 be seen entering the muscle bundles at regular intervals (Fig. 

 201), and sending out capillaries on all sides. The veins col- 

 lecting the blood from these capillaries are placed quite regu- 

 larly. The unit thus has the artery for its centre and the 

 collecting veins at the periphery. 



The nerves and their endings are spoken of in the section 

 on Nerve-endings. 



