70 



MUSCULAR TISSUE 



[CH. VI. 



FIG. 85. Three muscular fibres 

 running longitudinally, and 

 two bundles of fibres in trans- 

 verse section, M, from the 

 tongue. The capillaries, c, 

 are injected, x 150. (Klein 

 and Noble Smith.) 



muscle fibres, and the changes they undergo during contraction 

 have been the subjects of numerous hypotheses. One of two only of 

 these theories have been mentioned in the 

 preceding paragraphs, and it should be recog- 

 nised that even experts are content to leave 

 the matter very largely an open question at 

 present. 



Blood-vessels of Muscle. The arteries break 

 up into capillaries, which run longitudinally 

 in the intervening connective tissue, trans- 

 verse branches connecting them (fig. 85). No 

 blood-vessels ever penetrate the sarcolemma. 

 The muscular fibres are thus, like other tissues, 

 nourished by the exudation from the blood 

 called lymph. 



The motor nerves of voluntary muscle pierce 

 the sarcolemma, and terminate in expansions 

 called end-plates, to be described on p. 78. 



The sensory nerves of voluntary muscle 

 terminate in structures known as the neuro-muscular spindles, which 

 will be dealt with in the chapter on Motor ial Sensations. 



Red Muscles. 



In many animals, such as the rabbit, and some fishes, most of the 

 muscles are pale, but some few (like the diaphragm, crureus, soleus, 

 semi-membranosus, in the rabbit) are red. These muscles contract 

 more slowly than the pale muscles, and their red tint is due to haemo- 

 globin contained within their contractile substance. 



In addition to these physiological distinctions, there are histo- 

 logical differences between them and ordinary striped muscle, viz. : 



1. Their muscular fibres are thinner. 



2. They have more sarcoplasm. 



3. Longitudinal striation is therefore more distinct. 



4. Transverse striation is more irregular than usual. 



5. Their nuclei are situated not only under the sarcolemma, but 

 also in the thickness of the fibre. 



6. The transverse loops of the capillary network are dilated into 

 little reservoirs, far beyond the size of ordinary capillaries. 



Cardiac Muscle. 



The muscular fibres of the heart, unlike those of other in- 

 voluntary muscles, are striated ; but although in this respect they 

 resemble the skeletal muscles, they have distinguishing character- 

 istics of their own. The fibres which lie side by side are united at 



