OH. vi.] RED AND CARDIAC MUSCLE. 93 



appears dark. If an object on the microscope stage is doubly refracting it 

 will appear bright in this dark field ; if it remains dark it is singly refract- 

 ing. The sarcoplasm is singly refracting or isotropoiis : it remains dark in 

 the dark field of the polarising microscope. The muscle columns or sarco- 

 styles are in great measure doubly refracting or anisotropous. and appear 

 bright in the dark field of the polarising microscope. The sarcostyle, how- 

 ever, is not wholly doubly refracting; the sarcous elements are doubly 

 refracting, and the clear intervals are singly refracting. On contraction 

 there is no reversal of these appearances, though of course the relative 

 thickness of the singly refracting intervals varies inversely with that of the 

 doubly refracting sarcous elements. 



Ending of Muscle in Tendon. A tendon-bundle passes to each 

 muscular fibre, and becomes firmly united to the sarcolemma. 

 The areolar tissue between the tendon-bundles becomes also con- 

 tinuous with that between the muscular fibres (fig. in). 



Blood-vessels of Muscle. The arteries break up into capillaries, 

 which run longitudinally in the endomysium, transverse branches 

 connecting them (fig. 112). 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 

 lymph is removed by lymphatic vessels found in the perimysium. 



The nerves of voluntary muscle pierce the sarcolemma, and ter- 

 minate in expansions called end-plates, to be described later on 

 (see p. 102). 



Neuro-niuscular Spindles. Bundles of fine muscular fibres en- 

 closed within a thick lamellated sheath of connective tissue are 

 found scattered through voluntary muscles ; they are especially 

 numerous near the tendon and in the proximity of intra-muscular 

 septa. It is remarkable that they have not been found in the ocular 

 or tongue muscles. These structures are called neuro-muscular 

 spindles ; they vary in length from ^ to ^ inch, and are about 

 T - T inch in diameter. Each receives a nerve fibre which divides 

 into secondary and tertiary branches. The myelin sheath is lost, 

 and the tertiary branches encircle the muscular fibres, breaking 

 up usually into a network. There has been considerable discus- 

 sion as to the meaning of these spindles, but the belief is gaining 

 ground that they are sensory end organs in the muscle. (See 

 further, chapter on Touch.) 



Bed 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 haemoglobin contained within their contractile substance. 



