72 ANIMAL BIOLOGY. [Part I. 



more or less at right angles to the surface, or in gently curved 

 bundles, the curves at slightly different levels intersecting each 

 other. The individual prisms may be isolated by means of 

 hydrochloric acid. In the fang the encrusting material is called 

 cement, which resembles bone in structure. In it are large 

 lacunae, the canaliculi of which communicate with the ultimate 

 endings of the tubules of the dentine (Fig. 25, vii.). 



The tooth of the cod-fish differs markedly in structure from 

 the mammalian tooth above described. An inner zone has 

 large canals ; a thinner outer zone fine-branched tubules. 



7. Muscle. Muscle is divided into (a) striped or voluntary 

 muscle, and (b) un striped or involuntary. 



(a.) Striped. Of this kind are the most important muscles 

 of the trunk and limbs. The fibres are elongated, and are 

 surrounded and bound together by connective tissue (endo- 

 mysium\ in which run the capillaries of the blood-vessels. The 

 bundles into which the fibres are thus bound together are in 

 turn invested by stronger bands of connective tissue (peri- 

 mysium). 



Fresh striped muscle of the frog (cp. also that of an inverte- 

 brate such as the water-beetle, hydrophilus, or the cockroach), 

 mounted in normal saline, is seen to be composed of glassy or 

 slightly opalescent fibres surrounded by a delicate sheath, the 

 sarcolemma. Each is transversely striated, and contains fusi- 

 form muscle corpuscles. Treatment with acetic acid renders the 

 striation indistinct, and the corpuscles more conspicuous. They 

 are embedded in the midst of the fibre in frog's muscle, but lie 

 just beneath the sarcolemma in that of the rabbit. Dead fibres 

 tend to split up into a number of separate fibrillce, and a muscle 

 that is hardened in strong spirit very readily splits up into such 

 elements. Muscle fibres hardened in picric acid tend to break 

 up into discs. 



In a fresh muscle fibre teased in normal saline and examined 

 under the highest powers of the microscope, the following struc- 

 ture may, under favourable conditions, arid by careful focussing, 

 be observed : The fibre shows alternate bands of lighter and 



