46 COMPARATIVE ANATOMY chap. 



channels of the bone substance. The fibres are connected by fine 

 anastomoses. . . 



It is clear from the above that bone tissue shows much similarity 

 in its origin with gelatinous tissue, and dentine with epithelial cuticular 

 formations. 



The blood eeUs and lymph cells which float in the blood, the 

 lymph or the ccelomic fluid of animals, at first rise out of connective 

 tissue cells. The blood has even been described as fluid connective 

 tissue, the blood fluid representing the intercellular substance, the 

 blood corpuscles the connective tissue cells. Lymph corpuscles often 

 show amoeboid movement, and are capable of taking in solid niaterials 

 {e.g. products of excretion, food material, products of suppuration, and 

 foreign bodies). 



III. Neuromuscular and Muscle Tissue. 



The elements of both muscle and nerve tissue originally come from 



the epithelial cells. Both tissues appear simultaneously in the animal 



kingdom, and are connected in their origin. In its simplest form 



neuro-muscular tissue is met with among the lower Ccelenterata {e.g. 



.,^^ _. -- „..,-,,,„j^^^.,„„-^ Hydra). Here, in the outer epithelium, 



W^(Al^'^:f\^''/^^''-f:''^''^-f ^I'e found cells which form processes 



©f l||'i\ v-,'-:(^4^¥ '^'' inward, and produce a layer of fibres 



/*-:v( .Uu'SI l-fvu' «;"':( rf'i^ close under the epithelium'. These 



/ ]f'\'M '^•'1 )j'A';\l£'i«,;,-''<j!._ fibres are contractile, and represent 



.<mi=rii|;^>^i'ci,Qi:^-'-^'^'' o^ muscle processes of the epithelial cells. 



Fig. 46.-Neuro-musouiar cells of the The latter, which contain the nucleus, 



Hydra, after Kleinenterg. g, Muscle pro- take part with their companions in 



cesses of the same. limiting the surf ace of the body. These 



cells undertake the relations with the outer world, being suited to 

 receive external impressions. The stimulus created by such impres- 

 sions is carried through the protoplasm of the cell to the muscle pro- 

 cesses, which contract in consequence. The cells, with their processes, 

 are accordingly called neuro- muscular cells. In them the chief 

 functions are of nerve and muscle tissue localised, in the most general 

 undifferentiated manner, in different parts of one and the same cell. 

 In consequence of the intimate connection of the neuro-muscular cells 

 with one another, a locally created stimulus of one or more cells is 

 communicated to the neighbouring cells, and thus to their muscle 

 processes. The principle of the division of labour here again supplies 

 the key for understanding the further differentiation of neuro-muscular 

 tissue. One portion of the neuro-muscular cells undertakes principally 

 the function of contractility, and the greater part of its protoplasm 

 becomes differentiated into contractile substance ; another portion of 

 the neuro-muscular cells, while remaining in close organic connection 

 with the first, performs the function of receiving external impressions. 



