270 THE HORSE. 



ally present rough surfaces for this purpose, and are also closelj 

 incorporated with the periosteum. This union is so strong, that 

 it very rarely gives way ; and when extreme violence is used, 

 either the bone itself breaks, or" the tendon snaps in its middle. 

 Tendons are non-elastic. 



To the naked eye, an ordinal y muscle appears to be com- 

 posed of a number of small bundles of fibres, arranged in parallel 

 lines, and connected by a fine membrane. These bundles may 

 still further be separated into what seem at first to be elementary 

 fibres ; but when placed in the microscope, they are found to be 

 themselves made up of finer fibres united into fasciculi by delicate 

 filaments. These ultimate fibrillae are polyhedral in section, ac- 

 cording to the observations of Mr. Bowman, so as to pack closely 

 together, and are variable in size in different classes and genera of 

 animals. They also differ in appearance, one class presenting 

 stripes while the other is without them. The former includes all 

 the muscles whose movements are under the control of the will as 

 well as those of the heart, and some of the fibres of the oesophagus, 

 while the latter is composed of the muscles investing the stomach, 

 intestines, bladder, &c, which are comprehended under the gene- 

 ral term involuntary. 



The Sarcolemma is the name given by Mr. Bowman to the 

 areolar tissiie investing each fibre, sometimes also called myolemma. 

 It is very delicate and transparent, but tough and elastic; in 

 genera! it has no appearance of any specific structure, but some- 

 times it presents an aspect as if there was an interweaving of fila- 

 ments. 



When a fibrilla of striated muscle is examined under the 

 microscope of a high magnifying power, it is seen to present a 

 beaded appearance as if made up of a linear aggregation of dis- 

 tinct cells, alternately light and dark. When the fibrilla is 

 relaxed, each cell is longer than it is broad; but, during the 

 action of the muscle, it assumes the opposite dimensions, the increase 

 in one diameter being always in proportion to the diminution of 

 the other. As the contraction takes place the substance becomes 

 fiimer than before, but the bulk remains the same, the mass 

 merely gaining in thickness what it has lost in length. The 

 application of certain stimulating agents will produce the contraction 

 for a certain period after life is destroyed, varying according to 

 the vitality of the animal experimented upon and the nature cf the 

 individual muscle. This is called irritability in the striated 

 muscles, which exhibit powerful contractions, alternating with re- 

 laxations — while in the involuntary muscles a more steady, per- 

 manent, and moderate contraction is met with, to which the name 

 of tonicity has been given. 



Pure muscular fibre appears to be identical in composition 



