MUSCLE. 337 



Corresponding with the division of muscles into voluntary 

 <ind involuntary, there exist differences of structure: thus, 

 the muscles under the control of the will are all striped, 

 while those which are not under its influence are unstriped. 

 To this rule, however, one remarkable exception may be men- 

 tioned, viz. the muscles of the heart, the action of which is 

 to a great extent involuntary, and which are yet striped : this 

 exception is rather apparent than real, as will be seen here- 

 after. 



STRUCTURE OF MUSCLE. 



A striped muscle is made up of a number of unbranched 

 fibres, each of which is included in a distinct sheath, the 

 sarcolemma, and consists of a number of threads or fibrillse : 

 the fibres again are collected into sets or bundles called 

 lacerti; these are held together, and yet separated by a 

 mixed form of cellular tissue, which also contains fat vesicles 

 blood-vessels, and nerves. 



An unstriped muscle consists of fibrillse, intermingled with 

 fibrous tissue : these do not form fibres, and consequently 

 there is no sarcolemma. 



Between striped and unstriped muscle there is no essential 

 or specific structural difference: the one is not to be re- 

 garded as typically distinct from the other, but both should 

 rather be considered as different conditions in the develop- 

 ment of one and the same tissue. Of this position, evidence 

 will be hereafter adduced. 



According to the above view, muscular fibre presents 

 two grand stages of development ; the first of which is repre- 

 sented by the unstriped fibrilla, and the second by the 

 striped muscular fibre. 



We shall first describe the structure of the unstriped 

 muscular fibrilla, because it represents an earlier condition 

 of development than the striped. 



Structure of Unstriped Muscular Fibrilla. Unstriped 

 muscles consist of fibrillae which are unbranched, rather broad, 



