MUSCLES OF ANIMAL LIFE. 271 



the Animal body, not only as the instrument of the operation of the Nervous 

 System upon the external world, in which respect alone its action can be 

 said to form part of the Functions of Animal Life, but also to execute many 

 of those interior movements which the peculiar conditions of Animal existence 

 require for its own maintenance, such as the propulsion of the food along the 

 alimentary canal, and that of the blood through the vascular system. The 

 muscles concerned, however, in these operations, which are so immediately 

 connected with the maintenance of the Organic functions, differ essentially 

 from those strictly forming part of the apparatus of Animal life, both in their 

 own structure, and in the manner in which their contractility is called into 

 operation. The former are (like the contractile tissues of Plants) much more 

 susceptible than the latter, of being excited to action by a stimulus immediately 

 applied to themselves, and are with difficulty shown to be in any degree 

 under the influence of nerves ( 201) ; whilst the latter are readily thrown 

 into violent contraction, by a stimulus conveyed to them through the nervous 

 system. Hence a physiological distinction may be made, between these two 

 groups of muscles ; which is fully borne out by differences in the structure 

 and arrangement of their component parts. By some, the two classes have 

 been spoken of as those of Involuntary and Voluntary Muscles ; but this 

 distinction is not correct ; since every muscle ordinarily termed voluntary, is 

 susceptible of being called into action involuntarily. It is better to found the 

 distinction upon their nearer or more remote concern- in the functions of Or- 

 ganic Life : those which are immediately involved in their maintenance, and 

 over which the will can never exert any influence, the Heart, and Muscular 

 coat of the Intestinal Canal, for instance, being designated as the Muscular 

 System of Organic Life ; and those which can be employed by the Nervous 

 System to execute the commands of the Will, being included in the Muscular 

 System of Animal life. The structure peculiar to the latter will first be de- 

 scribed ; as it is evidently that which is most characteristic of Muscle. 



II. Muscles of Animal Life. 



367. When we examine an ordinary Muscle (from one of the extremities, for 

 example), with the naked eye, we observe that it presents a fibrous appearance ; 

 and that the fibres are arranged with great regularity in the direction in which 

 the muscle is to act. Upon further examination it is found, that these fibres 

 are united together in fasciculi or bundles of larger or smaller size, by means 

 of areolar tissue ; ana when the Microscope is applied to the smallest fibre 

 which can be seen with the naked eye, it is seen itself to consist of a fascicu- 

 lus, composed of a number of cylindrical fibres lying in a parallel direction, 

 and closely bound together. These pri- 

 mitive fibres present two sets of markings Fig. 60. 

 or striae ; one set longitudinal, the other 

 transverse or annular. By more closely 

 examining these fibres, when separated 

 from each other, it is frequently seen that 

 each may be resolved into fibrillg, by 

 the splitting of its contents in a longi- 

 tudinal direction, as shown in Fig. 60. 

 These fibrillae have a peculiar beaded ap- 

 pearance, which will be presently noticed 

 more particularly. It not unfrequentlv 

 happens, however, that when a fibre is 



, the fibres se 



drawn apart, its contents separate in the bundles of fibriiUe. 

 direction of the transverse striae; forming a 



