2 PHYSIOLOGY CHAP. 



sympathetic, but also the nerve-paths and centres of the cerebro- 

 spinal system. 



Nevertheless the muscular and nervous elements which play 

 a direct part in the functions of vegetative life have usually 

 certain morphological and functional characters which distinguish 

 them from those which make up the organs of animal life, and 

 regulate the relations of the organism with the external world : 



(a) Voluntary or skeletal muscles are almost always striated ; 

 involuntary muscles, i.e. those of vegetative life, are almost always 

 non-striated. 



(6) The former are controlled by the will, and only come into 

 play in response to nervous impulses ; the latter are nearly always 

 independent of the will, and may even function independently 

 of the central and peripheral nervous systems. 



(c) The voluntary muscles consist of long fibres, grouped into 

 large masses, each of which is an anatomical unit ; the involuntary 

 fibres, which are not grouped into separate muscles, almost always 

 form smooth layers that line vessels or tubes, or constitute sheaths 

 that surround certain special cavities. 



(d) Finally (and this appears the most important), the first 

 are almost always skeletal muscles, attached by tendons to bony 

 levers, by which they can lift weights and overcome resistance, 

 i.e. perform actual mechanical work; the second, on the con- 

 trary, are nearly all visceral muscles, and perform work that is 

 entirely confined to the interior of the body. 



The nerves that control the involuntary system, again, present 

 certain characters which distinguish them from those that 

 innervate the voluntary muscles. The latter consist of medullated 

 fibres which come directly from the spinal roots ; the former are 

 exclusively non-medullated, and come principally from the sym- 

 pathetic system, and make at the periphery an exceedingly fine 

 fibrillary network which surrounds the separate muscle cells. 



I. The skeletal muscles constitute the principal mass of the 

 body. Each muscle is an anatomical unit, a separate organ, 

 which can assume the most various shapes and sizes, but usually 

 consists of an elongated mass provided with tendons by which 

 it is attached to the skeleton. Each muscle consists of fibres 

 which are generally arranged parallel to its long axis, and converge 

 more or less towards the tendinous attachments. The muscle 

 fibres are united into bundles of varying size by connective 

 tissue, which is connected with the sheath or perimysium that 

 surrounds the whole muscle ; the blood and lymph vessels and 

 the nerves run through this connective tissue. 



The length and the diameter of the muscle fibres vary con- 

 siderably. On an average, the length does not exceed 30-40 mm., 

 but according to some authors it may reach 30 cm. The diameter 

 varies considerably even in the same muscle, and still more in 



