VERTEBRATES. 27 



the body is to be found in the origin of the motor nerves which supply tliem. 

 These are, however, interwoven in a very complex manner (forming the brachial 

 and sacral plexuses of spinal nerves) before they are distributed to the muscles 

 of the fore and hind limbs. It is somewhat easier to detect segmental arrange- 

 ment in tlie muscles of the trunk of higher vertebrates. They are no longer simply 

 disposed in myotomes, but are differentiated into groups and layers capable of 

 effecting more complex movements ; and this differentiation is of course greatest 

 in those regions — such as the neck — where the most complex movements are neces- 

 sary. The muscles are not confined to the outer surface of the skeleton, but cer- 

 tain verj' important muscles are attached to the ventral aspect of the vertebral 

 column. To this system belongs the diaphragm, a muscular partition, complete 

 only in mammals, dividing off the chamber containing the heart and lungs from 

 that containing the rest of the viscera. 



Although the branchial skeleton largely disappears from the higher forms, yet it 

 cannot be said that the musculature of the head becomes sinijiler in these, for the 

 development of a tongue is accomi)anied by a corresponding development of the mus- 

 cles attached to the hyoid bone — the representative of the branchial skeleton of the 

 lower forms. The muscles of the jaws belong naturally to the same series of those 

 of the rest of the visceral skeleton, but are specialized in the same degree as the bony 

 framework to which they are attached. 



While a considerable part of the trunk muscles on the dorsal aspect are derived 

 from the conversion of part of the walls of the mesoblastic somites — the muscle- 

 ])lates, the muscles of the lateral and ventral regions are derived from the lateral 

 plates of the embryo. In the head, on the other hand, some of the mesoblastic 

 somites give rise to no muscular structures at all, while others (the three foremost) give 

 rise only to the small muscles which move the eyeball. Tlie greater part of the mus- 

 culature of the head is thus derived from the parts corresponding to the lateral plates 

 in the ti'unk. 



In addition to the fibres of involuntary muscles which are present in the skin, 

 there are present in higher forms muscles which, arising generally from deeper parts, 

 are attached to the skin, and have the function of moving it. Such are the muscles 

 of expression of mammals, as well as larger sheets which may extend over consider- 

 able tracts of the body. The spines of the porcupine are erected by such muscles, 

 which are also those emplojxd by the mammals generally in shaking the coat. 



THE NERVOFS SYSTEM. 



Three constituent portions maybe distinguished — the central nervous system, the 

 peripheral nervous system and the connecting nerves. The last serve merely to trans- 

 mit outwards or inwards (centrifugally or centripetally) messages between the central 

 and peri|)heral systems. The latter is composed of the end organs of the centrifugal 

 and centripetal nerves. We have already considered some end organs of the centripe- 

 tal nerves — the lower sense-organs of the skin — and we shall describe the higher 

 sense-organs at the close of this chapter. The ends of the centrifugal nerves are in 

 the muscles, whether of the body, intestine, or blood-vessels, arid likewise in the vari- 

 ous glands, which are thus e.xcited to secretion. 



Only nerve-fibres are met with in the nerves, while nerve-centres or ganglia are 

 characterized by the presence of nerve or ganglion cells in addition. It is customary 



