THE SKELETON AND MUSCLES 195 



axis placed obliquely, and it is capable of moving in the glenoid 

 cavity either up and down, from side to side, or backwards and 

 forwards; so, too, man shares with apes, dogs and cats the 

 diarthrodial (lateral-ginglymus) joint between radius and ulna, 

 with apes the amphiarthrodial articulations between the bones 

 of the carpus, and with mammals generally the tarso-metatarsal 

 joint. 



The hip-joint in all mammals, including man, is a ball in 

 socket (enarthrodial) articulation. 1 



But it is by his shoulder-joint that man is distinguished 

 from all other Vertebrates, even the highest apes, for by its 

 peculiar ball and socket form free rotation of the upper limb is 

 possible, a movement of which no other mammal is capable. 



The study of the process of locomotion must begin with 

 the jelly-like projections (pseudopodia) in the Protozoa, 

 thence we pass to the water-animalculae, whose movements 

 depend on changes in specific gravity brought about by osmosis 

 and alterations in the air-content. When we consider move- 

 ment in its strictest sense, we have to deal with all the compon- 

 ent elements of the animal body which arise from protoplasm. 

 They are endowed with the common property of irritability, 

 that is to say, the power of contraction to certain stimuli. So 

 that we find possessed of this capacity for independent move- 

 ment leucocytes and lymph cells, mucous membrane and 

 connective tissue cells, ciliated cells, striped and unstriped 

 muscle fibres. 



It is the contraction of the striped muscle which in animals 

 that possess a skeletal frame, and in man, brings about changes 

 in the position of the joints and so procures movement. The 

 source of this muscular energy is not derived from disintegration 

 of nitrogenous material, but from the oxidation of carbon com- 

 pounds (the carbohydrate in the muscle). For the performance 

 of work all animals require a sufficiency of carbon-containing 

 food (carbohydrate, fat) ; the potential energy of any muscle is 

 by reason of the parallel arrangement of its fibres increased in 

 proportion to its transverse diameter, but it decreases as the 

 muscle contracts. 



Weber has estimated the amount of contraction, and has 



1 Munk-Schultz, lac. cit., pp. 379-83. 

 13' 



