THE MUSCULAR SYSTEM 63 



limb, traceable in one form or another throughout the higher 

 vertebrates, and also the serial homologies of the parts are indi- 

 cated in Fig. 34, while Fig. 36 illustrates the modifications which 

 have occurred in the bones of the wrist and ankle in a few familiar 

 species of animals. Such modifications (mainly reduction of the 

 number of separate bones by fusion) are always greater in the 



Carpus Tarsus Carpus Tarsus 



Primitive Snapping Turtle 



Qo^cP P0OC2P oSgdO 0(^ 



Rabbit* Cat 



^^^'' ^^P ogpo c^p 



Ulan Horse 



Fig. 36. Diagrams representing the carpal and tarsal bones in a primi- 

 tive condition and in live adult animals: c, centrale; f, tibiale; i, 

 intermedium; r, radiale; t, fibulare; u, ulnare; x, postminimus; 1-5, 

 distal carpals or tarsals. In the tarsals of the snapping turtle four 

 proximal and central elements are fused. In the carpus of the rabbit the 

 two centrals are fused and displaced into the distal row, and. as in 

 all the mammals, distal carpals 4 and 5 are fused to form the hamate 

 bone. Also the mammals represented have a sesamoid bone, the pisi- 

 form, added to the proximal row in about the position of the postmini- 

 mus of the turtle, but the pisiform and postminimus are not homologous. 

 The tarsus in all cases has the tibiale and intermedium fused as the 

 talus, and the fourth and fifth distal elements fused as the cuboid. 

 In the rabbit the first distal tarsal is fused with the second metatarsal. 

 Other fusions and losses are indicated in the representations of the 

 other species. 



posterior than in the anterior limb. In some kinds of turtles, the 

 wrist shows the primitive pattern practically unmodified, w^ith an 

 extra element (the postminimus) which may possibly reflect an 

 ancient condition when there were more than five digits represented. 



The Muscular System 

 Involuntary Muscle 



As would be expected considering the nature of their functions, 

 the contractile tissues are not arranged in a definite, continuous 

 system as are most other organ complexes of the body. Smooth 

 or involuntary muscle fibres, modified mesenchyme cells'^ of the 



