514 VERTEBRATE LIFE AND ORGANIZATION 



complexiLy of tetrapod appendiculai muscles, it is possible to divide 

 tliem into a dorsal group that evolved from the fish extensor and a 

 ventral group derived from the flexor. Our latissimus dorsi and triceps 

 (Fig. 25.9), for examjjle, are dorsal appendicular muscles, whereas the 

 pectoralis and biceps are ventral appendicular muscles. Segmentation 

 is lost for the most part as one ascends the evolutionary scale, though 

 traces of segmentation remain in the mammalian rectus abdominis. The 

 muscle layers on the flank became relatively thin, and some trunk mus- 

 cles, the serratus anterior, for example, became associated with the 

 pectoral girdle. 



Branchial muscles are well developed in fishes, and are grouped 

 according to the visceral arches with which they are associated (Fig. 

 25.8). Branchial muscles obviously become less important in tetrapods, 

 for the gills are lost and the visceral arches are reduced. Nevertheless, 

 certain ones are retained. Those of the mandibular arch remain as the 

 temporalis, masseter and other jaw muscles (Fig. 25.9). Most of those 

 of the hyoid arch move to a superficial position and become the facial 

 muscles that are responsible for smiling and other facial expressions. 

 Those of the remaining arches are associated with the pharynx and 

 larynx and some, e.g., the sternocleidomastoid and trapezius, even 

 acquire attachments onto the pectoral girdle. 



Questions 



1. Of what value is the accumulation of keratin in the skin of tetrapods? 



2. How would the structures in the skin that are concerned with thermoregulation in- 

 teract to reduce the body temperature of a mammal? 



3. Give an example of a bone in the human skull that is derived from each of the three 

 basic components of the skull. 



4. What changes are encountered in the visceral skeleton as one ascends the evolutionary 

 scale from fish to mammal? With what are these changes correlated? 



5. What changes in the muscular system are correlated with the changes in the method 

 of locomotion encountered between fish and mammals? 



Supplementary Reading 



Romer's The Vertebrate Body is an excellent reference for those wishing to pursue 

 further the morphologic aspects of the evolution of the organ systems described in this 

 and subsequent chapters. This book is also available in a condensed edition entitled A 

 Shorter Version of the Vertebrate Body. Comparable references to summaries of animal 

 physiology are difficult to find, but Prosser, Brown, Bishop, Jahn and Wulff, Comparative 

 Ammul Physiology, is an extremely valuable source book. Mammalian physiology is 

 considered in detail in such medical texts as Fulton's A Textbook of Physiology or Guy- 

 ton's Textbook of Medical Physiology. Less detailed and very readable accounts are to 

 be found in Carlson and Johnson, The Machinery of the Body, and in Cannon The 

 lUsdom of the Body. An excellent analysis of the role of the muscles in the various types 

 of vertebrate locomotion can be found in Gray's little book. How Animals Move. 



