THE ANATOMY OF THE RUFFED GROUSE* 



By David E. Davts 



The ruffed grouse is a member of the family Tetraonidae and. like other Galliformes, has 

 a generalized structure, as shown by the presence of two carotid arteries and a gall bladder, 

 and the absence of a penis. The intestines are looped in a primitive manner within the body 

 cavity. There is no hemipterygoid bone. The palate is schizognathous. However, the para- 

 sphenoid rostrum (basjsphenoid rostrum) has specialized knobs for the articulation of the 

 pterygoids which are not homologous with the basipterygoid processes. 



A notable specialization is a cutaneous muscle lying beneath each tract of ruff feathers. 

 This muscle is about two centimeters long by one centimeter wide at the base and is somewhat 

 triangular with the apex pointed ventro-anteriorly. The feather follicle extends into the 

 muscle, and the contraction causes the rachis to rise away from the neck. Edgew'orth'" states 

 that M. cucullaris is not developed in birds and its functions are taken over by M. cranio- 

 cervicalis. The ruff muscles are probably a derivative of M. cranio-cervicalis. 



The Skeleton 



Skull. The skull is illustrated in figure 77 a-c. The premaxillary bones are concave down- 

 ward and pointed anteriorly to form the top of the beak while posteriorly they terminate in 

 three prongs. The nasal processes are dorsal and pass between the nasal bones as far back as 

 the frontals. The maxillary process on each side passes backward along the border of the jaw 

 to form part of the jugal arch. The palatine jirocesses extend on the surface of the palate 

 to join the palatine bones on each side. 



The maxillary bones lie posterior to the premaxillary bones and each has two processes. 

 The ])alaliiie process lies along the palatine bone and the jugal process forms most of the 

 jugal arch. 



The nasal bones are broad and are sutured to the frontals. Each has two anterior pro- 

 cesses. The premaxillary process is a thin pointed sliver which lies along the external side 

 of the nasal process of the premaxilla. there being an open slit between the two. The maxil- 

 lary process extends downward to join the maxillary bone. The external nares are holorhinal, 

 formed by the premaxillaries in front and the nasals behind. The nasals are sutured to the 

 frontals in such a manner that a forward extension of the frontal lies under the nasals but is 

 not fused with them. 



The frontal, comprising the surface of the skull in the interorbital region, is concave up- 

 ward and somewhat narrowed. Behind this lies the parietal, a thick spongy bone which makes 

 up the hinder part of the top surface and extends down to the supraoccipital bone to form 

 most of the posterior aspect of the skull. The latter lies directly above the foramen magnum. 

 These bones of the top and back of the skull are paired during early development but in the 

 mature bird are so completelv ossified that neither the sutures between the |)arts nor between 



* The sttuly upon which this account is based was undertaken by Dr. Davis in collaboration with the New York State Con- 

 servation Department and under the auspices of the Museum of Comparative Zoology. CambridRe. Mass. The author 

 is indebted to the late Dr. Glover M. Allen for innumerable suggestions and constant advice, as well as to Miss 

 Katherine Wilson who assisted in the preparation of the drawings. .\cknowledgment is also due to the late John C. 

 Phillips for specimens utilized in dissection. 



