THE ANATOMY OF THE RUFFED GROUSE 727 



crest, parallel to the shaft. There is also another crest laterally. On the anterior aspect of 

 the distal end is a bony bridge through which passes the tendon of M. extensor digitorum 

 communis. Just lateral to this is a small tubercle. The articular surface of this end of the 

 tibia is distinctly pulley-like with the groove more pronounced anteriorly. 



The fibula is a long slender bone lying along the outer side of the tibia for nearly two- 

 thirds of its length and finally joining with it at its tip. Just below the head, it is fused with 

 the tibia for a distance of about 15 mm. where a ridge forms to meet it. Its head is enlarged 

 and articulates with the fossa of the external condyle of the femur, but the shaft narrows 

 quickly and tapers to a point distally. 



The tarso-metatarsus is grooved on its upper surface for the passage of tendons. At its 

 proximal end there is a small tubercle just internal to the groove to which the tendon of the 

 M. tibialis anticus attaches. On the lower surface at this end, there are two short flanges of 

 bone parallel to the shaft forming a passageway for the flexor tendons. Both Lagopus and 

 Tympanuchus have the medial one of these two flanges extending nearly to the distal end of the 

 tarso-metatarsus. The distal end of the tarso-metatarsus consists of three knobs, of which 

 only the middle (that of digit III I is a trochlea with two condyles. Between the tips of the 

 two projections, corresponding to digits III and IV, is a foramen with a distinct groove run- 

 ning up to it. The phalanges are all essentially similar. Except for the basal phalanges, the 

 proximal end of each has a facet with double indentations for the articulation of the two distal 

 condyles of the preceding member. The accessory metatarsal (bearing digit I, the hind toe) 

 articulates with a facet on the ventro-medial aspect of the tarso-metatarsus. 



In comparison with the other members of the Tetraonidae, it is seen that Bonasa is the 

 lightest built. Most of the differences, however, are merely of degree. Bonasa has a higher 

 pelvis than the others. The large posterior iliac cornice of Tympanuchus is characteristic of 

 that group. The other variations are slight. It seems of interest that there is greater variation 

 between the genera in the pelvis and the vertebral column than in the bones of the extremities. 



TABLE 99. AVEIIAGE SKELETAL MEASUUEMEMTS OF 16 ADULT GIJOLSl-: 



The Musculature* 



Leg. Garrod"" developed a leg muscle formula for use in classification in which A repre- 

 sents the presence of M. caudofemoralis; B, M. psoas; X, M. flexor cruris lateralis; and Y, M. 

 femorocruralis. The same author also considered the presence of M. ambiens and the vin- 

 culum between M. flexor longus digiti I and M. flexor profundus, but he did not represent 

 either of these by the use of a letter. Hudson°°' has added the letters Am (ambiens), V 

 (vinculum), C (M. iliacus, pars medius), and D (M. piriformis). In Bonasa all these ele- 



* Only the major voluntary muscles are discussed in this account. 



