THE PIGEON 193 



the large sartorius muscle, which forms this border; the much 

 smaller ilio-tibialis, a variable portion of the distal end of the 

 femoro-tibialis ; the very broad pubo-ischio-femoralis, which forms 

 the middle portion of the thigh ; the semimembranosus ; and the 

 semitendinosus, which forms the posterior border of the thigh. 



On the ventral surface of the middle division of the leg (the 

 shank) are the tibialis muscle, which forms its anterior portion, 

 and the gastrocnemius, which forms its posterior portion. 



Determine so far as possible which of these leg muscles are 

 extensors and which are flexors. 



Exercise 33. Draw an outline of the ventral aspect of the animal and 

 place in it those muscles which have been observed. 



The Skeletal System. This is made up of the exoskeleton and 

 the endoskeleton. The exoskeleton is composed of certain special 

 integumentary structures : the feathers, the horny covering of the 

 bill, and the scales and claws of the feet. The endoskeleton is the 

 bony and cartilaginous framework of the body. 



The Exoskeleton. Three kinds of feathers are present: the 

 contour feathers, the down feathers, and the filoplumes, or pin- 

 feathers, which were described on pages 160-161. Study the struc- 

 ture of a contour feather. It is made up of two portions: the 

 quill, the proximal cylindrical portion which projects from the 

 skin ; and the vane, the distal flattened portion. The quill is hol- 

 low and has two openings : one at the proximal end, the inferior 

 umbilicus ; and one at the base of the vane, the superior umbilicus. 



The vane is composed of a central axis, the shaft (the con- 

 tinuation of the quill), which is solid and is square in cross 

 section ; the barbs, lateral outgrowths of the shaft ; and the bar- 

 bules, minute projections of the barbs. The barbules are usually 

 provided with hooklike outgrowths which enable the overlapping 

 barbules of contiguous barbs to take firm hold of one another 

 and so convert the vane into an unbroken surface. At the base 

 of the vane these hooks are absent. 



Exercise 34. Draw a sketch of a contour feather showing the quill, 

 vane, shaft, and barbs. Draw also one or two barbs, with their 

 barbules and hooks, as seen under a microscope. 



