EXTERNAL PARTS OF BIRDS. — FEATHERS. 
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cprtain birds, as the ostrich tribe, penguins, and toucans. If we compare a bird's skin to a 
well-kept park, part woodland, part lawn ; then where feathers grow is the woodland ; where 
they do not grow is the lawn. The former places are called tracts or pterylcB (dimin. from Gr. 
TTTcpov, pteron, a plume) ; the latter, spaces or apteria (Gr. a privative, and nrepov) ; they 
mutually distinguish certain definite areas. Not only are the pterylce and apteria thus definite-, 
but their size, form, and arrangement mark whole families and even orders of birds ; so that 
pterylosis becomes available, and is indeed found to be important, for purposes of classification. 
Pterylography, or the description of this matter, has been made a special study by the cele- 
brated Nitzsch, who has laid down the general plan of pterylosis which obtains in the great 
majority of birds, as follows: 1. The spinal or dorsal tract {pteryla spinalis; fig. 24, 1), 
running along the middle of the bird above from the nape of the neck to the tail ; subject to 
great variation in width, to dilation and contraction, to forking, to sending out branches, to 
interruption, etc. 2. The humeral tracts {pt. humerales ; Lat. humerus^ the shoulder, or upper 
arm-bone : fig. 24, 2), always present, one on each wing; they are narrow bands, running from 
the shoulder obliquely backward upon the upper arm-bone, parallel with the shoulder-blade. 
Fig. 24. — Pterylosis of Cypselus apus, drawn by Coues after Nitzsch; right hand upper, left hand lower, 
surface. 1. spinal tract; 2. humeral; 3. femoral; 4. capital; 5. alar; 6. caudal; 7. crural; 8. ventral. 
3. The femoral tracts {pt. femorales ; Lat. femur, the thigh ; fig. 24, 3) : a similar oblique 
band upon the outside of each thigh, but subject to great variation. 4. The ventral tract (pt. 
centralis ; Lat. venter, the belly ; fig. 24, 8), which forms most of the plumage on the under 
part of a bird, commencing at or near the throat, and continued to the vent ; like the dorsal 
tract, it is very variable, is usually bifurcate, or forked into right or left halves, with a median 
apterium, is broad or narrow, branched, etc. ; thus, Nitzsch enumerates seventeen distinct modi- 
fications ! The foregoing are mostly isolated tracts, that is, bands nearly surrounded by com- 
plementary apteria ; the following are, in general, continuously and uniformly feathered, and 
thus practically equivalent to the part of the body they represent : Thus, 5, the head tract 
(pt. capitdlis ; Lat. caput, capitis, head ; fig. 24, 4) clothes the head, and generally runs 
into the beginning of both dorsal and ventral tracts. 6. The wing tract {pt. alaris ; Lat. ala, 
wing ; fig. 24, 5) represents all the feathers that grow upon the wing, excepting those of 
the humeral tract. 7- The tail tract (pt. caudalis ; Lat. cauda, tail ; fig. 24, 6) includes 
the tail-feathers proper and their coverts, and those about the elceodochon, and usually receives 
the termination of the dorsal, ventral, and femoral tracts. 8. The leg tract (pt. cruralis ; 
Lat. crus, cruris, leg ; figs. 24, 7) clothes the legs as far as these are feathered, which is 
generally to the heel, always below the knee, and sometimes to the toes or even the claws. — 
I need not enumerate the apteria, as these are merely the complements of the pterylae. The 
