PTERYLOSIS 15 



the reader is referred to the systematic part of this work. 

 In all, however, there are the following tracts present : — 



(1) The spinal tract runs from the head to the oil gland. 

 This tract is sometimes continuous at the sides of the neck 

 with the ventral tract, to be described next. It is sometimes 

 a single solid tract throughout, but more usually there is a 

 space developed in it, a n apterion in the back, which is of 

 greater or less extent. Commonly there is a break, more or 

 less distinct, between the anterior and the posterior portion 

 of the tract, which may be complete or may consist in an 

 abrupt transition between anterior stiffer feathers and 

 posterior slighter feathers. 



(2) The ventral tract is always a double tract, but the 

 median apterium may be very narrow. The anterior part, 

 of the tract may be single, but sometimes it is double from 

 its very origin. Very commonly on the pectoral region 

 each half of the ventral tract gives off a lateral branch. 



(3) The humeral tract is a band of stiff feathers running 

 across the humerus ; it is always present and shows no parti- 

 cular modifications. 



(4) The femoral tract is a corresponding band crossing 

 the thigh. There is sometimes, as in the barbets, a small 

 tract lying between the femoral and the spinal ; and, besides 

 the main tracts, the patagium and the lower leg are more or 

 less covered with contour feathers. The study of Pterylo- 

 graphy was first taken up in a systematic manner by Nitzsch ; 

 since his day the number of facts has largely increased, and 

 careful figures of the pterylosis of many birds, not figured 

 by NiTZSCH, have been published by a host of observers, 

 especially Gaeeod, Foebbs, Shufeldt, Pybcraft, Gadow,' 

 and others. For references to these see the descriptions of 

 the different orders of birds.^ 



The general facts of pterylosis must be used cautiously. 



> See also W. Makshall, ' Pterologisohe Mittheilungen,' Zool. Gart. xiv. xv. 

 xvi. 



^ For the musculature of the feathers see Helm, ' Ueber die Hautmuskeln 

 der Vogel,' &c., J.f. 0. 1884, p. 321. These skin muscles are either limited to 

 the skin, running from feather to feather, or are parts of skeletal muscles, such 

 as the cutaneous branch of the latissimus dorsi, (Sec. 



