HAIR. 



[ 311 ] 



HAIR. 



Edentata (figs. 23 & 24). The difference 

 between the hair of the three-toed sloth 

 (fig. 23) and that of the armadillo (fig. 24) 

 is well-marked. In the former , the cortical 

 cells take a remarkably oblique or radiating 

 course, whilst in the latter they run longitu- 

 dinally. 



Rodentia (figs. 25-35). In this Class the 

 pigment is met with sometimes in the me- 

 dulla, at others in the cortex. The arrange- 

 ment of the air-cells is often very beautiful, 

 and has rendered these hairs favourite micro- 

 scopic objects. Portions of a mouse-hair in 

 various parts of its length are represented in 

 fig. 27, a forming the free end. Fig. 28 dis- 

 plays two portions of the same hair as ana- 

 lysed by treatment with solution of potash. 

 The cortical parts have not been resolved into 

 their component cells ; whilst those of the 

 medulla have assumed their rounded and 

 natural form, and exhibit minute granules of 

 pigment, with larger globules of fat. The 

 arrangement of the medullary cells in two 

 rows is seen in fig. 286. The pigment 

 within the cells in situ is seen in fig. 31 b, 

 from the rabbit. The wool presents its cha- 

 racters in a marked degree ; the projection 

 of the outer layer of cortical cells, and the 

 distinctness of the medullary air-cells being 

 very evident. 



Marsupialia (figs. 36 & 37). In this 

 curious class the hair greatly resembles that 

 of the rodents. That of the Kangaroo pre- 

 sents very beautifully imbricated cortical 

 cells (fig. 36). 



Monotremata. The structure of the hair 

 of the Ornithorhynchus is as peculiar as that 

 of the animal in general. It presents that of 

 hair and wool combined (fig. 38). The basal 

 portion resembles wool, and is very long and 

 narrow ; the structure of two pieces in dif- 

 ferent parts of its length is seen in fig. 38 c 

 and d. At the end of this portion is attached 

 the proper hair containing the pigment 

 within the cortical substance (b) ; fig. 38 * 

 represents the surface-view of the hair, 

 showing the imbricated scales. 



In Birds the hair is replaced by FEATHERS. 



The hair of the Invertebrata does not 

 present the same structure as that of the 

 higher animals. Some physiologists have 

 therefore limited the term hair to the filiform 

 epidermic formations of the Mammalia; 

 whilst others admit the occurrence of hair in 

 all classes of the animal kingdom. At all 

 events, the hairs of the Invertebrata are not 

 usually composed wholly of epidermis. They 

 consist of an outer cortical or epidermic 



layer, frequently coloured, and upon which 

 their firmness depends ; lining this, is 

 sometimes a prolongation of the cutis ; at 

 others a colourless substance, which, when 

 the hair is dried, presents an irregular cell- 

 like appearance and contains air, so as to 

 resemble the air-cells of the hair of the 

 Mammalia. In other instances the hair is 

 completely solid, but exhibits no trace what- 

 ever of cell-structure. It remains to be 

 shown whether the latter may represent the 

 epidermis hardened in an amorphous state ; 

 and whether those lined with cutis may be 

 regarded as epidermic formations upon an 

 exserted papilla of the skin; whilst those 

 presenting the air-cells when dried correspond 

 to an outer hardened epidermic layer, and an 

 inner retaining its distinctly cellular state. 

 In those lined with cutis, the circulation can 

 sometimes be observed. 



We have only space to notice a few in- 

 stances of variety of form, many of which 

 occur, and have long rendered these hairs 

 interesting and elegant microscopic objects. 

 Thus, in some of the Arachnida they are 

 feathery, giving off slender lateral branches, 

 as in Lycosa (PI. 22. fig. 41), Epeira (PL 2. 

 fig. 8 a), Acarus (PI. 2. fig.l b), &c.; in others 

 these branches are directed forwards near the 

 middle of the shaft, but recurved at the end, 

 as in My gale (the bird-catching spider) (PL 

 22. fig. 4 1 ); or while the branches on the shaft 

 resemble the above, the end of the hair is 

 thickened, cylindrical and longitudinally stri- 

 ated, with minute setae arising from the striae, 

 as in fig. 42; again, some of them are simple, 

 but furnished with spiral striae (Epeira,~PL2. 

 tig. 86); in Trombidium they are sometimes 

 very elegantly feathery. 



In Insects, Arachnida, &c., they often ap- 

 pear to arise from a bulb at the base, but 

 the bulb is annular, not solid, and bears no 

 resemblance in structure to the bulb in the 

 Mammalia; it consists of a thickening or 

 fold of the epidermis of the skin, not of the 

 hair, from which it is separated by a white 

 ring, indicating thinness of this coat, and 

 often corresponding to a joint; the hair 

 arises from the base of a depression situated 

 within the annular bulb. The hair of the larva 

 of Dermestes is very beautiful, and is used as a 

 TEST-OB j ECT. Two forms are met with : in 

 one (PL 1 . fig. 1 c) the shaft is simply covered 

 with densely aggregated, minute, spinous, 

 secondary hairs; in the other (PL 1. fig. 1 

 a, b), the spines or scales upon the shaft are 

 narrow, acute, and placed in pretty regular 

 whorls ; in the uppermost whorl they are 



