TEGUMENTARY ORGANS 



429 



above the class of Fishes, in either Reptilia or Birds, but in Mammalia 

 there are structures which must, I think, be placed in the same category. 

 About the lips and nose of almost all mammals in fact, there are 

 •certain long, strong hairs, the vibrissse or "whiskers" {fig. 321). 

 These in their general structure resemble ordinary hairs, but the sac 

 of each, instead of lying free in the enderon, is enclosed in a second 

 thick sac, composed of firm, dense, connective tissue, which attains at 

 times an almost cartilaginous hardness. A looser areolated tissue 

 connects this with the outer 



surface of the proper hair sac, 

 and supports an abundant vas- 

 cular network proceeding from 

 vessels which enter at the deep 

 •end of the sac. Furthermore, a 

 very considerable nerve pierces 

 •one side of the " sclerotic " coat 

 near this end, and passes to the 

 surface of the proper hair sac, 

 upon which it spreads out and 

 forms a nervous expansion, its 

 fibrils dividing and subdividing, 

 and so terminating. 



Considering the different habits 

 of life of the mammal and the 

 fish, I think one cannot but be 

 struck with the similarity of plan 

 between their vibrissje and the 

 *' tactile " canals. The sensory 

 impression is conveyed to the 

 gelatinous contents of the canals 

 in the fish by the vibration of the 

 dense medium in which it lives ; 



while in the mammal the impulse is communicated by the contact 

 of some external object with a long elastic hair lever ; but the final 

 arrangement for the receipt and appreciation of the impressions is 

 essentially the same in each case, nor indeed does it differ from that 

 which is met with in the highest organs of sense. 



Muscles of the enderon. — In the Invertebrata the great majority of 

 the muscles are, as is well known, inserted into the integument, but 

 those which are attached to the chromatophora of molluscs and to 

 the spines of annelids and other worms, might be regarded as 

 belonging more especially to the integumentary system. 



Fig. 321. — Vibrissa from the snout of the 

 Mouse, a, " sclerotic " sac ; b, hair-sac ; 

 c, nerve-trunk ; rf, muscular fibres. 



