The connection of these sacs and canals with the corpuscula tactus and Pacinian bodies perfect connective tissue occupying the centre of the papilice, and further distinguished by having their endoplasts and imperfect elastic appears to me to be clear; for the knob which projects into the cavity of the mucous canal is homologous with the central " nucleus " of the Savian body, and this with the solid axis of the Pacinian body,and with the corpusculum tactits, so that the " tactile " sac of the Chi mmra, e. g., may be said to be a tactile cor puscle which is connected with the surface of the integument.
No organ at all resembling these has cer tainly been met with, above the class of Fishes, in either Reptilia or Birds, but in Mann:Ita lia 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 vi brissm 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 .vascular net work 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.
mammal and the fish, I think one cannot but be struck with the similarity of plan between their vibrissm and the " tactile" canals. The sensory impression is conveyed to the gelati nous 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 appre ciation 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 chromato phora of mollusks and to the spines of an nelids and other worms, might be regarded as belonging more especially to the integu mentary system.
In Fishes and Reptiles the superficial layer of striped muscles of the body is always more or less connected with the integument ; but hitherto no unstriped fibres appear to have been detected in it. In Birds, however, the unstriped muscles attain a very great develop ment, forming a thick layer whose bundles (c) run between and are attached to the sacs of the feathers (fig. 322.).
In the majority of Mammals there is a special tegumentary striped muscle, which attains an enormous development in the hedgehog, while a mere rudiment of it remains in man, as the platysma myeides. Ilere, however, the striped "peaucier" muscle is replaced by the unstriped bundles which, as Kelliker has shown, run from the upper layer of the enderon to the bases of the hair sacs, and effect the various movements of which the hairs are capable.
Considering the different habits oflife of the Calcareous deposits in the macron. —Deposits of this kind are very frequent in the Inverte brata. In the Pulmonate and some Gasteropod Mollusks, for instance, globular masses of car bonate of lime are scattered through the en deron, and would almost seem to take the place of fat. In nudibranchiate niollusks, such as the Doridx, spicula of like nature are met with, and these sometimes unite into true internal shells, as in the genus Villiersia. The greater part of the skeleton of the Actinoid polypes, and the whole of that of the Echinoderms, is composed of calcareous networks of this kind, and globular masses of calcareous matter are scattered through the enderon of the Twniadm, though the clear spherical bodies observed in these worms are by no means always of this nature. 'Whether these videronic calcareous deposits ever take place in the Vertebrata ap pears to me to be, as I have said above, an open question, only to be decided by a very careful examination of the mode of growth of their so-called "dermal" bones.