Letters and divers other mixt discourses in natural philosophy many of which were formerly published in the Philosophical transactions of Mr. Oldenburg, and part in the Philosophical collections of Mr. Hooke and else where : all which are now revised, augmented, and to them are added very many other matters of the same nature, not before published : also an intire treatis of the nature and use of colours in oyl. painting / written by M. Lister, F. of the R.S.

A Second Letter writ about the same time; 1673. to the same Person.

I did not think of explaning my sense of the use of the Caecum until I had had the Leisure and opportunty of purposely examining the I••testines of most kind of Animals. But because I am much mistaken by the person, who, as you tell me, is desirous to be anonimous to me, him I mean who raised the scruples you sent me, upon the Second Paragraph, which says, the use of the Intestinum Caecum to be subservient to that of the Colon, and Rectum; manifest in such Animals where Nature in∣tends a certain, and determinate Figure to the Excrements. I shall be forced to tell you, what I presume may prove, as neer the truth, as any one of the many con∣jectures extant in Authors, about the unknown use of this part. I understand by determinate figure. First, the Excrements divided into many small parts of a like shape, such as Sheep, Deer, Conies, Rats, Mice, Horses Catterpillers, Some Snailes &c. doe void. Secondly, in a greater Latitude, I oppose figured Excrements to Liquid, as C. Celsus in some place doth; Thus the dung of Pigeons, and Geese, of Men, Dogs, Cats, &c. may be said to be figured. Now the Caecum, in my opinion, is subservient in some measure to the figuration of both, but most manifestly in the first kind. My meaning is that pro∣bably the use of the Caecum is to keep the Excrements, that passe into its cavitie (and I believe all, or most part of them do in sound Animals) so long, until they are sufficiently drained, baked, hardned, or of a due consi∣stence, (as clay is temperd for the mould) to receive the Figure to be given from the Colon and rectum. This use I say of the Caecum, seems to me to be much more manifest in such Animals, as have figured Excrements of the first kind. In Ratts for example, (whose Excre∣ments

are the most elegantly and constantly a like figu∣red, of any Sanguineous Animal I have met with) the Caecum is more large, and capacious, than the stomack it self, and perhaps than all the small guts put toga∣ther. But its use in receiving the Excrements or ex∣hausted Chyle, is not more apparent, from its large capacity; than that other of further draining and tem∣pering them to a stiffnesse, for the service of the Co∣lon, from the admirable contrivance, and structure, of this latter Gut, which is a Phaenomon that deserves further consideration: it is I say to be noted, that im∣mediately under the Value of that Gut, in this Ani∣mal, are certain Spiral Fibres, which make a kind of screw. Now it seems to me, that the Excrements, after they are brought to a due Consistence by the Necessarie stay they make in the Caecum, and being car∣ried out thence into the Spiral folding, or screw of the Colon, cannot descend in a perpendicular, as formerly through the small Guts, but still gently glide very lei∣surly by the vinding of the screw; whence arises their Figure.

And I am apt to believe, that if the Caecum of a Ratt, or any of the first kind of Animals mentioned, was tyed up, or otherwise hindred from its receit, the Animals would unavoidably fall into a Diarrhaea: there being, I say, no reason, that I can foresee, why the yet liquid Excrements or exhausted Chyle, such as we constantly find it, even at the very bottom of the small Gut, should slop at the entrance of the Colon, and not speedly glide through the screw, in a down right descent, that is, elude the devise of nature, and make the configuration of that so curiously contrived part uselesse, we I say supposing the Experiment to have taken away the ne∣cessarie Diverticulum and repositarie of the unprepared Excrements, in tying up the Caecum.

I know not, whether the observation will hold good in general Terms, because I say, I have not yet pur∣posily examined divers Animals in nature, viz. That the more accurately figured the Excrements of any A∣nimal are, the more capacious is the Caecum, and on the contrary the lesse figured and liquid they are, the lesser the Caecum, or none at all. This is true certainly that some Animals, which are naturally loose have no Caecum at all or very little, as the Talpa, the Echinus terrestris the Gulo, a certian kind of voracious Woolfe, mentioned by Bartholine in his Observations.

We shall not trouble you at present with our obser∣vations concerning the different Figure of Excrements in the divers Species of Animals already by us examin∣ed, nor of the place and of their becoming so figured. Also we shall passe by our thought for the present of the manner of the Caecums reception and preparing the Excrements.

For the Colon, we likewise sorbear to offer some doubts we have, concerning natures end, in the neces∣sarie Figuration of Excrements in some Animals, as first to prevent Diarrhaeàs; Secondly to abide hunger the better; thus Snails in Winter rest with full Intes∣tines; Thirdly to heighten the firmentation and diges∣tion of the stomack and small guts.

What we have hastly writ at present, being only in∣tended for the better uuderstanding of that Paragraph, and not all that this subject would incite me to say.

If it shall be objected, that grant the reception of the exhausted Chyle to be made in the Caecum, before it passe into the Colon: yet it seems that either we must give a power of choice to the Caecum, or what just comes in, will first be thrown out, it still being uppermost, that is, the lesse prepared excrement. I answer, that I do not conceive, what choiee or distinction Sheep, can

make of the meat not ruminated, in the stomack, from that, which is but just now ruminated, and swallowed down, since all the many stomacks of a sheep are but one stomack and but one Gula, that is, in that respect of ruminating the stomack and Gula of a Sheep or Cow, is an other Caecum; and yet in ruminating nature has its aime, and chews not things oftner over, than needs must; the like we think of the office of the Caecum, which parts only with what is duly prepared, and retains the yet liquid Excerment.