目とは「光」という刺激を受け取る感覚器です。ここでは目の仕組みについてわかりやすく解説したいと思います。
「ものが見える」というのは目に光が入っているということ
普段、わたしたちは「ものが見える」ということを特別意識せずに生きています。しかし、ものが見えることには絶対に必要な要素が三つあります。
目・光・物体
ものが見えるとき、そこには必ず光があります。目は受容器、もしくは感覚器と呼ばれる動物特有の器官です。受容器とは動物が外からの刺激を受け取る器官のことです。目という受容器に光が入ることで、わたしたちは「ものが見える」と思うのです。
植物には目はありませんから、ものが見えることはありません。ただし植物も光を感じることはできます。植物と話すことはできませんので、植物がどのように光を感じているかはわかりませんが、それは動物が持つ視覚のような感覚ではないと想像されます。
光がなければものは見えません。完全な暗闇というのは滅多に存在しないと言ってよいでしょう。夜に電気を消していても、月明かりがありますし、新月の夜でも星明かりがあります。しかし、星明かり程度では、ものの輪郭がうっすらと見えるくらいでしょう。
そして、「ものが見える」には物体が必要です。物体そのものが光を発していて、その光が目に入る場合と、光を発していない物体が反射した物体が目に入る場合の2パターンがあります。前者は光源といって、太陽や電球やスマートフォンの画面がこれにあたります。後者は月や机や本やほとんどのものがそうですね。
物体は光を吸収することができるものが多いのですが、それぞれ吸収することのできる光の色が異なります。緑と黒と白の3色に塗られた傘があったとしましょう。太陽からはすべての色の光が届きます。そして、傘の緑の部分は緑色の光だけを反射していて、その緑色の光がわたしたちの目に入ることで「緑色だ」と感じることができるのです。赤いランドセルは赤色の光以外は吸収して、赤色の光を反射しているということなのです。
同じ傘の黒と白の部分はまたちょっと違います。黒はすべての色の光を吸収している部分なのです。なので、光がない夜の空は黒いのです。光があるときは、空気が青い光だけを反射させて、青い光が空全体に広がっているので、昼の空は青く見えます。
また白の部分は、すべての光を反射している部分です。すべての光が反射して合わさると白になるとはちょっとイメージがつきにくいですが、滝や波の水しぶきが白く見えるのは、水が白いからではありませんね。水は透明です。しかし、小さな空気の泡が多くあるとき、光が当たったら泡に反射して白く見えるのです。
なので、目と光と物体、この三つがそろってはじめて「ものが見える」ということになるのです。
わたしたちは過去を見ている
いまご説明したように、ものが見えるということは反射された光を見ているということなのですが、光がもっとも速度が速い物質なので、わたしたちは普段気にも止めていませんが、それでもある地点から別の地点へ行くのには時間がかかります。なので、見ている物体から反射した光がわたしたちの目に入る間にも、一瞬ですが時間がかかっています。なので、わたしたちが物質をみているとき、それはその物体の「過去を見ている」ということになるのです。
太陽と地球は1億4960万kmはなれています。この距離を光の速度で割ると、約8分になります。つまり、太陽から放たれた光が地球に届くまでには約8分間かかるということです。つまり、わたしたちが今見ている太陽は、8分前の太陽の姿なのです。なので、もしいまこの瞬間、太陽で大爆発が起こったとしても、わたしたちがそれを知るのは8分後になってしまうのです。
地球は天の川銀河という銀河の片隅にあります。天の川銀河の隣にある銀河はアンドロメダ銀河というもので、この銀河は地球から目視することもできます。もしアンドロメダ銀河に知的生命体がいて地球を観察できるような超強力な望遠鏡を持っていたとしたら、その知的生命体が見ている地球には恐竜が大地を闊歩していることでしょう。地球からアンドロメダ銀河に光が到達する時間はそれほどの時間がかかるのです。
光を受け取る目の仕組み
ものが見えるということがどういうことかわかったところで、光の受容器である目の仕組みについて考えていきましょう。
目には視細胞というものがあります。視細胞は光を感じることのできる細胞です。ミミズには目はありませんが、身体の表面に視細胞が散在しています。なので、全身で光を受容しているといえますね。しかし、ほとんどの動物は目という視細胞が1箇所に集まった光受容器官を持っています。
網膜には大きく分けて視細胞・連絡細胞・視神経という三つの種類の細胞があります。
まず、光を吸収すると視細胞が酵素を活性化させます。するとある物質の濃度が上がり、細胞膜の電位変化が起こります。その電気シグナルが連絡細胞を経由して、視神経に伝わり、さらにいくつかの神経を乗り継いで、最終的に大脳の後頭葉の視覚中枢に伝えられます。
光の刺激を受け取ることができるのは視細胞だけで、連絡細胞や視神経は刺激を受け取るはたらきはありません。
・ リチャード・ドーキンス『ドーキンス博士が教える「世界の秘密」』
・ 吉田邦久『好きになる生物学』
コメント
[…] 細胞ができてから死ぬまでの平均寿命は細胞によって異なります。 ヒトの細胞では、たとえば、寿命の短い細胞として小腸の上皮細胞があります。この細胞の寿命は約1日です。皮膚の表皮細胞や白血球は3～4日です。 寿命が長い細胞の… 摩訶不思議な目の仕組み – ものが見えるとはどういうことか […]
Im thankful for the article. Will read on…
I’ll right away snatch your rss feed as I can’t find your e-mail subscription hyperlink or e-newsletter service. Do you have any? Kindly permit me recognize so that I may just subscribe. Thanks.
I would like to thnkx for the efforts you’ve put in writing this site. I am hoping the same high-grade website post from you in the upcoming also. Actually your creative writing abilities has encouraged me to get my own blog now. Actually the blogging is spreading its wings fast. Your write up is a great example of it.
WONDERFUL Post.thanks for share..more wait .. …
After all, what a great site and informative posts, I will upload inbound link – bookmark this web site? Regards, Reader.
I know this if off topic but I’m looking into starting my own blog and was curious what all is required to get set up? I’m assuming having a blog like yours would cost a pretty penny? I’m not very internet smart so I’m not 100 certain. Any suggestions or advice would be greatly appreciated. Thank you
You made some decent points there. I looked on the internet for the issue and found most individuals will consent with your website.
Good day very nice web site!! Guy .. Excellent .. Superb .. I’ll bookmark your web site and take the feeds additionally?KI am glad to find a lot of useful information right here in the publish, we want develop extra techniques on this regard, thanks for sharing. . . . . .
This website is mostly a stroll-by way of for all of the information you needed about this and didn’t know who to ask. Glimpse here, and also you’ll positively uncover it.
I like this blog its a master peace ! Glad I found this on google .
Very interesting details you have mentioned, regards for putting up. “What the world really needs is more love and less paperwork.” by Pearl Bailey.
Great wordpress blog here.. It’s hard to find quality writing like yours these days. I really appreciate people like you! take care
Merely wanna remark on few general things, The website design and style is perfect, the content is really great. “To imagine is everything, to know is nothing at all.” by Anatole France.
Can I just say what a reduction to seek out someone who actually knows what theyre talking about on the internet. You positively know how one can bring a problem to gentle and make it important. More individuals have to learn this and perceive this facet of the story. I cant consider youre not more in style because you undoubtedly have the gift.
Everything is very open and very clear explanation of issues. was truly information. Your website is very useful. Thanks for sharing.
Greetings from Florida! I’m bored to death at work so I decided to browse your website on my iphone during lunch break. I love the info you present here and can’t wait to take a look when I get home. I’m shocked at how fast your blog loaded on my phone .. I’m not even using WIFI, just 3G .. Anyways, awesome blog!
Everyone loves what you guys tend to be up too. Such clever work and exposure! Keep up the fantastic works guys I’ve added you guys to our blogroll.
Awesome blog. Will read on…
I do consider all of the concepts you’ve introduced on your post. They’re very convincing and can certainly work. Still, the posts are very short for newbies. May just you please lengthen them a little from subsequent time? Thank you for the post.
Hi my friend! I wish to say that this post is amazing, nice written and include approximately all significant infos. I would like to see more posts like this.
I am continuously invstigating online for posts that can facilitate me. Thx!
Enjoyed studying this, very good stuff, appreciate it.
buy atorvastatin 20mg online order lipitor 10mg generic atorvastatin over the counter
Wow, wonderful blog layout! How long have you been blogging
for? you make blogging look easy. The overall look of your website is great, let alone the content!
You can see similar here sklep internetowy
You are my inhalation, I have few blogs and occasionally run out from to post : (.
order cipro 500mg pill – ethambutol 1000mg usa augmentin 375mg ca
buy ciprofloxacin no prescription – buy ethambutol generic buy augmentin 1000mg generic
ciplox generic – chloromycetin pills erythromycin 500mg uk
where can i buy metronidazole – metronidazole 400mg pills buy azithromycin no prescription
A person necessarily lend a hand to make critically posts I would state. That is the first time I frequented your web page and so far? I amazed with the analysis you made to make this actual publish amazing. Magnificent activity!
ivermectin 12mg online – order aczone pills order tetracycline 500mg online cheap
buy ivermectin 6mg for humans – cefixime 200mg pill how to buy tetracycline
buy valacyclovir online – buy nemasole zovirax 400mg cheap
acillin oral purchase ampicillin pills amoxil buy online
Loving the info on this site, you have done outstanding job on the posts.
metronidazole over the counter – buy azithromycin 250mg sale azithromycin 250mg over the counter
buy lasix without a prescription – tacrolimus canada generic captopril 25 mg
where can i buy glycomet – buy combivir cheap order lincocin 500mg sale
Heya i am for the first time here. I came across this board and I find It really useful & it helped me out much.
I’m hoping to present something back and aid others such as you aided me.
Also visit my blog post: vpn special code
zidovudine 300mg for sale – metformin 1000mg uk allopurinol cost
buy generic clozapine over the counter – order frumil 5mg online pepcid order online
I got what you mean ,saved to my bookmarks, very nice website .
I would like to thnkx for the efforts you have put in writing this blog. I am hoping the same high-grade blog post from you in the upcoming as well. In fact your creative writing abilities has inspired me to get my own blog now. Really the blogging is spreading its wings quickly. Your write up is a good example of it.
seroquel 50mg usa – buy effexor pill order eskalith generic
buy clomipramine 25mg online cheap – brand imipramine pill sinequan 75mg
For the reason that the admin of this site is working, no uncertainty very soon it will be well-known, due to its quality contents.
Review my web-site; vpn coupon code 2024
atarax 10mg uk – pamelor medication buy endep cheap
augmentin 1000mg pills – trimethoprim pill purchase baycip pill
buy cheap generic amoxicillin – buy erythromycin 250mg generic cipro 1000mg cost
azithromycin 500mg canada – azithromycin medication ciprofloxacin 500 mg cost
order cleocin 150mg pill – order cleocin 150mg how to buy chloromycetin
What Is FitSpresso? As you may know, FitSpresso is a natural weight loss supplement that comes in capsule form.
What is Boostaro? Boostaro revolutionizes romantic performance enhancement through its reliance on the wisdom of natural ingredients
I love it when people come together and share opinions, great blog, keep it up.
What Is Aizen Power? Aizen Power is presented as a distinctive dietary supplement with a singular focus on addressing the root cause of smaller phalluses
Helpful info. Lucky me I discovered your website accidentally, and I am surprised why this coincidence didn’t came about earlier!
I bookmarked it.
Feel free to visit my blog post; vpn special coupon
You made some good points there. I did a search on the subject matter and found most persons will approve with your site.
ivermectin 3 mg for sale – buy eryc 250mg for sale order cefaclor pill
purchase albuterol generic – purchase albuterol inhalator sale cost theophylline
I conceive this internet site has some rattling great information for everyone :D. “When you get a thing the way you want it, leave it alone.” by Sir Winston Leonard Spenser Churchill.
Great work! This is the type of information that should be shared around the net. Shame on Google for not positioning this post higher! Come on over and visit my website . Thanks =)
buy methylprednisolone online – medrol 4mg tablets azelastine sale
This is the right blog for anyone who wants to find out about this topic. You realize so much its almost hard to argue with you (not that I actually would want…HaHa). You definitely put a new spin on a topic thats been written about for years. Great stuff, just great!
I don’t normally comment but I gotta admit thankyou for the post on this one : D.
Hi, I think your site might be having browser compatibility issues. When I look at your website in Safari, it looks fine but when opening in Internet Explorer, it has some overlapping. I just wanted to give you a quick heads up! Other then that, fantastic blog!
Hey there! This post couldn’t be written any better! Reading this post reminds me of my old room mate! He always kept chatting about this. I will forward this page to him. Fairly certain he will have a good read. Many thanks for sharing!
Hey very nice blog!! Guy .. Excellent .. Amazing .. I’ll bookmark your blog and take the feeds additionally…I am satisfied to seek out numerous useful information here in the submit, we’d like work out more techniques in this regard, thanks for sharing.
naturally like your web-site however you need to check the spelling on quite a few of your posts. Several of them are rife with spelling issues and I to find it very bothersome to inform the truth on the other hand I will definitely come back again.
desloratadine where to buy – order zaditor online cheap purchase albuterol generic
Wow, wonderful weblog layout! How long have you been running a blog for?
you made blogging look easy. The full glance of your site is wonderful, as smartly as the content material!
You can see similar here najlepszy sklep
order generic metformin 1000mg – buy januvia generic precose 50mg drug
Hey there just wanted to give you a quick heads up and let you know a few of the pictures aren’t loading correctly. I’m not sure why but I think its a linking issue. I’ve tried it in two different internet browsers and both show the same results.
buy generic glyburide – purchase forxiga pills dapagliflozin cheap
Puravive is a weight loss supplement that targets the root cause of weight gain issues in men and women.
repaglinide 2mg sale – order jardiance 10mg generic order generic empagliflozin
Fitspresso stands out among the crowded health supplement market as an exceptional product.