i:vr. 



absorbed. In thin situation then U a vacant spaoe partly occupied 

 by UM caruncle, called the laous lachrymal!* (Sy. 11, rf) ; it i a sort 

 of reservoir or rather link fur the trail. Above and below, at the 

 entrance to this space where the ciliary margins terminate, there is a 

 (mall prominence on the inner edge of both (Jtyt. 1 1 , a ; 1 4, a), centrally 

 punctured by mnall orifice*. These are tlio puncta lachrymolia. 

 Their inward aapect in well shown in.liy. IS. They an- the eniuncto- 

 rie* of the eye ; anil their function is to absorb the fluids presented 

 to thorn, and convey them by two converging canals (Jig. 14, a) to the 

 lachrymal sac (fy. 14, r), which they enter by a common orifice 

 (.';/. 14, 6). This is a membranous bag about as large as a kidney- 

 bean lodged in a groove in the lachrymal bone, behind the tendon of 

 the orbicular muscle. The lachrymal sac entering a vertical channel 

 in the bone at the end of the groove is narrowed into the lachrymal 

 canal (fy. 14, rf), and passes directly downwards into the inferior 

 meatus or chamber of the noae, which it enters on the outer side by 

 a slit in the mucous lining. It is not exactly understood in what 

 way the puncta absorb whether by capillary attraction or by some 

 vital force of suction. The side of the lachrymal sac is connected 

 with the tendon of the orbicularis, which may aid in producing the 

 effect by suddenly drawing its membranous, surfaces apart We all 

 know the effect of repeated winking when the eyes are filled with tears. 



Nervous and Vascular Constitution of the Eye. Enough has been 

 already said, for general information, with respect to the blood- 

 vessels distributed to the eyeball, and it is not necessary to mention 

 those which supply the appendages. With respect therefore to 

 vascular arrangements we have only to add, that although there are 

 abundant proofs of the existence of an active absorption within the 

 globe, no lymphatic vessels especially destined to that function have 

 been hitherto found in it The optic or second cerebral nerve has 

 been already described. All the straight muscles, with the exception 

 of the rectus externus, the inferior oblique, and the levator pal pe one, 

 are supplied by the third nerve. The fourth is wholly distributed 

 to the trochlearU, and the sixth to the rectus externus. The orbicular 

 muscle is supplied, like most of those of the face, by the portio 

 dura of the seventh pair. All these, except the optic, are muscular 

 or motor nerves. The fifth nerve supplies the whole organ in common 

 with many other parts with ordinary sensation. Any account of the 

 intricate nervous constitution of the iris would be here quite out of 

 place. The third and sixth nerves are mainly concerned in it. Thus 

 of the ten cerebral nerves, the second, third, fourth, and sixth are 

 wholly, and the fifth and seventh partially distributed to the organ 

 of vision ; a fact which may give some idea of the elaborate organi- 

 sation and varied exigencies of the parts which compose it 



Comparative Anatomy of the Eye. The sense of sight is undoubt- 

 edly developed amongst the lowest class of animals, but it ia difficult 

 to point out the exact members of the series in which this faculty is 

 first exercised. The moving zoospores of plants are attracted 

 towards the light, and Ehrenberg regards as eyes the red spots seen 

 in some organisms, such as Voltox, which are now regarded as plants. 

 All anatomists are agreed on regarding the ocelliform spots situated 

 at regular intervals along the margin of the disc of Pulmograde 

 Meiltmr as eyes. They are composed of a collection of pigmentary 

 granules, superposed upon an enlargement of a nervous thread, which 

 comes from the central circle of nervous matter in the animals. 

 Similar spot* have been observed at the extremities of the rays in 

 the Atttriadtf by Professor E. Forbes, In many of the lower 

 Articulata no higher development obtains. In the Entozoa no visual 

 organ has been detected. Distinct eye-spots are seen in the Leeches, 

 the Rotifcrir, and the Dorsi branchiate Annelida. Amongst the mass 

 of Molliuca these organs are not observed. They are however very 

 distinct in the Pectens and other swimming forms of Lamelli- 

 branchiate Mollusks. 



" The eyes of most of the higher articulated animals are constructed 

 upon the composite type, each of the masses that U situated upon 

 either side of the head, being made up of an aggregation of simple 

 eyes, every one of which is in itself a complete visual instrument, 

 but is adapted to receive and to bring to a focus only those rays 

 which come to it in one particular direction. In most insect* each 

 composite eye forms a large hemispherical protuberance, which 

 occupies a considerable part of the side of the head, and when 

 examined with a microscope, its surface is seen to be directed into a 

 vast number of facets, which are usually hexagonal. The number 

 of these facet*, every one of which is the cornea of a distinct eye, is 

 usually very great Behind the cornea is a layer of dark pigment, 

 which take* the place and serves the purpose of the iris in the 

 eye* of the Vtrttlrata, and this is perforated by a central aperture 

 or pupil, through which the rays of light which have traversed 

 the cornea gain access to the interior of the eye. When a vertical 

 section is made of one of these composite eye*, it U seen that each 

 separate ere is the frustum of a pyramid, of which the cornea 

 form* the large end or base, whilst the small end abut* upon a 

 bulbous expansion of the optic nerve. The interior of this pyramid is 

 occupied by a transparent substance which represent* the vitreous 

 humour, and -the pyramids are separated from each other by a 

 layer of dark pigment which completely inclose* them, save at the 

 pupillary apertures and also at a corresponding set of apertures at 

 their smaller ends, where the pigment it perforated by the fibre* 



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of the optic nerve, of which one proceeds to each ceparate eye. 

 Kach facet of the common cornea or ' oomeule ' i* usually 

 on both it* surfaces, aud thus act* a* a lens, the focus of which has 

 been ascertained by experiment to be equivalent to the length <( the 

 transparent pyramid behind it ; so that the image pro.lur, ,1 I.y the 

 It-iis will fall upon tlio extremity of the filament of the optic 

 which posse* to it* truncated end. The rays which have passed 

 through the several corneulea are prevented from mixing with each 

 other by mean* of the layer of black pigment which surrounds each 

 cone ; aud thun no rays, except those which correspond with the axis 

 of the cone, can reach the fibres of the optic nerve. Hence it is 

 evident that each separate eye must hare an extremely limited range 

 of vision, being adapted to receive but a very small pencil of rays 

 proceeding from a single point in any object ; and as these eyes are 

 usually immoveable, they would afford but very imperfect information 

 of the position of surrounding objects, were it not for their enormous 

 multiplication, by which a separate eye is provided (so to speak) for 

 each point to be viewed. No two of the separate eye*, save those 

 upon the opposite sides of the head which are directed exactly 

 forwards, can form an image of the same point at the same time. 

 but the combined action of all of them may give to the insect, it 

 may be imagined, as distinct a picture as that which we obtain by a 

 very different organisation. At any rate it seems certain, from 

 observation of the movement of insects, that the vision by which 

 they are guided must be very perfect and acute. 



" Although the foregoing may be considered the typical structure of 

 the eyes of insects, yet there are various slight departures from it in 

 the different subdivisions of the class. Thus in some cases the 

 posterior surface of each corneule is concave, and a space is left 

 between it and the iris which seems to be occupied by a watery tluid 

 or aqueous humour ; in some instances again this space is occupied 

 by a double convex body which seems to represent the crystalline 

 lens ; and there are cases in which this crystalline lens is found 

 behind the iris, the number of eyes being reduced, and each individual 

 eye being larger, so that the entire aggregate approaches, both in its 

 structure aud mode of action, to that of Arachitida and certain 

 Cruttacea. Besides their composite eyes, insects usually posses* a 

 small number of rudimentary single eyes resembling those of the 

 A rachnida : these are seated upon the top of the head, and are called 

 stemmata. Their precise use is unknown, but that they bore con- 

 siderable influence in the direction of the movements appears from 

 the fact that if the stemmata of a bee be covered with paint, on 

 being let go it will fly continually upwards a fact which seems 

 related to those already mentioned in regard to the influence of visual 

 sensations upon automatic movement*. It is remarkable that the 

 larva. 1 of insects which undergo a complete metamorphosis only posses* 

 simple eyes, the composite eyes being developed at the game time 

 with the wings and other parts which ore characteristic of the imago 

 state during the latter part of the pupa condition. In the higher 

 Cruttacea the structure of the eyes is nearly the same as in insects ; 

 but the compound masses are not so large relatively to the bulk of 

 the body, and the number of distinct eyes is not nearly so great In 

 the lower Cruttacea however, as in Myriapoda, the visual organs much 

 more closely approximate the type of structure which they present 

 in the Arac/initta ; each aggregate mass being composed of a small 

 number of simple eyes, of which every one has its own separate 

 cornea as well as its own crystalline lens and vitreous humour, and 

 these in some instances being altogether detached from each other. 

 Among some of the Suctorial Uruttacta the visual organs are altogether 

 wanting in their state of full development, although they ore uniformly 

 present in their early condition ; and tlio same may be said of the 

 Cirrhipedia. Among Araclmiila, which in this as in many other 

 respect* present an approximation to Ycrttbrata, we find a great 

 reduction in the number of eyes, which are never more than eight in 

 number (sometimes being only two), aud are to be compared with the 

 stemmata of insects rather than with their compound eyes. These 

 eyes are sometime* collected into one mass on the summit of the 

 cephalo-thorax, and are sometimes symmetrically aud separately on 

 the two sides of the median line. In the Scorpions we find two large 

 eyes placed on the dorsal aspect of the cephalo-thorax near the median 

 line, and three pairs of smaller ones, which are placed on the outer 

 margins of the same division of the body. The larger eyes are 

 described by Miiller a* each possessing a cornea which is convex 

 anteriorly and concave posteriorly; and a nearly globular crystalline 

 lens resembling that of fishes, whose anterior surface lies in tint 

 hollow of the cornea, while it* posterior rests upon the vitreous 

 humour, without being imbedded in it The vitreous humour is a 

 nearly hemispherical mass of soft granular matter, being almost flat 

 in front and very convex behind ; over it* posterior surface is spread 

 the retina, or expansion of the optic nerve; and this is covered by 

 a thick layer of pigment which passes inwards in front of the vitreous 

 humour so as to form a sort of iris, the pupillary aperture of which 

 however exceeds the diameter of the crystalline lens. Among 

 clauses which constitute the higher division of the Molluscous seriev 

 in virtue of the possession of a distinct head, the presence of visual 

 organs is by no means constant ; many Oatteropoda and Ptcropoda 

 being destitute of them altogether, and others possessing ocellil'orm 

 spot*, which may be concluded to be rudimentary eye* from their 



