Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/951

Rh usclea of eball. EAK.] rudiment of the third eye-lid, or membrana nictitans, so well developed in birds. The palpebral conjunctiva has small papillce scattered over its surface ; its epithelium is stratified, with scaly cells on the free surface and elongated cells in the deepest layer. In the sub-epithelial tissue are small branched mucous glands, which are numerous in the caruncula. Little masses of adenoid tissue (p. 849) with lymphatic vessels are also found in it, and the conjunctiva of the front of the eyeball is thinner than the palpebral part. It is not glandular, and its nerves terminate in end-bulbs (p. 862). The palpebral conjunctiva, and in part that of the eyeball, receive their blood-vessels from those of the eye-lids, but the portion of the conjunctiva next the cornea is supplied by the arteries of the sclerotic coat. The Lachrymal Apparatus is engaged in the secretion of the tears, and in conveying them away from the front of the globe. The lachrymal gland occupies a depression in the outer part of the roof of the orbit. It is smaller than an almond, is sub-divided into lobules, and belongs to the group of compound racemose glands. It consists of the ramifications of short ducts, which terminate in small sacculi. The wall of each sacculus consists of a delicate membrana propria, and the cavity contains the polyhedral secreting cells. Outside the membrana propria is a capillary network derived from the lachrymal artery, but Giannuzzi and Boll have recently described a space between this network and the membrana propria which they believe to bo continuous with the lymphatic system. Pfliiger has described nerves as terminating in connection with the secreting cells. The excretory ducts of the gland are from six to eight, and open on the back of the upper eye-lid. The tears are washed over the surface of the globe by the involuntary wink ing of this lid. When the secre tion is increased in quantity, in the act of crying, the tears flow over the cheek, but in ordinary circum stances they are conveyed away by two slender tubes, the lachrymal canals, which open by minute orifices, thepuncta lachry- malia, one at the inner end of the free border of each eye - lid. These tubes open at their opposite ends into a small reservoir, the lachrymal sac, situated in a hollow in the lachrymal bone. From this sac a duct, the nasal or lachrymal duct, proceeds which opens into the inferior meatus of the nose, and here the tears mingle with the mucous secretion of that cavity. Muscles of the Eyeball. The sclerotic coat of the eyeball has six muscles inserted into it. Four of the muscles are called recti, and are situated, one superior, one inferior, one external to, another internal to, the globe. They all arise from the rim of bone which bounds the optic foramen ; the external and internal muscles are inserted vertically into the sides of the sclerotic, but the superior and inferior recti have oblique insertions into its upper and lower aspects. The other two muscles are called obliqui. The superior oblique arises along with the recti, passes to the inner end of the upper border of the orbit, where its tendon goes through a pulley, and is directed back to be inserted FIG. 79. Lachrymal canals and duct. 1, orbicular mnsclo; 2, lachrymal canal; 3, punctum; 4, caruncula; 5, lachrymal sac; 6, lachrymal duct; 7, angular artery. 891 obliquely into tlie upper and outer part of the sclerotic. The inferior oblique arises from the lower border of the orbit, passes outwards and upwards to be inserted obliquely into the sclerotic. These muscles roll the eyeball in the orbit, and, without entering into a minute analysis of their actions, their office may be stated generally as follows : The internal rectus rolls it inwards, the external outwards, about its vertical axis ; the superior rectus rolls it upwards, the inferior downwards, about its transverse horizontal axis, though from the obliquity of their insertions they give it at the same time a slight inward or outward movement as the case may be ; the superior and inferior oblique roll the globe around its antero-posterior or sagittal axis, the superior upwards and outwards, the inferior downwards and out wards. Periosteal Muscle of the Orbit. The periosteum of tho orbit contains, as H. Miiller and Turner have described, a layer of non-striped muscular fibre in the part which covers over the spheno-maxillary fissure. In man it is rudimen tary, but in the sheep, deer, elephant, &c., where the osseous wall of the orbit is deficient, this muscle forms a well-defined structure. It has been suggested that it acts as a protractor muscle of the globe. The EAE, or organ of hearing, is a complex acoustic ap- Ear. paratus, situated in connection with the temporal bone. It is divided into three parts, named external, middle, and internal ear. The External Ear consists of the pinna or auricle and Exten the external auditory meatus. The auricle is the oblong ear- convoluted body situated at the side of the head. Its incurved outer border is named the helix. Within this lies a curved ridge, the anti-helix, in front of which is a deep hollow, the concha, which leads into the external meatus. The concha is bounded in front by a prominence, the tragus, and behind by a smaller prominence, the anti-tragus ; below the anti-tragus is the lobule, which forms the most depending part of the auricle. The framework of the auricle is formed of yellow elastic fibro-cartilage invested by integument, except the lobule, which consists merely of a fold of integument containing fat. Attached to the car tilage are not only the three auricular muscles referred to on FIG. 80. Tho car as seen in section, a, helix ; &, anti-trapus ; e, anti-helix ; d, concha ; e, lobule ; / mastoid process ; g, portio dura ; h, styloid process ; k, internal carotid artery; I, Eustachiar. tube; m tip of petrous process; n, exter nal auditory meatus; o, membrani tympani; p, tympanum; 1, points to mal leus; 2, to incus; 3, to stapes; 4, to cochlea; 5, 6, 7, the three semicircular canals; 8 and 9, portio dura and portio mollis. page 836, but also certain smaller muscles called the proper muscles of the pinna. Thus the greater muscle of the helix is placed on its anterior border ; the lesser muscle of the