COMPARATIVE. HISTORICAL. 88l 



the sac and the canal contains numerous large venous radicles. In expiration, 

 especially in forced expiration, these radicles swell, and press the walls of the 

 ducts together. For this reason air cannot be driven into the lacrimonasal 

 canal, even by forced pressure. If strong inspiratory efforts are made, as in 

 the act of deep frequent insufflation, the veins are emptied, and as the walls again 

 retract they exert an aspiratory influence on the tears. 



The secretion of tears results from direct irritation of the lacrimal 

 nerve, the subcutaneus malae, the facial, and the cervical sympathetic, 

 which have been designated the secretory nerves. Reflex secretion of 

 tears may be brought about by irritation of the nasal mucous membrane 

 on the same side. The ordinary secretion in the waking hours is 

 probably a reflex result of irritation of the anterior surface of the eye- 

 ball (by the air, by evaporation of the tears) ; the cornea and the con- 

 junctiva possess sensibility to pain and touch, to cold and heat. In- 

 tense irritation by light also produces a reflex flow of tears through the 

 intermediation of the optic nerve. In the rabbit the center does not 

 extend further forward than the origin of the trigeminus, but it extends 

 downward to the fifth vertebra. During sleep the factors mentioned 

 are absent, and the tears dry up. Reichel under the direction of Heid- 

 enhain found that the active gland, after injection of pilocarpin, con- 

 tains cloudy, granular diminutive cells, with obscure outlines, and spher- 

 ical nuclei, whereas in the resting gland the cells are light and slightly 

 granular, with irregularly formed nuclei. The overflow of tears pro- 

 duced by emotion is still unexplained, -as is also that caused by hearty 

 laughing. In coughing or vomiting, the secretion of tears is increased 

 by reflex influences, and the drainage of the tears is impeded by the 

 expiratory pressure. 



The tears moisten the eyeball, protect it from desiccation, and carry 

 off small particles, with the assistance of winking. Atropin diminishes 

 their quantity. 



The tears are alkaline in reaction and have a salty taste ; they represent a 

 "serous" secretion, containing from 98.1 to 99 per cent, of water, 1.46 of organic 

 substances (o.i of albumin and mucin, o.i of epithelial cells), from 0.4 to 0.8 

 of salts (principally sodium chlorid). 



Pathologically, bacteria arc present; in the secretion within the lacrimal 

 canal, streptothrix. 



COMPARATIVE. HISTORICAL. 



Comparative. The simplest form of visual apparatus consists of deposits 

 of pigment in the external covering of the body connected with the terminations 

 of afferent nerves. The pigment absorbs the light-rays and undergoes a chemical 

 change as "visual substance," and, as a result of the action of the luminiferotts 

 ether, it discharges kinetic energy, which stimulates the terminations of the nervous 

 end-apparatus. Deposits of pigment with efferent nerves, and in addition a bright, 

 refractive body, are found in the margins of the swimming-bells of the higher 

 medusae, while the lower forms have spots of pigment only at the base of the 

 tentacles. In many of the lower worms there are spots of pigment near the 

 brain. In the earthworm the head is sensitive to light because of the presence 

 of light-cells, the caudal end less so. In others the pigment surrounds the nerve- 

 endings, which are represented by the so-called crystalline rods, or crystalline 

 balls (for example the rotifera, or wheel-animalcules). In leeches the eyes. 

 which are usually situated in the head, are not typically developed. Many of 

 the lower worms, and especially the parasites, possess no visual apparatus whatever. 

 In starfishes the eyes are in the ends of the arms, and consist of spherical crystal- 

 line organs, surrounded by pigment, and supplied with nerves. In all the other 

 echinodcrms, only deposits of pigment are present. Among the articulates 

 eyes in different stages of devejopment are met with: i. Eyes without cornea that 

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