COMPARATIVE AND HISTORICAL. 437 



bladder ; the latter sometimes occurs after delivery, in consequence of the pressure of the child 

 against the bladder. 3. After section of the spinal cord (p. 435). 4. Where the voluntary 

 impulses are unable to act upon the inhibitory apparatus of the sphincter urethral reflex, as well 

 as when the sphincter urethral reflex is increased. 



Incontinence of urine (stillicidium urine) occurs in consequence of 1. Paralysis of the 

 sphincter urethra?. 2. Loss of sensibility of the urethra, which of course abolishes the reflex of 

 the sphincter. 3. Trickling of the urine is a secondary consequence of section of the spinal 

 cord, or of its degeneration. 



Strangury is an excessive reflex contraction of the walls of the bladder and sphincter, due to 

 stimulation of the bladder and urethra ; it is observed in inflammation, neuralgia [and after 

 the use of some poisons, e.g., cantharides]. 



Enuresis nocturna, or involuntary emptying of the bladder at night, may be due to an 

 increased reflex excitability of the wall of the bladder, or weakness of the sphincter. 



282. COMPARATIVE AND HISTORICAL. Amongst vertebrates, the urinary and genital 

 organs are frequently combined, except in the osseous fishes. The Wolffian bodies which act 

 as organs of excretion during the embryonic period, remain throughout life in fishes and 

 amphibians and continue to act as such. Fishes. The myxinoids (cyclostomata) have the 

 simplest kidneys ; on each side is a long ureter with a series of short-stalked glomeruli with 

 capsules arranged along it. Both ureters open at the genital pore. In the other fishes, the 

 kidneys lie often as elongated compact masses along both sides of the vertebral column. 

 The two ureters unite to form a urethra, which always opens behind the anus, either 

 united with the opening of the genital organs, or behind this. In the sturgeon and hag-fish, 

 the anus and orifice of the urethra together form a cloaca. Bladder-like formations, which, 

 however, are morphologically homologous with the urinary bladder of mammals, occur in fishes, 

 either on each ureter (ray, hag-fish), or where both join. In amphibians, the vasa efferentia of 

 the testicles are united with the urinary tubules ; the duct in the frog unites with the one on 

 the other side, and both conjoined opens into the cloaca, whilst the capacious urinary Madder 

 opens through the anterior wall of the cloaca. From reptiles upwards, the kidney is no longer 

 a persistent Wolffian body, but a new organ. In reptiles, it is usually flattened and elongated; 

 the ureters open singly into the cloaca. Saurians and tortoises have a urinary bladder. In 

 birds, the isolated ureters open into the urogenital sinus, which opens into the cloaca, internal 

 to the excretory ducts of the genital apparatus. The urinary bladder is always absent. In 

 mammals, the kidneys often consist of many lobules, e.g., dolphin, ox. 



Amongst invertebrates, the mollusca have excretory organs in the form of canals, which are 

 provided with an outer and an inner opening. In the mussel, this canal is provided with a 

 sponge-like organ, often with a central cavity, and consisting of ciliated secretory cells, placed at 

 the base of the gills (organ of Bojanus). In gasteropods, with analogous organs, uric acid has 

 been found. Insects, spiders, and centipedes have the so-called Malpighian vessels, which are 

 excretory organs partly for uric acid and partly for bile. These vessels are long tubes, which 

 open into the first part of the large intestine. In crabs, blind tubes connected with the 

 intestinal tube, perhaps have the same functions. The vermes also have renal organs. 



Historical. Aristotle directed attention to the relatively large size of the human bladder 

 he named the ureters. Massa (1552) found lymphatics in the kidney. Eustachius (t 1580) 

 ligatured the ureters and found the bladder empty. Cusanus (1565) investigated the colour 

 and weight of the urine. Rousset (1581) described the muscular nature of the walls of the 

 bladder. Vesling described the trigone (1753). The first important chemical investigations on 

 the urine date from the time of van Helmont (1644). He isolated the solids of the urine and 

 found among them common salt ; he ascertained the higher specific gravity of fever-urine, and 

 ascribed the origin of urinary calculi to the solids of the urine. Scheele (1766) discovered uric 

 acid and calcium phosphate; Arand and Kunckel, phosphorus; Rouelle (1773), urea; and it got 

 its name from Fourcroy and Vauquelin (1799). Berzelius found lactic acid; Seguin, albumin in 

 pathological urine ; Liebig, hippuric acid ; Heintz and v. Pettenkofer, kreatin and kreatinin ; 

 Wollaston (1810), cystin. Marcet found xanthin ; and Lindbergson, magnesic carbonate, 



Functions of the Skin. 



283. STRUCTURE OF THE SKIN, HAIRS, AND NAIL. The skin (3 3 to 

 2'7 mm. thick ; specific gravity, 1057) consists of 



[1. The epidermis ; 



2. The chorinm, or cutis vera, with the papilhe (fig. 294).] 



The epidermis (0*08 to 0*12 mm. thick) consists of many layers of stratified epithelial cells 

 united to each other by cement substance (figs. 293, 294). The superficial layers stratum 



