PACHYMA. 



[ 572 ] 



PADINA. 



PACHY'MA, Fr. A supposed genus 

 of Sclerotiacei, but probably a condition of 

 certain roots, the substance being converted 

 into pectic acid. It is well known in the 

 United States under the name of Tucha- 

 hoo. 



BIBL. Fr. Syst. Myc. ii. 242 ; Berk. Int. 

 Crypt. Bot. 254. 



PACHYMATIS'MA, Bowk. A genus 

 of marine Sponges. 



Distinguished by the fleshy, crust-like, 

 not cellular nor elastic mass, covered by a 

 thick skin, and perforated by scattered ori- 

 fices ; the interior beset with siliceous aci- 

 cular and stellate spicula. P. Johnstonia. 

 (BoAverbank, Brit. Sponq., Ray Soc.) 



PACHYPHLCE'US, Tul. A genus of 

 Tuberacei (Ascomycetous Fungi), with a 

 common warty integument opening by a 

 terminal aperture with a distinct base, cla- 

 vate asci, and spherical s-poridia. Three 

 species occur in Great Britain. 



BIBL. Tul. Fung. Hyp. 130 ; Berk, and 

 Br. Ann. N. It. xiii. 359, xviii. 79 j Berk. 

 Outl. 377 ; Cooke, Handb. 743. 



PACHYT'ROCHA, Kent. A genus of 

 Peritrichous Infusoria. Like Cothurnia, 

 but with a fleshy pad closing the carapace. 

 P. cothurnoides ; pond-water. (Kent. Inf. 

 729.) 



PACINIAN CORPUSCLES. These 

 curious organs form terminations or appen- 

 dages of the spinal nerves in the skin and 

 subcutaneous tissue of the palm of the 

 hand, the sole of the foot, the fingers and 

 toes, in the sympathetic semilunar ganglia, 

 the mesentery, &c. 



They are elliptical or pear-shaped, whitish, 

 and about 1-25 to 1-6" in diameter. Each 

 consists of from twenty to sixty concentric 

 layers of connective tissue (fig. 553), sepa- 

 rated by interspaces, those between the 

 outer layers being considerable, those be- 

 tween the inner being small ; each is lined 

 with epithelium. They surround a cavity 

 filled with soft, abundantly nucleated and 

 very easily alterable material, which under- 

 goes coagulation after death, and into the 

 interior of which the nerve-fibres penetrate. 

 They are filled with a clear serous liquid ; 

 each is also furnished with a stalk, contain- 

 ing a slender nerve-fibre, which passes into 

 the central space, in which it terminates, 

 frequently in two or three branches, each 

 with a granular tubercle. 



The Pacinian corpuscles are met with 

 also on the nerves of many Mammalia, and 

 are very numerous in the skin, the beak, 



Fig. 553. 



A human Pacinian corpuscle, a. Btalk ; b nerve-fi 

 within it ; c, outer, d, inner layers of the sheath ; e, p 

 nerve-flbre in the central cavity ; /, its termination. 



fibre 

 pale 



Magnified 350 diameters. 



and the tongue of birds. They are readily 

 examined in the mesentery of the cat 



BIBI, KolKker, Mik. An. ii. ; Schultze, 

 Strieker's Hist. 



PADI'NA, Adanson. A genus of Dic- 

 tyotacese (Fucoid Algse), containing one 

 species, P. Pavonia (fig. 554), found rarely 

 in summer and autumn on the south coast 

 of England. The fan-shaped or reniform 

 fronds grow in tufts, and are 2 to 5" hio-h 

 sometimes entire, sometimes cleft (fig. 554)' 

 They are marked with concentric zones'. 

 The substance is parenchymatous the 

 number of layers of cells diminishing, with 

 the thickness and solidity, from the base to 

 the edges. The back of the frond is covered 



