ENTOZOA. 67 



commence on eacli side of the mouth, but, from their extreme 

 fineness, can with difficulty be perceived ; they slightly enlarge as 

 they pass downwards to about one-third of a line in diameter in 

 large specimens, and then gradually diminish to the sides of the caudal 

 extremity. They are occasionally of a red colour, and denote the 

 situation of the principal vessels of the body. The dorsal and ab- 

 dominal longitudinal lines are less marked than the preceding, 

 and by no means widen in the same proportion at the middle of 

 the body. They correspond to the two nervous chords, hereafter 

 to be described. 



The last species of human Entozoon which remains to be noticed 

 is the Ascaris vermicularis (Jig. 32.), a small worm, also noticed by 

 Hippocrates under the name of acncaptg, and claiming the attention of 

 all physicians since his time, as one of the most troublesome parasites 

 of children, and occasionally of adults ; in both of whom it infests 

 the larger intestines, especially the rectum. The size of the male 

 Ascaris vermicularis is two or three lines, that of the female is five 

 lines. 



The integument in the Nematoid parasites of the human subject, and 

 in almost all the order, is smooth ; it consists of a thin compact epidermis, 

 and a cellular corium firmly attached to the outer transverse mus- 

 cular fibres. The epiderm is developed in the Strongylus horridus 

 of the water hen, into four longitudinal rows of reflected hooklets ; 

 and similar spines are arranged in circular groups upon the anterior 

 part of the Gnathostoma spinigerum. 



M. Cloquet, in his elaborate monograph on the Ascaris lumhri- 

 coides, correctly states that the exterior layers of muscular fibres are 

 transverse, and the internal longitudinal. In this large specimen of 

 the Strongylus gigas, which I have dissected for the muscular sys- 

 tem, you will perceive that a very thin layer of transverse fibres ad- 

 heres strongly to the integument, the fibres being imbedded in 

 delicate furrows on the internal surface of the skin ; within this 

 layer, and adhering to it, but less firmly than the transverse fibres do 

 to the integument, there is a thick layer of longitudinal fasciculi, 

 which are a little separated from one another, and distributed not in 

 eight distinct series, but pretty equally over the whole internal cir- 

 cumference of the body. Each fasciculus is seen under a high 

 magnifying power to be composed of many very fine fibres ; but these 

 do not present the transverse striae which are visible by the same 

 power in the voluntary muscular fibres of the higher animals. The 

 inner surface of the stratum of longitudinal fibres is covered with a 

 soft tissue composed of small obtuse processes, filled with a pulpy 

 substance, and containing innumerable pellucid globules. 



F 2 



