LYMPHATIC SYSTEM. 125 



the lymphatics, as is rendered probable from what Dr. Haller 

 says of his having found them irritable in his experiments, 1 

 and also from what is observed on seeing them distended with 

 their lymph in living animals, in which case they appear of 

 a considerable size ; but upon emptying them of their contents 

 they contract so much as not to be easily distinguished. This 

 is an experiment which I have frequently made in the trunk of 

 the lacteals in a goose and on the lymphatic vessel on its neck, 

 both of which, when distended with their natural fluids, are as 

 large as a crow-quill ; but on emptying them in the living 

 animal, I have seen them contract so much that it was with the 

 greatest difficulty I could distinguish them from the fibres. 2 



1 Sur le Mouv. du Sang, ex. 295, 298. 



2 See also Haller, El. Phys. lib. ii, sect. 3, $ iii. The celebrated Nuck thought he 

 could separate the coats of the lymphatics, Adenographia, cap. iii (LXIV). 



empty ; but expanding and recovering its cylindrical form when opened 

 either in the air or under water. 



Mr. Hunter a describes the arteries as possessing both a muscular 

 and an elastic power, the former chiefly acting in the tranverse direc- 

 tion of the tube ; and the elasticity, " when the muscular action ceases, 

 being exerted to dilate the vessel and restore it to a middle state again, 

 becoming the elongator or antagonist of the middle coat, and by that 



means fitting it for a new action There appears to be no 



muscular power capable of contracting an artery in its length, the whole 

 of that contraction being produced by its elasticity." Mr. Hunter's 

 experiments on the functions of the arteries are supported by the latest 

 and best observations on their structure. Professor Henle 5 describes the 

 proper contractile or middle coat, the muscular coat of Hunter, the middle 

 or elastic of former writers, as composed of fibres encircling the vessel, 

 and possessing much the same ultimate structure as the organic muscle 

 of the stomach and intestines ; while the yellow tissue, or elastic coat 

 of Hunter, so remarkable in the larger arteries, is situated immediately 

 external to the contractile coat, has all the microscopic characters of 

 elastic tissue, and its fibres variously directed. The external or cellular 

 coat consists of longitudinal filaments of common cellular tissue. 



It is well known that the blood may be propelled by the action of 



the vessels alone. The heart is sometimes wanting even in a large 



human foetus, of which a remarkable instance was described by the 



second Dr. Monro. c 



(LXIV) The larger lymphatic and lacteal vessels' 1 are similar in structure to 



a Works, ed. by Palmer, iii, 164-169. c Essays, &c. page civ, ed. by his Son. 

 b Anatomic Generate, tr. par Jourdan, 8vo, Edin. 1840. 



t. ii, pp. 28-34. 8vo, Paris, 1843. d Henle, op. cit. torn, ii, p. 89. 



