MINUTE ANATOMY OF THE VEINS. 275 



external elastic tunic of Henle* exists as an independent 

 membrane in the smaller and medium-sized arteries, with few 

 exceptions (internal spermatic, splenic, renal, hepatic, brachial, 

 crural, popliteal and plantar arteries). 



The basilar artery, the muscular tissue of which is poor in 

 elastic fibres, loses this membrane completely, and presents 

 instead a very loose network of fine elastic fibres in the tunica 

 adventitia. The dorsal artery of the penis contains similar 

 and numerous elastic networks. The aorta, axillary, carotid, 

 subclavian, and pulmonary arteries, whilst they present largely 

 developed elastic laminse in the muscular tunic, do not possess 

 a proper external elastic membrane. 



Speaking generally, this membrane is formed by a network 

 of fine elastic fibres, which is sharply defined internally towards 

 the muscular layer, but which externally anastomoses with the 

 elastic network of the tunica adventitia. The remaining por- 

 tion of the adventitia consists of decussating fasciculi of con- 

 nective tissue, with networks of elastic fibres. 



After what has been said, it is obvious that, whilst a descrip- 

 tion can be given which shall be applicable to individual 

 arteries, and to groups of arteries, no general statement can be 

 given that is appropriate to the entire arterial system ; there 

 is, in fact, a certain antagonism between the elastic element of 

 the tunica adventitia and that of the circular muscular layer, 

 as is well shown in the case of the basilar artery. Still more 

 frequently, again, there exists a certain antagonism between 

 the muscles and the elastic elements of the circular muscle 

 layer. If, in any vessel, the muscles preponderate, the elastic 

 fibres diminish and recede towards the adventitia. 



THE VEINS. 



Veins differ from arteries in possessing thinner walls, less 

 elastic and muscular tissue, and for the most part a stronger 

 tunica adventitia. 



* Allgemeine Anatomie, p. 502 ; Handbuch der Anatomic des Menschen, 

 Band iii., p. 73 ; Gewebelehre, von Kolliker ; Luigi Fasce, Istologia delle 

 arterie e delle vene degli animali vertebrati. Palermo, 1865. 



