VEIN. 



1375 



cular tunic is less distinct, thinner than the 

 areolar, with which it is more or less mixed 

 up, and the latter is more abundant and more 

 developed than in small arteries. 



In passing to vessels of larger size, the 

 fibrous tissue is seen to have a more conspi- 

 cuous development. The longitudinal stri- 



Fig. 860. 



A, Small Portion of the Margin of a compressed 

 specimen of the Renal Vein of a Rabbit, treated 

 with Acetic Acid. 



a, external longitudinal coat, exhibiting yellow 

 fibres and persistent nuclei of areolar tissue ; b, 

 middle circular tunic, showing nuclei of muscle 

 cells ; c, internal longitudinal coat (striated mem- 

 brane), the limit of the vessel's cavity ; d, corre- 

 sponds to a small portion of the cavity of the vein. 



B, Ulinitte Vein from Mesentery of Rabbit, exhibit- 

 iif/ the three Tunics Jess distinctly. " (Magnified '200 

 diameters.) 



ations and fibres on the lining membrane form 

 a perfect striated membrane, and, in still larger 

 vessels, increase so as to constitute a compact 

 coat of fibres, of more or less density the 

 internal tunic of longitudinal fibres. In the 

 middle coat, the muscular element becomes 

 involved and mixed up with areolar tissue, 

 some fibres of which run parallel, and some at 

 right angles to tiie muscular nuclei. The 

 external tunic of areolar tissue becomes in- 

 creasingly developed, and yellow fibrous tissue 

 conspicuous and abundant.* 



In fg. 8GO. A, is represented part of the 

 renal vein of a rabbit. The vessel is not cut 

 open or seen in section, but is simply laid 

 upon, glass, treated with acetic acid, and a 

 small portion of the margin submitted to mi- 

 croscopical scrutiny- The internal membrane 

 is densely striated, and the other coats de- 

 veloped in proportion. 



* In the comparative examination of small and 

 large blood vessels, there is much to confirm Henle's 

 doctrine of nuclear and celt fibre. In the capillaries, 

 which are mere tubular cells, there is no fibre, and 

 the nucleus is persistent; in larger vessels, where 

 fibrous tissue is developed, the nucleus disappears 

 and yellow fibre is found : but in those veins which 

 retain abundant persistent nuclei, as the cerebral 

 veins, even in those of larger size, no nuclear fibre 

 is detected. 



There are certain regions in which the veins 

 undergo striking modifications in structure, 

 which require description ; and the remarkable 

 organs valves placed in their interior are 

 yet to be considered. 



These points will be examined as follows: 



I. Veins, at their junction with the heart. 



II. Cavse, passing through the diaphragm 

 and pericardium. 



III. Cerebral sinuses. 



IV. Umbilical vein. 



V. Venous valves. 



I. At the junction of the veins with Ike heart 

 there is a gre it alteration in their composition, 

 and this as it respects both cavte, and the pul- 

 monary veins : they assume a muscular cha- 

 racter, and become thicker and red, which 

 arises from a prolongation of the muscle of the 

 auricle into the fibrous structuie of the vein. 



This anatomical fact, as far as the cavse are 

 concerned, was described as early as 1664, by 

 Borelli ; subsequently (1700), by Bidloo, who 

 gave figures of it ; and afterwards (1779), by 

 Goiter, who imagined that the muscle was 

 continued into the smaller vessels, and that it 

 there assumed a paler colour. 



This muscular layer (though its existence 

 is denied by Cruveilhier) may be followed 

 upon the superior vena cava as far as the cla- 

 vicle, upon the inferior as far as the diaphragm, 

 and upon the pulmonary veins as far as the 

 division of the trunks into branches. (Henle.) 

 It exists on the superior cava in greater 

 amount than on the inferior, and it is there 

 best submitted to examination. When viewed 

 with the naked eye, the vena cava superior, 

 at its junction with the heart, is seen to be 

 red and muscular, and thicker than elsewhere. 

 In following this away from the heart, the 

 muscular character gradually diminishes, and 

 the ordinary fibrous tissue assumes the pre- 

 ponderance till the former is altogether lost. 

 At ton lines from the junction of the vessel 

 with the heart the mass was removed from 

 which the accompanying drawing was taken. 



The cavae of some birds are sufficiently thin 

 and transparent to be viewed with transmitted 

 light when entire, and when slit up and held 

 to the light a beautiful arrangement of these 

 fibres is seen : they do not form one flat, 

 even, circular, covering, but are arranged in 

 numerous, successive, interrupted, rings, very 

 fine and red, gradually diminishing as thev are 

 seen further from the heart. I have observed 

 this in the red-throated diver (Colymbus sep- 

 (entiiunalis), moor-hen (Galfinula chloropvs), 

 and various other birds. According to Cuvier, 

 in the ostrich, muscular fibres extend the 

 whole length of the posterior cava, and dis- 

 appear suddenly opposite the kidneys. 



When a little mass of this structure is ex- 

 amined with the microscope, it is found to 

 consist of muscle of the same character as that 

 of the heart, the fibres being small, striped 

 (though, perhaps, rather less distinctly so than 

 that of the auricle or ventricle), and, as far as 

 I have seen, destitute of sarcolemma. There 

 is a good deal of wavy fibrous tissue, inter- 

 mixed with the muscle ; i:i this respect, con- 



