TISSUES OF THE BODY. 



371 



Fig. 365. Vessels from the human retina, a, arterial 

 6, capillary network ; c, venous twig. 



It is easy to conceive that, 



In other vertebrates, likewise, the size of the capillaries must be greater 

 to correspond with the larger dia- 

 meter of the blood corpuscles. 



Touching now the distances 

 between the tubes, and the 

 greater or less vascularity of a 

 part depending thereon, the 

 most remarkable variety pre- 

 vails. The lungs, the glands, the 

 mucous membranes, and the skin 

 are the most vascular of all struc- 

 tures ; whilst other parts, such 

 as the serous and fibrous mem- 

 branes and the nervous trunks, 

 are very poor in vessels. The 

 vascular networks of the lungs 

 (fig. 364) and of the retina 

 (fig. 365) afford examples, 

 although the latter membrane 

 cannot be reckoned among 

 those poorest in blood in the 

 body. 



Finally, we know of tissues 

 which contain no blood-vessels, 

 such as the cornea, the lens 

 cartilage, the epithelial structures, and nails, 

 owing to the minuteness of the 

 elements of form, only consider- 

 able groups of the latter can be 

 surrounded by the capillary net- 

 works in organs with a small 

 amount of vascularity. But even 

 in those parts most abundantly 

 supplied with blood the capillary 

 tube always lies external to the 

 elementary structure, and never 

 penetrates into the interior; at 

 the very most, is each individual 

 form element surrounded with a 

 single loop, as in the case of 

 the fat cell ( 122) and muscle 

 fibre ( 168). The forms under 

 which capillary networks pre- 

 sent themselves are very nume- 

 rous, and at the same time fre- 

 quently so characteristic of the 

 various parts to which they be- 

 long, that the practised eye can 

 often recognise an organ from a 



c;pr>rirm nf it* sn'hstanpp wViiVVi Fig. 366. Vessels from about fat cells. A, an arterial 



twig at a, and venous at 6, with round capillary 

 has been injected. network of a fat globule. B, the capillaries of three 



These forms are chiefly deter- free cells of the latter ' 

 mined by the texture of the part and the grouping of its structural elements, 



