18 THE BLOOD. 



and the elements of a vascular tissue, although placed at an appreciable distance 

 from the vessels, may still be organised and living structures, and withui the 

 dominion of the nutritive process. How far the .si^here of nutrition may, in 

 certain cases, be limited, is a question that still needs fmiher investigation ; in 

 the cuticle, for example, and its appendages, the nails and hairs, which are 

 placed on the surface of the body, we must suppose that the old and dry part, 

 which is aboiit to be thrown off or worn away, has passed out of the limits of 

 nutritive influence ; but to what distance beyond the vascular surface of the 

 skin the province of nutrition extends, has not been determined. 



Regeneration. — When part of a texture has been lost or removed, 

 the loss may be repaired by regeneration of a new portion of tissue of 

 the same kind ; but the extent to which this restoration is possible is 

 very different in differeut textures. Thus, in muscle, a breach of con- 

 tinuity may be repaired by a new growth of connective tissue ; but the 

 lost muscukr substance is not restored. Eegeneration occurs in nerve ; 

 in bone it takes place readily and extensively, and still more so in 

 fibrous, areolar, and epithelial tissues. The special circumstances of the 

 regenerative process in each tissue will be considered hereafter ; but 

 we may here state generally, that, as far as is known, the reproduction 

 of a texture is efFected in the same manner as its original formation. 



In experimental inquiries respecting regeneration, we must bear in 

 mind, that the extent to which reparation is possible, as well as the 

 readiness with which it occurs, is much greater in many of the lower 

 animals than in man. In newts, and some other cold-blooded verte- 

 brata, indeed (not to mention still more wonderful instances of re- 

 generation in animals lower in the scale), an entire organ, a limb, for 

 example, is readily restored, complete in all its parts, and perfect in all 

 its tissues. 



In concluding what it has been deemed advisable in the foregoing pages to 

 state respecting the development of the textures, we may remark that, besides 

 what is due to its intrinsic importance, the study of this subject derives great 

 interest from the aid it promises to afford in its application to pathological 

 inquiries. Researches which have been made within the last few years, and 

 which are still zealously carried on. tend to show that the structures which con- 

 stitute morbid growths are formed by a process analogous to that by which the 

 natural or soimd tissues are developed : some of these morbid productions, 

 indeed, are in no way to be distinguished from areolar, fibrous, cartilaginous and 

 other natural structures, and have, doubtless, a similar mode of origin ; others, 

 again, as far as yet a^ipears, are peculiar, but still their production is with, much 

 probability to be referred to the same general ijrocess. The prosecution of this 

 subject, however, does not fall within the scope of the present work. 



THE BLOOD. 



KlYSICAL AND ORGANIC CONSTITUTION. 



The most striking external character of the blood is its well-known 

 colour, which is florid red in the arteries, but of a dark purple or 

 modena tint in the veins. It is a somewhat clammy and consistent 

 liquid, a little heavier than water, its specific gravity being 1-052 to 

 1'057 ; it has a saltish taste, a slight alkaline reaction, and a peculiar 

 faint odour. 



To the naked eye the blood appears homogeneous ; but, when 

 examined with the microscope, either while within the minute vessels, 

 or when spread out into a thin layer upon a piece of glass, it is seen to 



